Factors to Consider When Selecting Trees and Shrubs

Spring is an excellent time to plant trees and shrubs. However, before going to the garden center or nursery, gardeners should do some preparatory work. Gardeners should examine the planting site, determine their landscape needs, and obtain pertinent information on possible plant materials. Some important plant characteristics are size, hardiness, susceptibility to insects and diseases, and soil conditions. Careful plant selection can create an attractive landscape and prevent future maintenance problems.

Size

Knowledge of the mature height and spread of trees and shrubs can prevent many landscape problems. The mature height and spread of trees and shrubs will vary somewhat due to soil conditions and other factors. However, knowledge of their approximate mature size can prevent overcrowding, interference with overhead utility wires, obstruction of vehicular and pedestrian traffic, and other problems. Properly selected plant materials also have longer life spans and require less maintenance.

Hardiness

An important consideration when selecting trees and shrubs is their cold hardiness. Iowa occupies USDA Hardiness Zones 4 and 5. The average minimum temperature in Zone 4 is -20 to -30 F. The average minimum temperature in Zone 5 is -10 to -20 F. Select trees and shrubs that are reliably hardy in your area.

Tolerance to summer heat and drought also can be important. The white-barked birches and the European mountain ash prefer cool, moist environments. Hot, dry weather weakens the trees. The weakened trees are then destroyed by insects or diseases. Because of their poor tolerance to heat and drought, the white-barked birches and European mountain ash are not recommended for Iowa.

Pest Susceptibility

Another important consideration when selecting trees and shrubs is their susceptibility to insects and diseases.

In the last 10 to 15 years, pine wilt has become a serious disease of Scotch pine (Pinus sylvestris) in Iowa and other midwestern states. Pine wilt is a fatal disease that quickly kills infected Scotch pines. Other pine species that are occasional victims of pine wilt include jack (Pinus banksiana), mugo (Pinus mugo), Austrian (Pinus nigra), and red (Pinus resinosa) pine. Unfortunately, there are no effective treatments for the prevention or control of pine wilt. The best strategy is to contain the disease by promptly removing and destroying dead or dying Scotch pines. Because of pine wilt, Scotch pines are no longer recommended for windbreaks and home landscapes. White pine, spruces, firs, and arborvitae would be better choices for Iowa.

Apple scab is another common disease in the home landscape. A fungal disease, apple scab causes heavy leaf drop on susceptible crabapple varieties. Heavily defoliated trees survive, but are unattractive. Apple scab can be prevented by several fungicide applications in spring. Fortunately, problems with apple scab can be avoided by selecting disease-resistant crabapple varieties. 'Hopa' 'Radiant' 'Royalty' and 'Vanguard' are very susceptible to apple scab and are not recommended for the home landscape. Resistant varieties include 'Adams' 'Adirondack' 'Donald Wyman' 'Prairifire' 'Profusion' 'Purple Prince' 'Sugar Tyme' and others.

Soil Conditions

Soil conditions at the planting site strongly affect trees and shrubs. Most trees and shrubs grow well in soils with a pH of 6.0 to 7.5. However, there are exceptions. Pin oaks require a pH of 5.5 to 6.5. Another important soil characteristic is soil porosity. Select appropriate plants for wet and dry sites. For example, river birch, green ash, sycamore, and redosier dogwood do well in wet sites. Red cedar, crabapple, hawthorn, and potentilla tolerate dry soils.

Good sources of information on landscape trees include PM 1429d, Low-Growing Trees for Urban and Rural Iowa , PM 1429e, Street Trees for Iowa , and PM 1429g, Conifer Species for Iowa. These publications are available at your local county extension office. One of the most comprehensive reference books available is Manual of Woody Landscape Plants: Their Identification, Ornamental Characteristics, Culture, Propagation and Uses, by Michael Dirr. Another good reference book is Growing Shrubs and Small Trees in Cold Climates, by Nancy Rose, Don Selinger, and John Whitman. These books may be available at your local public library or can be purchased at bookstores or online.

This article originally appeared in the April 18, 2003 issue, p. 42.

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