Extension News

Choose Wisely When Selecting Trees

Note to media editors: This is the ISU Extension Garden Column for the week of May 23.


By Richard Jauron
Iowa State University Extension


Spring is an excellent time to plant trees. 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. Careful plant selection can create an attractive landscape and prevent future maintenance problems. 



Knowledge of the mature height and spread of trees can prevent many landscape problems. The mature height and spread of trees 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 will have longer life spans and require less maintenance. 



An important consideration when selecting trees 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 that are reliably hardy in your area. 


Tolerance to summer heat and drought also can be important. The white-barked birches and European mountainash prefer cool, moist environments. 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 is their susceptibility to insects and diseases. 


Apple scab is a 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.  When purchasing a crabapple, select a variety that is resistant to apple scab. 


While the emerald ash borer is not currently in the state, it will likely arrive in the next few years. Its arrival may mark the beginning of the end for ash trees in Iowa as the emerald ash borer attacks and destroys all species of ash. Accordingly, it is no longer advisable to plant ash trees in the landscape. Maples, oaks, lindens, honey locust, and hackberry would be good alternatives.  Also, consider under-used trees, such as Kentucky coffeetree, Turkish filbert, American hophornbeam, ginkgo, common horsechestnut and baldcypress. 


Soil Conditions

Soil conditions at the planting site strongly affect trees. Most trees 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 trees for wet and dry sites. For example, river birch, swamp white oak, baldcypress and sycamore do well in wet sites.  Red cedar, crabapple and hawthorn 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 ISU Extension office or online at www.extension.iastate.edu/store.


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.  Manual of Woody Landscape Plants  may be available at your local public library or can be purchased at bookstores or online. 



Contacts :

Richard Jauron, Horticulture, (515) 294-1871, rjauron@iastate.edu

Jean McGuire, Extension Communications and External Relations, (515) 294-7033, jmcguire@iastate.edu