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 requirements.

Size. Knowledge of the mature height and spread of trees and shrubs can prevent many landscape problems. The mature height and spreadof 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. Proper selection of plant materials will also increase their life span and reduce maintenance chores.

Hardiness. An important consideration when selecting trees and shrubs is their cold hardiness. The importance of cold hardiness should be clear to all Iowans after experiencing the record cold temperatures of early February 1996. 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 can also be important. The white-barked birches and the European mountainash prefer a cool, moist environment. Hot, dry weather weakens the trees. The weakened trees are then destroyed by insects and/or diseases. Because of their poor tolerance to heat and drought, the white-barked birches and European mountainash are not recommended for Iowa.

Pest Susceptibility. Another important consideration when selecting trees and shrubs is their susceptibility to insects and diseases. Pests damage and occasionally kill susceptible plants. Fortunately, some pest problems can be avoided by careful selection of trees and shrubs.

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

Some insect problems can also be a avoided by careful plant selection. Since 1981, many honeysuckles have been damaged by honeysuckle witches-broom aphid. The aphids feed on the tips of new growth and suck sap from the leaves. As the aphids feed, they inject a toxin which leads to a proliferation of small, weak side shoots or witches-broom at the stem tips. The Tatarian honeysuckle (Lonicera tatarica) and Zabel's honeysuckle (L. tatarica 'Zabelii') are highly susceptible and are badly damaged by the witches-broom aphid. These types should be avoided in the home landscape. Aphid-resistant honeysuckle varieties, such as 'Freedom', 'Honey Rose', and 'Arnold's Red' are better choices. If a tree or shrub has serious insect or disease problems and resistant varieties are unavailable, gardeners should consider alternate trees and shrubs.

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, American hornbeam, and redosier dogwood do well in wet sites. Juniper, crabapple, hawthorn, and potentilla tolerate dry soils.

Careful plant selection can create an attractive landscape and prevent future maintenance problems. --Richard Jauron, Department of Horticulture

This article originally appeared in the March 29, 1996 issue, p. 42.

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