Selecting and Using Plant Growth Regulators on Floricultural Crops

Optimizing Results

Plant growth regulators (PGRs) are chemicals that are designed to affect plant growth and/or development (figure 1). They are applied for specific purposes to elicit specific plant responses. Although there is much scientific information on using PGRs in the greenhouse, it is not an exact science. Achieving the best results with PGRs is a combination of art and science — science tempered with a lot of trial and error and a good understanding of plant growth and development.

For best results, PGRs should be handled as production tools — like water and fertilizer. PGRs should be an integrated part of your crop production cycle. They should not be used as crutches for poor management of other cultural practices. However, PGRs should be used in conjunction with a number of nonchemical control options to manipulate plant growth so well-proportioned, compact plants are produced.

Biological Control

Selecting shorter growing cultivars is often the first step available to growers for reducing the occurrence of overgrown plants. While this works well in theory, it may not be commercially practical. Customer demand for specific color or growth-form characteristics may limit your choices. However, response to PGRs depends on species and cultivar selection. In general, slow-growing or dwarf cultivars will require less PGR than more vigorous cultivars. Some plant species or cultivars are responsive to specific PGRs, but not all PGRs. Research your crop, including its responsiveness to PGRs.

Environmental and Cultural Control

Knowing how the growing environment and cultural practices can affect plant growth will help in managing a crop’s growth. There are a number of factors that can be manipulated in the greenhouse or nursery to reduce plant growth: container size, timing of transplant or seeding, irrigation practices, nutrient management, mechanical conditioning, light quality and quantity, pinching, and temperature. How these factors are manipulated will affect whether chemical control is necessary, and if so, the amount of PGR required for optimum growth control.

Container Size

Root restriction can be used to control plant growth by utilizing a small container or by increasing the number of plants per pot. This method works especially well when other production parameters, such as ample light, wide spacing, and proper nutrition are provided. Plants grown in small pots at close spacing will require more chemical growth regulation for adequate growth control than those receiving ample light.

Timing

One of the most effective methods of controlling excessive plant growth is by crop timing. The simple method of staggering the finish time of a crop at two- to three-week intervals is very effective with many crops, like bedding plants. This ensures that a new supply of plants will be available, which avoids the need to hold a crop beyond its market window, when it generally becomes “leggy.”

Irrigation Practices

A traditional method of controlling plant growth is to withhold water. Drought stress can be used on a number of crops, including impatiens and tomatoes. Allow the plants to wilt slightly between irrigations, but do not allow them to reach the permanent wilting point. Drought stress will lead to shorter plants, but excessive stress or drought stress of sensitive crops may have the undesirable effects of reduced plant quality and delayed flowering. Drought stress may also cause premature bolting of some crops, such as ornamental cabbage and kale (cole crops).

Growers who tend to “run their plants dry” will use less PGRs than growers who run their plants wet. The method of irrigation can affect the plants’ response to PGRs. Plants grown on subirrigation trays or water collectors should be treated with lower rates of soil-active PGRs because the overspray from the treatment will be available to the roots during subsequent irrigation events.

Nutrient Management

Reducing or withholding fertilizer tends to slow overall plant growth. Limiting the amount of nitrogen to 50 to 100 parts per million (ppm) will help control the growth of many crops, such as bedding plants. The type of nitrogen supplied can also impact plant growth. Relying on nitrate-nitrogen instead of ammoniacal-nitrogen or urea-nitrogen forms — which encourage lush growth — will also help.

Phosphorus is the primary element that promotes stem elongation. Plug producers commonly use low-phosphorus fertilizers like 13-2-13 Cal-Mag or 15-0-15, which does not contain phosphorus to help limit stem elongation. As with “dry” plants, nutrient-deficient plants require less growth regulator for growth control than lush plants.

Mechanical Conditioning

Brushing plants is a very effective way of controlling plant height (30 percent to 50 percent reductions) of many vegetable transplants or herbs. Currently, only one PGR (Sumagic, Valent USA) is labeled for chemical growth control of fruiting vegetable transplants.

Brushing involves the movement of a PVC pipe, wooden dowel rod, or burlap bags over the top third of the plant. Research at the University of Georgia suggests that plants should be brushed daily for about 40 strokes to obtain the greatest effect. The foliage should be dry to avoid damage to the leaves. This method is not effective on plants such as cabbage or broccoli. It should not be used if foliar diseases or, in many cases, flowers are present. Evaluate the degree of growth regulation provided by brushing before adding a PGR treatment. Also, be aware that the effects of brushing on plant growth dissipate within three to four days after you stop applying the treatment. So, you may want to apply a low rate of a PGR to provide continued growth control during the shipping, handling, and retail phases.

Pinching

Pinching can be used to improve the shape of the plant, increase branching, and control excessive stretch. However, the labor costs of  pinching and the subsequent delay in plant development may not make it an economically feasible option of controlling growth of many crops (figure 2).

Light Quality and Quantity

Higher light quality tends to limit plant elongation, resulting in shorter plants. Low light quality caused by late spacing or crowding of the crop or too many hanging baskets overhead can lead to leggy plants and should be avoided.

