Measuring Nursery Plant Water Use in Containers

Outdoor nursery growers produce a diverse catalogue of container stock. Inevitably, these nursery plants can differ in their water requirements and needs. Changes in weather throughout the growing season, plant growth over time, the type of substrate used, or even the region in which a plant is situated can drive differences in plant water-use requirements.

Aside from references in the literature, little information is available to answer questions about unique plant-, substrate-, or region-specific water needs. One effective method is to directly measure the volume of water the plants are using. But what is a practical method to do so?

Understanding plant water loss

After an irrigation event and once water stops draining, moisture is lost from a container through the drainage holes and from the substrate surface via evaporation and transpiration by the plant.

Evaporation

Evaporation from the substrate occurs through pores at the surface. The size of substrate particles usually dictates the size of the pore. Since bark substrates (primarily used in nurseries) traditionally have large particles (sometimes greater than 2 inches), they typically consist of a greater percentage of large- diameter pores than finer substrates, like peat moss. These large pores, called macro-pores or gravitational pores, readily release water. Thus, hot or sunny days can cause water to evaporate quickly out of the pores.

Most of this evaporation occurs in the upper bark layers, since the top substrate surface is mainly exposed to the atmosphere. While evaporation and water loss can occur through the drain holes at the bottom, most evaporative loss occurs from surface evaporation.

Transpiration

Surface evaporation is an important contributor to water loss from the container, but transpiration is the primary contributor.

Nearly all water used and lost by a plant is driven through its stomata opening and closing. Plant leaves need to keep stomata open to allow carbon molecules to enter leaf tissues for photosynthesis. As a result of open stomata, water molecules are naturally lost. This water- loss process is called transpiration. Thus, transpiration is a delicate balance of water loss versus carbon efficiency.

In addition to gas exchange and carbon accumulation, when water evaporates from the leaf surface via transpiration, water and mineral nutrients in the root zone are pulled inward by the roots and up through the plant. Thus, transpiration influences the water storage reservoir in the substrate and can dictate how often and how much irrigation is needed. Several factors influence transpiration rates:

• Crop type and canopy size make a difference; some plants have greater transpiration rates than others, and larger canopies often, but not always, increase water loss.
• High temperatures lead to greater evaporative loss.
• Warm temperatures can stimulate stomata to open and drive water loss.
• High light intensity stimulates stomata to open, escalating water loss.
• Low relative humidity dries the water boundary layer on the leaf surface, increasing transpiration rates.
• Wind dries the water boundary layer on the leaf surface.

Measuring evapotranspiration

Since both evaporation and transpiration contribute to substrate water loss and neither occur independently in practice, they are often grouped with the process called “evapotranspiration,” or ET.

Evapotranspiration rates will differ due to daily changes in weather patterns, different types of nursery crops, the stage of growth or time of year, container size, canopy and root mass size, and cultural practices such as specific substrate type or amendments, container spacing, or pruning.

Measuring evapotranspiration is relatively easy and can be important in understanding plant water requirements. To measure evapotranspiration, wait for a sunny period with no rain in the forecast for several days. Select a few nursery crops that are known or suspected to be high and low water users.

Heavily irrigate the containers to fill the substrate pores with water, then let the substrate drain for approximately one hour (fig. 1). After an hour of drainage and moisture equilibrium, weigh the containers. Then, wait 24 hours and reweigh the containers. Afterward, simply subtract the weight of the recently irrigated container from the new weight to calculate the volume of water lost per 24 hours. Since each fluid ounce of water equals 1 ounce by weight, this is the volume of water needed to reirrigate and fill the substrate pores.

To take it a step further, once the plants were reweighed after 24 hours, withhold watering the plants (if possible) until they begin to wilt. Once wilting occurs, follow the same steps as described above [i.e., subtract the weight of the container after 1 hour of drainage (step 3 in fig. 1) by the weight of the container once a plant has wilted]. This will identify the total amount of available water the plant can use (the middle ground between the volume of water at maximum substrate water storage and at plant wilt when the rootzone is too dry).

Understanding evapotranspiration rates for specific nursery crops can provide valuable information for informed substrate and irrigation management decisions, such as better substrate selection or grouping plants by water requirements.

Practical tips

Knowing plant-specific evapotranspiration rates and how quickly they lose water on hot or sunny days can help optimize irrigation scheduling or load sizes. It begins with understanding how much water, or the rate of application, the irrigation system is using.

Operations that apply irrigation via overhead sprinkler systems can conduct a leaching fraction test as described in Virginia Cooperative Extension’s publication “Leaching Fraction: A Tool to Schedule Irrigation for Container-Grown Nursery Crops” (SPES-128P), https://www.pubs.ext.vt.edu/SPES/SPES-128/SPES- 128.html, with some slight modifications. First, the leaching fraction test will help identify the volume and rate of water applied by the irrigation systems. Second, weighing the containers before and after the leaching fraction test allows calculation of the amount of water stored after the irrigation application.

Operations that use drip emitters or micro-irrigation can simply place the emitter in a measurable beaker for a specific period of time to calculate the volume of water per minute.

Conclusions

Understanding plant water use can help nursery managers optimize other production practices, such as production systems (container type, species-specific needs, substrate properties), irrigation type and scheduling, and cultural management decisions (spacing and grouping). By measuring evapotranspiration, growers can better understand how much water their plants are using and how long they’ll need to irrigate to resupply the root zone.

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