Trees are a big investment for the landscape. Not only are trees and shrubs more expensive than other types of plant material, like perennials, they are also long-lived in the landscape. This makes selecting and purchasing the best woody plants an important task.
When it comes time to plant new trees and shrubs, gardeners should do some preparatory work before going to the garden center or nursery. 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.
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.
An important consideration when selecting trees and shrubs is their cold hardiness. Most of Iowa occupies USDA Hardiness Zone 5, with small parts of southern Iowa in Zone 6a and small parts of northern Iowa in Zone 4b. The average minimum temperature in Zone 5 is -10 to -20 F. The average minimum temperature in Zone 4b is -20 to -25°F and in Zone 6a is -5 to -10°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.
Learn more about hardiness zones in Iowa in this publication: Gardening in Iowa Zones.
Another important consideration when selecting trees and shrubs is their susceptibility to insects and diseases.
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.
Some insect problems can also be avoided by careful plant selection. The Japanese beetle is an introduced pest found across much of Iowa. This beetle feeds on dozens of different plant species but does have a few favorites, including lindens (Tilia). While Japanese beetle feeding rarely kills the tree, they skeletonize the leaves in mid-summer, leaving the tree stressed and unattractive. Using insecticides to control this pest is often impractical and expensive. Selecting a different species like Kentucky coffeetree (Gymnocladus dioicus), Ginkgo (Ginkgo biloba), or Hackberry (Celtis occidentalis) can prevent the extra maintenance and damage the Japanese beetle causes.
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, sycamore, and redosier dogwood do well in wet sites. Red cedar, crabapple, hawthorn, and potentilla tolerate dry soils.
When looking to buy trees and shrubs from a garden center or nursery, many gardeners are not familiar with the basics for selecting high quality nursery stock. Here are some suggestions on how to select healthy trees and shrubs. Using these guidelines when purchasing plant material will increase the chances that your plants will thrive in the landscape.
The first step is to observe the overall appearance of the tree or shrub. Does the plant have off-color foliage (yellow, brown, or grey-green leaves)? Is it wilting? A wilting plant is a good indication that the plant has not received proper care or there is something wrong with the root system. Vigorous trees will have a healthy foliage color and a full, lustrous appearance.
Some plants naturally have yellow-colored foliage (‘Princeton Gold' maple) or droopy leaves (black maple). Ask garden center personnel if you are not certain of the plant's normal leaf color or growth habit.
Look for plants that are evenly branched on all sides. If the tree or shrub is flat on one side, it will be difficult to prune in such a way to make it symmetrical. In most cases, if the branches are clustered at the top of the tree, the plant will not produce more branches below. Also, look to see if there are any broken or rubbing branches. This indicates poor care or pruning.
If you are shopping in early spring and the trees are still dormant, scrape away a tiny bit of the bark on a branch with your fingernail. The tissue beneath the bark should be green. Brown tissue usually indicates a dead branch. Additionally, a live branch is usually flexible, while a dead branch will snap. Buds present since last summer should be swollen and plump. Trees with obvious insect or disease problems should be avoided.
Look for trees with healthy growth from year to year. You can check last year's growth by looking for the bud scale scars (scars encircling the twig created when the bud scales on the terminal buds fall off in spring). From the tip of the twig to the bud scale scar indicates the amount the tree grew last year. Small amounts of growth (less than 4 inches) are sometimes an indication the tree or shrub is in trouble. This is highly dependent of species as some plants naturally grow at a slower rate. Ask the salesperson how rapidly the tree or shrub typically grows.
A healthy trunk should be straight, undamaged, and have no signs of injury. If the trunk is wrapped, ask garden center personnel to remove the wrap so you may look for sunken areas of bark or any other injury.
Avoid trees that appear too large for their container and balled and burlapped plants with extremely small root balls. There is a possibility that the tree could be root bound. Root bound trees and shrubs often have circling roots which can girdle a tree or shrub if left uncorrected. Industry standards state that a tree that is 1 inch in caliper should have a root ball 16 inches in diameter.
Check that the tree or shrub is well-rooted. Grasp the trunk near the base and try to move the tree or shrub in the container. Well-rooted plants should not create a hole in the soil when the stem is moved; the container and the plant should move together. With the help or consent of garden center personnel, examine the plant's root system by carefully removing the pot. Healthy roots are firm and lighter in color than the surrounding soil. Roots that have a foul odor or are mushy indicate a disease problem.
Balled and burlapped trees and shrubs are basically checked in the same manner as container-grown shrubs and trees. Look for cracks in the root ball or large amounts of loose soil. If you see this, it could be an indication of poor handling which could mean problems in the future.
When transporting your trees and shrubs, be sure to carry them by their container or root ball rather than by their trunk to avoid damaging their root system. Lay the trees down in the bed of the truck if possible. Exposure to strong winds in the back of an open truck bed can dry and tear foliage, so cover or wrap plants that will be transported in a truck or other open vehicle. If you are unable to cover the plants, drive at a slower speed (less than 35 mph) to prevent serious injury. Also, be sure to water the plant thoroughly when you get home.