Photoperiod also can be used to control the growth of many plants. This practice is widely used with pot chrysanthemums by providing taller cultivars with only one week of long days (LD) to limit vegetative growth when compared to shorter cultivars, which receive three weeks of LD to promote growth.

Light quantity also affects plant growth. Higher light levels improve plant growth and quality, as well as branching. Spacing will often determine the need for and amount of additional chemical control necessary for optimum height control under high light levels.

Temperatures

Temperature manipulation can be used very effectively to control plant growth. Lower temperatures slow plant growth. Remember to account for the effects of lower temperatures on the developmental processes of the plant, i.e., lower temperatures may delay flowering, so you may need to adjust your crop schedules to hit your market window.

Lower rates of PGRs are required for plants grown under lower  temperatures. Conversely, higher temperatures enhance plant growth and higher rates of PGRs are necessary for adequate growth regulation.

Some growers use differential day/night temperatures (DIF) to control growth. In the South and Mid-Atlantic regions, we are typically limited to a DIP in temperatures, where we reduce the predawn temperatures to 5 to 10 degrees Fahrenheit (F) lower than the night temperature setting and hold it for up to four hours. This treatment reduces growth at the time of day that cell elongation is greatest and therefore controls plant height. Obviously, do not drop the temperature low enough to injure cold-sensitive crops.

Optimizing plant growth control requires an understanding of the effects of environmental and cultural conditions on plant growth. Experience and in-green-house trials will allow you to combine PGRs with a number of nonchemical control options to manipulate plant growth to produce high-quality, compact plants.

Choosing the Correct PGR

The selection of PGRs and their application rates will be affected by the vigor of the cultivars selected and how your crop is grown. Especially with very vigorous plants, higher fertility and irrigation levels will increase the amounts of growth regulator required to prevent excessive growth. Shading, lower light levels, or tight plant spacing — especially under higher growing temperatures — will also increase plant stretch and reduce lateral branching. For the highest-quality plants, the use of PGRs must be integrated into your production plan.

PGRs are most effective when applied at the appropriate times to regulate plant growth or development. In other words, growth retardants cannot “shrink” an overgrown plant. They must be applied before the plant is overgrown to avoid plant stretch. When planning PGRs in your production schedule, consider what you want to accomplish with the treatment.

• Do you want to regulate shoot growth of the plant, resulting in a sturdier, more compact plant with improved color? If so, you probably want a growth retardant.
• Do you want to increase plant branching for enhanced cutting production or for a bushier potted plant or hanging basket? If so, you probably want to use a branching agent or “chemical pincher.”
• Do you want to enhance flower initiation or synchronize flowering? If so, you probably want to use chlormequat chloride or gibberellic acid.
• Do you want to remove flowers from stock plants to increase the number of vegetative cuttings? If so, you probably want to use an ethylene-generating compound.

Answering these questions will indicate which type of PGR you need to use to accomplish your goal and the most appropriate timing of the application. Then you will need to select a specific PGR in that class and determine the appropriate dosage and application method to attain the desired response.

Regulating Shoot Growth

Most PGRs used in the greenhouse or nursery are used to regulate shoot growth of containerized crops. These PGRs are referred to as “growth retardants.” Typical growth retardants are ancymidol (Abide or A-Rest), daminozide (B-Nine or Dazide), chlormequat chloride (Citadel or Cycocel), flurprimidol (Topflor), paclobutrazol (Bonzi, Downsize, Pac O, Piccolo, or Piccolo 10 XC), and uniconazole (Concise or Sumagic).

Now that most of the PGR chemistries are off-patent, there are several options available (table 1). These PGRs control plant height by inhibiting the production of gibberellins, the primary plant hormones responsible for cell elongation. Therefore, these growth-retardant effects are primarily seen in stem, petiole, and flower stalk tissues. Lesser effects are seen in reductions of leaf expansion, resulting in thicker leaves with a darker green color (figure 3).

* ppm = parts per million

Other benefits of using these PGRs in plant production include improved plant appearance by maintaining plant size and shape in proportion with the pot and increased shipping capacity with the smaller plants. Plant growth retardants also increase the tolerance of plants to the stresses of shipping and handling, as well as retail marketing, thereby improving shelf life and extending plant marketability.

Remember, growth retardants do not reduce plant size. They limit the plant’s growth rate. You must apply the growth retardant prior to the “stretch.” Look for recommendations on the PGR label for time of application. These recommendations will be given in terms of plant development or plant size, as opposed to production time.

For example, uniconazole (Concise or Sumagic) labels specify that pansies should have attained a minimum height of 4 inches prior to application. Paclobutrazol (Bonzi, Pac O, Piccolo, or Piccolo 10 XC) labels state that bedding plant plugs should be treated at the one to two true-leaf stage and bedding plants (after transplanting) at 2 inches of new growth or when the plants reach marketable size.

Generally, growth-retarding PGRs should be applied just prior to rapid shoot growth. This is generally one to two weeks after transplanting a plug, after the roots are established, and as the plant resumes active growth. On pinched plants, it is after the new shoots are visible and starting to elongate.

