Hardiness is the ability of a tree to survive in a particular geographic area, with temperature being the major determining factor. Trees in Virginia are subjected to winter air temperatures ranging from average minimal sub-zero lows in USDA Hardiness Zone 5b (-10°F to -15°F -- parts of Bland, Giles and Tazewell counties) to the relatively mild average minimal annual lows of Zone 8a (15°F to 10°F -- Chesapeake and Virginia Beach). All areas of the state may experience summer air temperatures up to 105°F or more, with the Richmond area generally being the hottest.
In the landscape, soil buffers temperatures at the cold extreme, with soil temperatures below a frozen crust seldom dropping below 20°F to 30°F. Tree roots are generally several degrees less cold hardy than tree stems. This means that roots of trees in containers and planters, whose stems might ordinarily be hardy to sub-zero temperatures, may die when substrate temperatures drop below freezing. If the substrate freezes, wind may cause leaves on evergreen trees to dry out because roots are unable to absorb water from the frozen root ball. Wind can also speed container substrate temperature decreases.
Tree roots in containers and planters may die during the summer when substrate temperatures frequently exceed air temperatures, often going above 120°F. Wind, as well as heat from pavement, can increase water evaporation from containers, leading to dried out leaves.
Containers and planters for landscape trees can be constructed of a variety of materials in many different shapes and configurations. Coordinate the appearance of containers and planters with the landscape site. Also, determine how much time and money is available for container and planter maintenance and select accordingly.
With regard to container or planter size, a confined tree root system ideally should have two cubic feet of substrate for every square foot of tree canopy. Consider the mature size of the desired tree and design or select containers or planters with adequate root volume. Remember that the greater the substrate volume in the container or planter, the greater the temperature buffering, and the reserve of water and nutrients. In exposed and windy areas it may be desirable to line containers and planters with supplemental insulation.
Drainage is very important when designing or selecting containers and planters. For containers, and planters with constructed bottoms, be sure that channels exist to drain away excess water. Planters that are open to the ground should be built atop soil through which water readily drains.
If containers must be used to permit seasonal relocation to more temperate conditions during winter months, select small containers and trees. To avoid container damage or structural collapse if containers are used on above ground patios, rooftops, etc., keep the total weight of the container, substrate, trees and water in mind.
Even with careful container and tree selection, trees may outgrow the volume of their containers. Periodic tree removal for root pruning and substrate replacement is one method of controlling size (a technique used in bonsai). Removal and root pruning can be labor intensive, however, especially with large trees. Rather than root pruning, consider transplanting trees into larger containers or planters, or into the landscape, if they grow too large.
One advantage of growing trees in containers and planters is the opportunity to blend a substrate that may be better than the existing site soil. Substrates for containers and planters should resist changes in volume and compaction due to rain and irrigation. The substrate should have good aeration and drainage while retaining adequate water. Field soil is a poor container substrate because gravity cannot pull excess water out of the container, leaving field soil too wet for container use. In addition, field soil can be contaminated with weed seeds, insects and diseases.
A variety of organic and inorganic components can be used to blend a substrate including pine bark, compost, sand and expanded clay products. Avoid using peat moss that is difficult to rewet if it dries out, and perlite and vermiculite that are light and may float. All soilless substrates need to be supplemented at least yearly with a complete slow release fertilizer with micronutrients.
Substrate composition should be tied to irrigation method. If the only available water will be rain, use a substrate with smaller pore spaces to retain more water. To increase the water-holding capacity of a substrate, add sand to create smaller pore spaces in which water will be held. If regular watering or irrigation will be available, a lighter substrate may be adequate.
Selection of a suitable tree depends upon location (environmental and size), container or planter size, and desired tree form or type. Trees that will remain in place should be selected for their year-round appearance, maintenance requirements, and size. They should be aesthetically compatible with their container/planter and surroundings. Trees classified as dwarf, semi-dwarf, and small are the most appropriate size selections.
|Trees recommended for containers and planters|
|Common Name||Latin Name||Cultivars and Comments|
|Trident maple||Acer buergeranum|
|Hedge maple||Acer campestre|
|Amur maple||Acer ginnala||'Flame'|
|Paperbark maple||Acer griseum|
|Fullmoon maple||Acer japonicum|
|Japanese maple||Acer palmatum||Dwarf cultivars|
|Serviceberry||Amelanchier spp.||Numerous cultivars|
|River birch||Betula nigra||Large planters only|
|Eastern redbud||Cercis canadensis||'Forest Pansy', 'Silver Cloud', 'Texas White'|
|American hornbeam||Carpinus caroliniana||Large planters only|
|Falsecypress||Chamaecyparis spp.||Dwarf cultivars; evergreen|
|Chinese fringetree||Chioanthus retusus|
|Kousa dogwood||Cornus kousa|
|Corneliancherry dogwood||Cornus mas|
|Cockspur hawthorn||Crataegus crusgalli||var. inermis is thornless|
|Downy hawthorn||Crataegus mollis|
|Honeylocust||Gleditsia triacanthos||'Sunburst'; large planters only|
|Common witchhazel||Hamamelis virginiana|
|Possumhaw, deciduous holly||Ilex decidua||'Council Fire'|
|Nellie R. Stevens holly||Ilex x 'Nellie R. Stevens'||Evergreen|
|Foster's Hybrid holly||Ilex x attenuata||'Foster's'; large planters only; evergreen|
|Juniper||Juniperus spp.||Tree-form cultivars; evergreen|
|Goldenraintree||Koelreuteria paniculata||Medium to large planters|
|Crape myrtle||Lagerstroemia indica||Smaller cultivars; provide winter protection above Zone 7b|
|Southern magnolia||Magnolia grandiflora||'Little Gem'; evergreen|
|Ann magnolia||Magnolia x 'Ann'|
|Galaxy magnolia||Magnolia x 'Galaxy'||Large planters only|
|Saucer magnolia||Magnolia x soulangiana||Large planters only|
|Star magnolia||Magnolia x stellata|
|Centurion crabapple||Malus x 'Centurion'||Large planters only|
|Japanese crabapple||Malus floribunda||Large planters only|
|Indian Magic crabapple||Malus x 'Indian Magic'||Large planters only|
|Sargent crabapple||Malus sargentii|
|Sourwood||Oxydendrum arboreum||Medium or large planters|
|Lacebark pine||Pinus bungeana||Evergreen|
|Swiss stone pine||Pinus cembra||Evergreen|
|Japanese red pine||Pinus densiflora||Large planters only; evergreen|
|Mugo pine||Pinus mugo||Evergreen|
|Japanese black pine||Pinus thunbergiana||Large planters only; evergreen|
|Purpleleaf plum||Prunus cerasifera||Numerous cultivars|
|Kwanzan cherry||Prunus serrulata||'Kwanzan'; large planters only|
|Yoshino cherry||Prunus x yedoensis||Large planters only|
|Callery pear||Pyrus calleryana||Large planters only; avoid 'Bradford'|
Virginia Cooperative Extension materials are available for public use, re-print, or citation without further permission, provided the use includes credit to the author and to Virginia Cooperative Extension, Virginia Tech, and Virginia State University.
Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Alan L. Grant, Dean, College of Agriculture and Life Sciences; Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; Jewel E. Hairston, Administrator, 1890 Extension Program, Virginia State, Petersburg.
May 1, 2009