Trees are a significant investment for the landscape. Not only are trees and shrubs more expensive than other types of plant material, like perennials, but they are also long-lived in the landscape. This makes selecting and purchasing the best woody plants an important task.
Selecting the Right Species | Purchasing High-Quality Plants | Transporting | More Information
Selecting the Right Tree or Shrub Species
Before heading to the garden center or nursery, do a little preparatory work. Examine the planting site, determine its 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. Thoughtful 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, vehicular and pedestrian traffic obstruction, reduce maintenance needs, and other problems.
Hardiness
For long-term survival in the landscape, select trees and shrubs that are cold-hardy for your area. The USDA divides the country into zones based on the average minimum temperature each year. Most of Iowa occupies USDA Hardiness Zone 5 (-10 to -20°F), with small parts of southern Iowa in Zone 6a (-5 to -10°F) and small parts of northern Iowa in Zone 4b (-20 to -25°F).
Tolerance to summer heat and drought is also an important consideration. Some species are much more tolerant of warm temperatures and high humidity than others. Trees such as white-barked birches and the European mountain ash prefer cool, moist environments, so they would not do well in open, hot locations. Unfavorable environmental conditions weaken trees, making them more prone to insect and disease problems.
Learn more about hardiness zones in Iowa in this publication: Gardening in Iowa Zones.
Pest & Disease Susceptibility
Another important consideration when selecting trees and shrubs is their susceptibility to insects and diseases. Some tree and shrub species are susceptible to certain diseases or insect issues. Selecting a species that is not affected by these pests and diseases means trees are healthier, easier to care for, and more attractive.
For example, linden (Tilia) is frequently damaged by Japanese beetles. While these pest insects don't kill 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. Similarly, pine wilt is a serious disease of Scotch pine (Pinus sylvestris) that quickly kills it and other select pine species. There are no effective treatments for preventing or controlling pine wilt, so the best strategy is to select a species that is not affected by pine wilt, such as white pine, spruces, firs, or arborvitae.
Sometimes, a disease issue can be avoided by selecting an appropriate cultivar. For example, apple scab is a fungal disease that causes heavy leaf drop on susceptible crabapple (Malus) varieties. Heavily defoliated trees survive, but are unattractive. Apple scab can be prevented by several fungicide applications in spring, but this is a lot of work and chemical use. By selecting a disease-resistant crabapple variety, this disease can be avoided.
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. For example, pin oaks require a pH of 5.5 to 6.5 and become yellow and unattractive when grown in soil with a higher pH.
Another important soil characteristic is soil moisture. Certain species of trees and shrubs grow best in wet locations, while others do better in dry spots. By selecting the appropriate species for the soil moisture in your landscape, the tree can grow well and have fewer problems. For example, planting species like river birch, sycamore, and redosier dogwood, which grow best in wet soils, will result in better-looking plants with fewer issues than if species like red cedar, crabapple, hawthorn, and potentilla, which do best in dry soils, are planted in that same wet location.
Learn more in this article: Landscape Plants for Difficult Sites.
Maintenance Considerations
Some species of trees and shrubs have specific maintenance requirements. Consider the work and the nuisances that things like fruit, thorns, or growth habits can create. Oak and walnut trees can drop fruit that can needs to be picked up. Hawthorn, roses, and barberry can be difficult to work around because of their thorns. Low-branched trees are difficult to navigate around when planted near sidewalks or driveways.
For example, selecting a male cultivar of ginkgo or Kentucky coffeetree will prevent the formation of nuisance fruit that would need to be picked up. Selecting a redbud instead of a crabapple will eliminate the potential for messy fruit and the mess left by the birds attracted to it. Planting a compact cultivar of ninebark instead of barberry keeps thorns out of the landscape and columnar forms of trees keep branches out of the street and sidewalk.
Nativity
Plants native to Eastern North America, the Great Plains, or Iowa are not automatically better for a particular location in your landscape than non-native plants. Urban and suburban landscapes are not "native" spaces. They have highly altered soils and very different pest and disease pressures. Even when planting natives, they must be matched to the site. For example, the native Colorado spruce (Picea pungens) needs well-drained soils and will struggle in wet, compacted soils common in suburban neighborhoods.
However, when matched to their site conditions, native plants can do well and better support native insects, birds, and other animals.
Learn more in this article: Plants Native to Iowa.
Invasiveness
Certain plant species from other parts of the world are invasive, but are still used in landscapes. By avoiding these species, you can prevent their spread to nearby natural areas where they can outcompete native species. However, not all non-natives are invasive. For example, lilac is native to Eurasia but has not spread to natural areas in North America.
While not considered invasive, some natives become aggressive spreaders in disturbed or altered soils. These native plants must be managed to prevent them from overwhelming a garden space, which requires extra work and time. By choosing a plant that does not spread readily, less time is spent maintaining the plant. For example, planting a native trumpet creeper will require frequent cutback and sucker removal, but a large-flowered clematis does not.
Learn more in this article: Alternatives to Common or Invasive Landscape Plants.
Purchasing a High-Quality Tree or Shrub
When buying trees and shrubs from a garden center or nursery, many gardeners are unfamiliar 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.
Overall Appearance
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 indicates that the plant has not received proper care or has 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.
Branches
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.
Living Tissue
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. 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.
Growth
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) sometimes indicate the tree or shrub is in trouble. This is highly dependent on species as some plants naturally grow at a slower rate. Ask the salesperson how rapidly the tree or shrub typically grows.
Trunk
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.
Root System
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 indicate poor handling which could mean problems in the future.
Transporting
When transporting your trees and shrubs, 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. Even dormant plants can see damage to buds from harsh conditions while in transport. 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.
More Information
- Tips for Shopping and Selecting Quality Plants
- Guidelines for Selecting Trees (PDF)
- Tree Planting Basics
- How to Plant Bare Root Trees
- Planting in Late Summer and Early Fall
- Plant Shopping (video)
- Gardening in Iowa Zones
- Spring Garden Tasks
- Fall Garden Tasks
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