This is where the art of plant growth regulation is most important. You must learn how your crop grows and when to intervene to obtain the desired results. Remember to note details of crop development in your records of PGR treatments. For example, due to weather conditions, next year you may need to treat at seven days after transplanting instead of at 10 days after transplanting, which you used this year. You must gauge when rapid elongation will likely occur and treat to counter it.

Many growers use multiple applications of growth retardants to better control plant growth. A single application at a high rate early in the plant production cycle may be excessive if growing conditions are not as good as expected. An early application at a lower rate provides more flexibility, but the tradeoff is the additional labor involved with a second application if it becomes necessary. Some growers improve crop uniformity by using multiple applications of lower rates to affect small corrections in plant growth.

Be aware that excessive rates of many of these PGRs can cause persistent growth control in the flat or even in the landscape. It is always a good idea to evaluate the long-term effects of your treatments by growing some out for yourself and talking with your customers.

Be careful to avoid late applications, especially of paclobutrazol or uniconazole, because they may delay flower opening on bedding plants. However, drench applications of paclobutrazol have provided excellent control of poinsettia height very late in the production cycle without causing the reduction in bract size accompanying late spray applications. Learn the art of using PGRs for plant growth regulation.

Ancymidol (Abide or A-Rest; Restricted-entry interval [REI] = 12 hours) is a more active compound than daminozide or chlormequat chloride. Ancymidol is active as a spray or a drench, so application volume affects plant response. In addition, ancymidol is labeled for chemigation, i.e., distribution through the irrigation system via flood, sprinkler, or drip systems. Follow all label directions. Abide is not labeled for spray applications in shadehouses or nurseries, but drench applications can be made indoors and outdoors. A-Rest is labeled for use as a spray or drench on containerized plants in greenhouses, nurseries, shadehouses, and interiorscapes.

Ancymidol is widely used as a foliar spray for treatment of plants in the plug stage. Its relatively high activity and toning ability produces excellent plugs. Many growers consider ancymidol to be the product of choice for pansy production. Rates vary with cultivar or series. For example, the Delta series is more responsive to PGRs than the Sky/Skyline series.

Daminozide (B-Nine REI = 24 hours or Dazide REI = 12 hours) was one of the first PGRs labeled for use in the floriculture industry and is still widely used. In general, it is not phytotoxic and has a short-term effect that seldom results in overstunting of treated plants. The low activity of daminozide and its lack of soil activity make it easier to get consistent, predictable responses than with the newer, more potent PGR chemistries. Plants should be well-irrigated prior to treatment, but foliage should be dry at the time of treatment. Do not irrigate overhead for 18 to 24 hours after treatment.

The low activity also means that daminozide must be applied more frequently to maintain control over vigorous crops. Generally, foliar sprays of 2,500 to 5,000 ppm are applied every 10 to 14 days, as necessary. Daminozide is labeled for use on containerized or bed-grown crops in the greenhouse and on containerized plants grown outdoors under nursery conditions. Frequency of application may need to be increased to weekly for more vigorous cultivars grown outdoors.

Chlormequat chloride (Citadel or Cycocel; REI = 12 hours) is another PGR with a long history in floriculture. Note that the product-use labels for these chlormequat chloride products vary in application limits. See the label for your product for the specific rates and sites of application (table 1). Chlormequat chloride is generally applied as a foliar spray at 200 to 3,000 ppm with a maximum of three to six applications per crop cycle, depending on which product you use. Rates above 1,500 ppm often cause chlorosis on young, treated leaves of floricultural crops. Chlormequat chloride also promotes earlier flowering and greater flower numbers on Hibiscus and geranium (Pelargonium).

Chlormequat chloride is also labeled for drench applications at rates of 2,000 to 4,000 ppm when applied inside a greenhouse, depending on the specific product label (table 1). Chlormequat drenches may not be economically advantageous.

Of these chlormequat chloride products, only Cycocel is labeled for use on containerized plants in the outdoor nursery where it may be applied at a maximum spray rate of 3,000 ppm up to three times in any crop production cycle. This limit includes any applications of Cycocel combined with daminozide. Drench applications of Cycocel are not permitted in the outdoor nursery, even on containerized plants. Read the pesticide label for your product: It is the law for application sites and rates. Chlormequat chloride is not labeled for application through the irrigation system.

A daminozide/chlormequat chloride tank mix has more PGR activity than either daminozide or chlormequat chloride alone and generally causes less phytotoxicity than chlormequat chloride applied by itself.

Both the daminozide and chlormequat chloride labels have approved tank mix instructions. The combination provides activity that ranges from low (800 ppm daminozide plus 1,000 ppm chlormequat chloride) to very high (5,000 ppm daminozide plus 1,500 ppm chlormequat chloride).

This tank mix has been tested on a wide variety of perennials. For example, three-lobed coneflower (Rudbeckia triloba) was very responsive to daminozide applied twice at 5,000 ppm but was not responsive to chlormequat chloride at rates up to 4,000 ppm. However, a tank mix of 5,000 ppm daminozide with increasing rates of chlormequat chloride resulted in height control similar to the daminozide treatments with a single application.

Although the rate of daminozide is usually adjusted to increase or decrease activity, changing the chlormequat chloride rate also affects activity. Single applications of the tank mix are frequently more effective than multiple applications of daminozide alone. However, multiple applications of the tank mix may be required for the more vigorous crops.

Flurprimidol (Topflor; REI = 12 hours) is similar in chemistry to ancymidol but much more potent. Its activity is similar to that of the triazoles. Many floricultural crops are responsive to flurprimidol. With spray applications, flurprimidol rates are similar to those used with paclobutrazol. However, in soil applications, its activity is more similar to that of uniconazole. Topflor is labeled for use as a spray, drench, or chemigation on containerized ornamental plants grown in nurseries, greenhouses, and shadehouses. Topflor is only recommended for a few bedding plant plugs and should never be used on the plugs of sensitive crops like begonia, pansy, salvia, or vinca. Flurprimidol is very active on most bulb crops, like tulips, Oriental lilies, callas, caladiums, and hyacinths, where it is applied as a drench when the new growth is about 1 inch tall or as a bulb soak prior to planting.

The triazole class of PGRs includes paclobutrazol (Bonzi, Downsize, Pac O, Piccolo, or Piccolo 10 XC; REI = 12 hours) and uniconazole (Concise or Sumagic; REI = 12 hours). These compounds are much more active than most of the previous compounds. Uniconazole is more potent than paclobutrazol.

As mentioned above, the activity of flurprimidol (Topflor) is between these two triazoles, depending on application method. These PGRs are rapidly absorbed by plant stems and petioles or through the roots. Excess spray dripping off treated plants acts as a drench to the substrate, increasing the activity of the treatment. For foliar sprays of triazoles, uniform application of a consistent volume per unit area is critical to uniform and consistent crop response.

Both compounds (see table 1) are labeled for application to the media surface prior to planting plugs. In this case, the PGR is applied as a spray to the surface of the medium in filled pots. The PGR moves into the medium with subsequent irrigations and effectively behaves as a drench.

Take care with applications to sensitive plants. In some cases, excessive stunting can be persistent. These compounds must be used carefully and appropriately. Especially when working with the triazoles, thoroughly test your application methods and rates on a small number of plants before treating your entire crop. Avoid late applications of the triazoles. They should be applied prior to flower initiation when possible.

Paclobutrazol has a broad label for ornamentals that includes use on greenhouse or outdoor-grown, containerized crops. See table 1 for label restrictions for the different products. All of the paclobutrazol products are labeled for application through the irrigation system, including ebb/flow or flooded floor systems. Do not use paclobutrazol on annual vinca (Catharanthus roseus), because it causes spotting, or on fibrous begonias, which exhibit severe stunting with exposure to paclobutrazol.

To establish rates for plants not listed on the product label, treat a small number of plants with 30 ppm as a foliar spray or 1 ppm as a drench. In many cases, multiple treatments with lower rates have been more effective, with less chance of overstunting, than a single application at a higher rate.

Uniconazole also has a broad label for ornamentals, but its use is limited to containerized plants grown in greenhouses, overwintering structures, shadehouses, or lath houses. It is not labeled for outdoor nursery use. Uniconazole also is not labeled for application through any irrigation system. It has been very effective on a large number of floricultural crops. As with paclobutrazol, avoid using it on fibrous begonias. It is very potent, so pay special attention to proper mixing, uniform application, and proper volumes. Use caution in the higher rates or on more sensitive species, because uniconazole effects can be persistent even after the plant has been transplanted into the landscape.

NOTE: Ancymidol, flurprimidol, paclobutrazol, and uniconazole are persistent on plastic surfaces and in soil. Do not reuse flats, pots, or soil from treated plants — especially for plug production of sensitive crops.

Enhancing Lateral Branching

Another group of PGRs used in floricultural crops are those that enhance branching, including ethephon (Collate; Florel), benzyladenine (BA; Configure), dikegulac sodium (Atrimmec), and methyl esters (Off- Shoot-O; table 2). These PGRs are frequently called chemical pinchers because they generally inhibit the growth of the terminal shoots or enhance the growth of lateral buds, thereby increasing the development of lateral branches. They can be used to replace mechanical pinching of many crops like Vinca vine, Verbena, Lantana, and English ivy (Hedera).

Ethephon (Collate; Florel brand Growth Regulator; REI = 48 hours) is a compound that breaks down in plant tissue after application to release ethylene, a natural plant hormone. As with ethylene, its effects can vary depending on the species and the stage of growth at time of application. Collate and Florel have broad-use labels for increasing lateral branching of floricultural crops and are commonly used on zonal and ivy geraniums and poinsettia to increase branching (figure 4).

You may need to consider combinations of PGRs. For example, if you apply ethephon to enhance the branch development of Wave petunias in a hanging basket, you may still need to follow up with a treatment of a plant growth retardant to control the elongation of those new laterals.

Ethephon should be applied to actively growing plants prior to flower development. If flowers are present at the time of application, they are likely to abort. Ethephon may delay flowering by about one to four weeks, particularly if applied close to the time of flower initiation. Ethephon should not be applied to plants that are heat- or drought-stressed.

The pH of water used for the spray solution can be important. If the pH is too high, the ethephon will convert to ethylene before it gets to the plant, and activity will be reduced. Collate and Florel contain sufficient acidifiers and buffers to maintain a pH of 5.0 or lower when mixed with most greenhouse water supplies. In general, water that has sufficient quality for irrigation of greenhouse crops (moderate pH and alkalinity) is suitable for mixing ethephon. However, if you are acidifying your water prior to irrigation, use the acidified water for mixing the ethephon as well. The solution should be applied within four hours of mixing.

Benzyladenine (6-BA Configure; REI = 12 hours). Configure is a synthetic cytokinin (6-benzyladenine), which is a plant hormone that stimulates lateral branching. It is a relatively inexpensive PGR that enhances branching of a wide variety of floricultural crops. BA stimulates, but does not cause, an increase in branching. Therefore, timing of the application is critical to a good branching response. Read the label for details on when to apply for optimum response because BA has a short period of activity and no residual in the plant. So, multiple applications may be useful with many crops. Furthermore, BA is not well-translocated in the plant, so thorough coverage is required.

Depending on the timing of the application, BA increases branching of the phylloclades or — when applied during floral initiation — increases the number of flower buds breaking on Christmas cactus. Configure at 500 to 3,000 ppm increased basal branching of Hosta and at lower rates, 300 to 600 ppm, for increased basal branching of Echinacea.

Further screening trials with other annuals and herbaceous perennials have identified a large number of crops with increased basal or lateral branching in response to BA. However, pansy is very sensitive to spray applications of BA with long-lasting leaf yellowing even at low rates (50 to 100 ppm). So, consider multiple applications at low rates. Due to leaf yellowing, do not use Configure on exacum. Several growers report successful use of Configure to increase the number of shoots on plugs and liners.

Although the primary objective with BA is to increase branching, it has resulted in growth reduction in some crops. However, if additional growth control is necessary we have found that growth retardants may be tank mixed with BA or be used immediately following the BA treatment, without reducing the branching response.

Dikegulac sodium (Atrimmec; REI = 4 hours) is a compound that delays terminal growth by interfering with cell-wall synthesis, which is required for new growth. By primarily inhibiting terminals, apical dominance is reduced, which enhances the production of lateral branches.

Atrimmec (PBI Gordon) has a broad-use label for containerized greenhouse ornamentals as well as for container-or field-grown ornamentals and trees.

Atrimmec should be applied to actively growing plants with at least two nodes to provide sufficient lateral development. In addition to creating a fuller plant, enhancing the number of laterals in a pot generally reduces the overall height of the plant due to the greater distribution of resources. Dikegulac sodium usually causes leaf chlorosis and may delay growth resumption. Therefore, it is critical to apply it early in the production cycle. Responses of herbaceous floriculture crops are very species-specific, so test several rates (400 to 1,600 ppm) under your growing conditions.

Methyl esters (Off-Shoot-O; REI = 4 hours) are labeled for chemical pinching of actively growing azalea, cotoneaster, juniper, ligustrum, Rhamnus, and Taxus. Ensure good coverage of growing points to physically burn soft tissue. Do not apply to the same plants more than once, and do not apply to herbaceous plant material.

Plant Flowering

Plant growth regulators can be used to enhance flowering. To improve flowering, Florgib 4L, ProGibb T&O, or GA₃ 4%, which contain the growth promoter gibberellic acid (GA₃), can be used to substitute for all or part of the chilling requirement of some woody and herbaceous ornamentals typically forced in the greenhouse, including azalea for florist crops and Aster for cut flowers. These compounds also can improve flowering and/or bloom size of camellia and baby’s breath (Gypsophila), promote earlier flowering and an increased yield of statice (Limonium) and induce flowering of Spathiphyllum. Gibberellic acid is also used to promote growth and increase stem length of other cut flowers like stock (Matthiola), Delphinium, and ‘Sweet William’ (Dianthus). See product labels for specific uses and recommended rates.

Again, timing is critical, because late applications or excessive rates may cause excessive plant stretching, resulting in weak, spindly stems. Chlormequat chloride (a plant growth retardant) used to control stem height of hibiscus and geranium also improves early flowering of these crops.

* ppm = parts per million

Removal of Flowers

Flower removal is especially desirable for stock plants maintained for cuttings of vegetatively propagated ornamentals, like Verbena or Lantana. Ethephon (Collate; Florel) is the primary compound used for flower removal. Once ethephon is absorbed by the plant, it is converted to gaseous ethylene — a natural plant hormone effective in many plant processes. Ethylene is the primary hormone responsible for flower senescence and fruit ripening. It is the “postharvest” hormone. With proper rates and timing, it will remove unwanted flowers from stock plants, cuttings, or plugs. Flower removal diverts more energy into vegetative growth, increases the number of laterals available for cuttings on stock plants, and promotes increased branching of plugs and finished plants, which increases fullness in the container. Because initiation and development of flowers require time, ethephon should not be used on crops within six to eight weeks of marketing.

Other PGR Uses

Another specific application of the gibberellin and cytokinin products (Fascination or Fresco) is the reduction of lower leaf yellowing on Easter, Oriental, and LA hybrid lilies. See the label for detailed instructions. These products also may be used to increase bract expansion in poinsettias. Fascination, Fresco, ProGibb T&O, and GA₃ 4% are labeled to promote the growth of plants that have been overregulated by plant growth retardants. These PGRs are very potent growth promoters. Start with low rates, 1 to 3 ppm, and apply at five-day intervals as necessary.

Read the Label!

Plant growth regulators are classified as pesticides. Therefore, they are subject to all of the same U.S. Department of Agriculture (USDA) recordkeeping and Worker Protection Standard (WPS) rules as all of your other pesticides. Their use is governed by the manufacturer’s label, as with other pesticides. The label not only contains information on restrictions, but also much information on how to use the product effectively. Before going to the time and expense of applying PGRs to your crop, answer these questions:

• Is the chemical labeled for the crop you wish to treat? Most PGR labels have undergone revisions that apply to a broad range of similar crops not specifically listed on the label, with the user taking responsibility for determining appropriate rates. This provides label permission to use the compound on these crops without the manufacturer accepting the responsibility for the rate selection.
• Is the chemical labeled for the area you wish to treat? Many of the PGRs are only labeled for use inside a greenhouse or other growing structure.
• Are there any potential side effects, such as phytotoxicity? Note that you may need to look elsewhere for this information for your specific crop.
• Are there label warnings regarding the PGR’s effect on plant flowering? For example, many branching enhancers delay flowering. Collate or Florel causes flower bud abscission prior to enhancing branching; therefore, is not recommended within six to eight weeks of marketing. Side effects are frequently affected by the timing of the application; e.g., late applications of growth retardants may delay flowering.

Always follow the label for mixing and application instructions. Many of these products require thorough shaking before dispensing. For best results, use only clean equipment that is dedicated to PGRs. Do not use sprayers that may contain other pesticide residues. In general, PGR labels restrict the addition of wetting agents and tank mixing with other pesticides or fertilizers. See the label for specific applications that recommend additional adjuvants. Follow label directions exactly when mixing PGR solutions, and apply them on the same day as they are prepared. Store PGRs tightly sealed in their original containers in a cool, dry, dark place.

Application Guidelines

Spray Applications

Plants to be treated with PGRs should be healthy, turgid, and unstressed – never wilted. The label will identify the target tissue for that PGR. For example, daminozide is only effective as a foliar spray, whereas paclobutrazol and uniconazole sprays must reach the stems. When making spray applications, look at the growth and development of the plant to see that there is sufficient development to make the treatment effective and accomplish your goal. Generally, there should be sufficient foliage or stems to absorb the PGR.

Uptake and effectiveness of a PGR also depend on selecting the application technique that will ensure proper coverage of the target tissue. Daminozide is not soil-active. Therefore, a foliar spray application, wetting most of the foliage, is necessary to provide a uniform reduction in growth. Leaf surfaces should be dry for foliar applications, and the best uptake of PGRs from spray applications will occur under low-stress, low-drying conditions. This is more critical for daminozide and ethephon than for some of the newer chemistries, like the triazoles. Overhead irrigation after treatment with daminozide or ethephon should be delayed for 18 to 24 hours to avoid washing the material off of the leaves.

The triazoles — paclobutrazol and uniconazole — are absorbed primarily by stem tissue and then translocated upward in the plant. Therefore, consistent and complete coverage of the stems is necessary for uniform effects. In other words, if the stem of one lateral receives an inadequate amount of spray, it will grow faster than the others, resulting in a poorly shaped plant, most noticeable in potted crops like poinsettia or chrysanthemum. Ancymidol and flurprimidol are taken up by both foliage and stems. In addition, all four of these compounds are very “soil-active,” which means they may be adsorbed to particles in the media and become available to the plant through root uptake. Therefore, drenching is a very effective application method for these chemicals in crops where it is economically feasible (see Applying Drenches, below).

The label will provide a recommended application volume for sprays, especially for chemicals that are soil-active. All foliar applications of PGRs should be applied on an area basis, i.e., uniformly spray the area where the plants are located with the recommended volume of solution. DO NOT spray individual plants or spray to reach a subjective target like “spray to glistening.” Since every applicator will have a slightly different definition of these goals, there will be no way of recommending appropriate rates or obtaining predictable results.

For soil-active PGRs, dosage is dependent on both the concentration of the solution and the volume of that solution applied in the treated area. Therefore, to improve predictability, the label-recommended spray application rates are generally set at 2 quarts of finished spray per 100 square feet, which is sufficient to cover the plant and permit a small amount of runoff onto the substrate. It also is considered to be a comfortable walking pace for applicators with hand-held sprayers. This is the same application volume recommended for daminozide, which is not soil-active.

With the soil-active PGRs, precautions should be taken to avoid overapplication with sprays. Spray applications require more attention to detail, because over-spray material lands or drips onto the medium. The overspray from a 2 quart per 100 square feet application is a part of the recommended dosage. However, if your application volume exceeds that recommendation, then your application dosage also exceeds the recommendation (figure 5).

Recognizing that stem coverage is necessary for the triazoles, you may need to apply a higher-than-recommended volume to large or dense plants to obtain adequate coverage. In fact, the paclobutrazol label recommends a spray volume of 3 quarts per 100 square feet for “larger plants with a well-developed canopy.” Adjust the concentration you apply accordingly. This suggests the importance of recordkeeping (see Recordkeeping, below). Always consider the rates presented in the appendix, on PGR product labels, or from any other resource, to be guidelines to assist you in developing your own rates based on your growing conditions and application methods.

The relationship of rate and volume can be exploited when treating multiple crops with different PGR needs. With a single solution of PGR in the spray tank, you can apply the label-recommended volume to attain your basic application dosage, or you can apply additional volume to crops that need additional growth regulation to attain a higher dosage. Application volume is another tool you can use to maximize your efforts and reduce time mixing or reloading higher concentrations of PGR solutions.

Spray Equipment

To assure proper spray volumes, your compressed air sprayer should be equipped with a pressure gauge and regulator, and you should consistently use the same nozzle for all PGR applications. Your sprayer should be calibrated by determining the output of the chemical with the selected nozzle at the selected pressure within a specified time period. Using this information, you can apply a known amount of material to a known area.

Spray droplet size also affects response, with smaller droplet sizes providing better coverage — but only up to a point. Mist- or fog-type applicators do NOT provide adequate volume for coverage of plant stems and the medium, and therefore, have not been effective when used with compounds like paclobutrazol and uniconazole. PGR applicators should be trained to uniformly apply a given amount of clear water in the greenhouse before they make PGR applications. Uniformity of the application is critical to the uniformity of the crop response.

Applying Drenches

Although drench application has several advantages over sprays, traditional drenches are seldom used on perennials due to the higher application costs of handling individual pots. Drenches generally have a less negative effect on flowering or flower size and tend to provide longer-lasting growth regulation than sprays. Drenches are easier to apply uniformly than sprays because the drench volume is easily measured; when applied to moist substrate, it is easy to obtain good distribution of the PGR in the media. Therefore, the resulting growth regulation is frequently more uniform.

The product label specifies the recommended volumes for drench applications to different size pots or types of media. In general, 4 fluid ounces of drench solution is applied to a 6-inch “azalea” pot, and that volume is adjusted up or down with pot size to obtain a volume where about 10 percent of the solution runs out of the bottom of the pot when the substrate is moist.

Remember that the amount of active ingredient (a.i.) applied to plants depends on both the concentration (ppm) of the solution and the volume applied. Read the label. Table 3 provides a general table of volume recommendations for drench applications.

Alternative methods of applying PGRs directly to the substrate have been developed and are described on the label. For example, ancymidol, flurprimidol, and paclobutrazol are labeled for application through the irrigation system (chemigation). These are generally labeled for flood (subirrigation), drip irrigation, and overhead sprinkler systems. Again, rates vary with the volumes used and method of application. Paclobutrazol applied once by subirrigation requires 50 percent to 75 percent of the amount of paclobutrazol that is applied in a typical drench application.

Pressure-compensated drippers are recommended for use with PGRs to more accurately regulate the volume of solution applied to each pot. Read and exactly follow the label for chemigation applications, especially with regard to safety of municipal water supplies.

Three other methods of providing a drench-type application of soil-active PGRs on a more economical scale are being used by growers: substrate surface application sprays, sprenches, and watering in.

Substrate surface application sprays are spray applications made to the surface of the substrate of filled flats or pots. The treatment is applied at normal-to-high spray volumes, but because it is applied to the substrate surface, it is activated by irrigation and is available to the plant in the root zone. Both paclobutrazol and uniconazole are labeled for this method of application. Rates are lower than those used for sprays but higher than those used for drench applications.

Sprenches, the second method, is a high-volume foliar spray that results in additional runoff into the substrate, providing a drench effect. Rates are lower than for recommended for spray rates.

Watering in is a type of chemigation where the PGR is injected into the irrigation water and applied at each irrigation at very low rates of active ingredient. Only PGRs labeled for chemigation can be used for watering in.

All of these application methods use the relationship between rate and volume to provide the desired control. Again, you must develop techniques that fit your production methods and your growth management preferences.

Liner soaks or drenches are another specialized way to use soil-active growth retardants. The root system of rooted liners or plugs is dipped into a solution of the PGR (or they may be thoroughly drenched in the plug tray). Extensive work has been conducted at the University of Florida on this application method:

• Liners should be “dry,” which is defined as the root ball being ready for irrigation plants but not under drought stress.
• Time in the solution is not critical; 30 seconds to two minutes is sufficient for saturation of the rootball.
• Liners may be planted immediately or held up a few days without loss of PGR effect.
• There is no loss of effectiveness of the soak solution during treatment.

Advantages of the liner soak include early control of very vigorous crops and flexibility of the treatment with respect to not having to handle plants during the restricted-entry interval. The liner dip is especially useful in combination plantings where the more vigorous plants can be treated prior to planting without reducing the growth of the slower plants in the group. The liner dip rates should be selected to provide early control of plant growth. Additional PGR applications can be made as necessary for longer term crops.

Be Aware of Bark

For many years, the adage in PGR drenches has been, “Bark ties up soil-active PGRs.” However, new research shows that this is not necessarily true. As long as the bark is properly aged before the media is mixed, it has little effect on the availability of these soil-active PGRs to the plant roots. Again, you must identify PGRs and rates that work with your production system.

Growing Conditions

Look also for label recommendations on time of day or condition of the plant for optimum treatment response. Generally, a healthy, unstressed plant growing under low evaporative conditions, e.g., early in the morning or late in the afternoon, is most responsive to treatment.

To maximize uptake, the chemical must remain in contact with the leaf long enough to be absorbed. This time varies for the different PGRs, but generally, foliar uptake is enhanced with slower drying conditions, which in turn increases the effectiveness of the treatment. This is especially important with foliar uptake of PGRs, such as daminozide, chlormequat chloride, BA, or ethephon. Plants treated with daminozide or ethephon should not be overhead irrigated for at least 18 to 24 hours after treatment, but plants treated with flurprimidol, paclobutrazol, or uniconazole may be irrigated one hour after treatment. Read the label for any warnings on how irrigation or environmental conditions will affect plant response to the PGR treatment.

Recordkeeping

Keeping notes on your application methods and the results of your PGR treatments will allow you to improve the consistency of your own application methods and establish rates and volumes appropriate for your production system. Note the concentration and volume applied, the stage of development of the crop (number of leaves, approximate height, presence of flowers), and the environmental conditions under which the PGR was applied. It is always recommended to keep a few untreated plants for comparison, especially if you are new to using PGRs (figure 6).

Summary

The degree of growth regulation caused by PGRs is impacted by all other phases of plant culture. Remember that you have to fit PGRs into your own production program. Plan ahead to achieve the best results from PGRs; do not use them as an afterthought when the plants are out of control. You cannot shrink an overgrown plant!

The multitude of variations possible in application methods, cultivar and species grown, and growing conditions makes it impossible to recommend specific rates for all operations. Use the product labels and the appendix as resources for the use of PGRs on a variety of crops. Use the lower of suggested effective rates for starting your own trials.

There are two general rules for using rate recommendations from other sources:

1. Southern growers use higher rates and more frequent applications than Northern growers. Rates for Virginia/North Carolina tend be closer to the Southern rates.
2. Outdoor applications usually require higher rates or more frequent applications than plants grown under cover.

Always consider any rate recommendation as a starting point for your own trials and keep records of your successes and failures with PGRs. When you treat your crop, hold back a few untreated plants so you can judge the effectiveness of your treatment (figure 7). Remember that methods of application have significant effects on results. Develop your own program, then test and refine it. Watch for PGR compounds new to the floriculture market and for expanded labeling of current products as we develop more guidelines for their use on floricultural crops.

Recommended Resources

PGR Calculator

For a ready resource on preparing PGR solutions for both spray and drench applications, use the online PGRMix Master app, a plant growth regulator calculator developed by floriculture specialists from North Carolina State University and the University of New Hampshire: http://e-gro.org/mixmaster/.

This online calculator allows you to enter your own PGR costs and calculate solutions based on the rate desired and the amount of area to be treated. The program includes information on both spray and drench applications.

OHP PGR Calculator

OHP, Inc. also has a PGR mixing app for iPhone, iPad or iPod. The PGR calculator app is available for free from the Apple App Store. Product information bulletins and other publications are also available with this app. See www.ohp.com for more information.

For an extensive list of crops and their responses to different PGRs, see the pdf publication "PGR Guide for Annuals" under the Grower Resources tab at https://www.fine-americas.com/.

Helpful Conversions

Volume

1 gallon (gal) = 128 fluid ounces (fl oz)

1 fl oz = 30 milliliters (ml)

1 gal = 3,785 ml = 3.785 liters

1 cup = 48 teaspoons

1 tablespoon = 3 teaspoons

1 fl oz = 2 tablespoons = 6 teaspoons

Weight

1 ounce (oz) = 28.3 grams (g)

1 pound (lb) = 16 oz = 454 g

Concentration

1% = 10,000 ppm
1 ppm = 1 milligram (mg) per liter

Disclaimer

Commercial products are named in this publication for informational purposes only. Virginia Cooperative Extension does not endorse these products and does not intend discrimination against other products which also may be suitable.

Appendix. Growth regulators for floricultural crops in greenhouses.

This table lists labeled rates of plant growth regulators for greenhouse crops, as well as recommendations based on research at North Carolina State University and recommendations by suppliers. Read labels for a complete listing of precautions. The degree of control can vary depending on a number of factors, including plant type, cultivar, stage of development, fertilization program, growing temperatures, and crop spacing.

When using a PGR for the first time, it’s good to test the rate on a few plants prior to spraying the entire crop. Keep accurate records and adjust rates for your location.

General recommendations: Plug culture and flat culture have different recommended rates. The rates in this table include recommendations for both plug (lower rates) and flat culture (higher rates). Apply ALL foliar sprays of plant growth regulators using 0.5 gallon per 100 square feet of bench area.