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Increase life span of urban trees

URBANA, Ill. – Urban communities responded to an increasing lack of interaction with nature by planting trees. Trees provide environmental benefits, along with their beauty, such as cooling buildings through shade and cleaning the air and water through filtration.

Mulch improperly mounded close to tree truck resembles a "tree muffin" and can cause damage.

“Many urban trees only live about 20% of their life due to external issues, such as pests and disease, said University of Illinois Extension horticulture educator Kelly Allsup, "but most stress can be linked back to improper care and installation.” 

A recent USDA paper analyzing tree life expectancy in urban areas found the typical street tree lived between 19 and 28 years. To compare, the ideal life span of a white oak is 600 years and the average life span of a red maple can be between 75 to 150 years in the Illinois wilds.

Urban trees must withstand pollution, poor soils, limited legroom for roots, and pressure from insects and disease. What’s worse, most are planted incorrectly, and their health and cultural requirements – sunshine, water, soil, and climate – are not monitored.

Some basic knowledge of tree stresses can help your urban tree live longer.

Plant the right tree in the right place. It is essential that certain growing parameters be considered when choosing what kind of tree to plant. Allsup suggests, answering the following questions before choosing a tree: How much space does the tree require at maturity? What are the culture requirements of the tree? What is the soil like? What is the soil pH needed and soil type (well-drained or compact)? Does the site get full sun or partial sun? How much water does that tree need? What is the growth rate of the tree? What kinds of things does the tree tolerate (salt, drought, flooding)?  What are the insect and disease issues that tree faces? What are the planting needs and pruning needs of the tree?

Don't plant trees too deep. “Trees planted too deep look like a telephone pole and their natural root flare cannot be seen above the soil line,” she says. This leads to roots not getting enough oxygen and the potential for strangling roots. The proper planting hole should be two to three times as wide as the root ball and no deeper. Rough up the sides with your shovel so roots will spread easier. Sometimes, the tree is planted too deep in the container or root ball.  It is essential that you dig a little to find the top root and plant at that depth and only backfill with the existing soil. “Amending soil may have good results in the beginning but ultimately cause roots not to expand and may cause issues with watering,” Allsup says.   

The worst and best thing for a tree in an urban environment is pruning. Improper pruning practices, such as topped trees and stubbed branches, cause the tree to grow weak branches and create a potential avenue for insect and disease issues. Proper pruning allows good structural growth, protection from wind and reduces the risk of tree failure. The reason urban trees need to be pruned while forest trees do not is because urban trees grow massive side branches that would usually be shaded out in a forest environment. Learn the basics of tree pruning and start corrective pruning at the time of planting.

Use the proper mulching technique. Mulch should not contact the trunk of the tree.“In the industry, we say mulch like a bagel and not like a muffin,” Allsup says. Use organic mulch, such as hardwoods or cypress. Extend a 2- to 4-inch-deep mulch ring as far out as you are willing to go. Properly mulched trees will require less frequent irrigations, less competition with grassroots, and keep trees safe from lawn mower damage.

Reduce strangling or girdling roots. Girdling roots circle the base of a tree rather than spreading out. These roots cut off the sap flow from the stems and leaves and water and nutrient flow from the roots to the leaves and branches causing the tree to decline at a rapid rate, most likely within five to 20 years. Some tree species are naturally prone to girdling roots like maples, ashes, crabapples, lindens, pines and oaks.

Girdling roots are caused by planting trees too deep, the planting hole is not wide enough, sides of planting hole not roughed up, improper mulching, pot-bound circling roots at time of planting, obstructions like sidewalks and curbs and poor soils. Signs of girdling roots can be no trunk flare, leaning trunks, bark cracks, branch dieback, and leaf issues. Research has shown that a third to half of trees that fall after a storm can be attributed to girdling roots. It is much easier to prevent girdling roots rather than fix it when tree starts showing signs of decline. If a tree is pot bound, cut off circling roots, make sure roots lay straight and follow good planting practices and monitor trees to avoid girdling roots in the future.

Space your trees properly for their mature size. Small trees that grow 10 to 30 feet tall typically need a 20-foot diameter. Medium-sized trees that grow 30 to 50 feet typically need a 30-foot diameter. Large trees that grow more than 50 feet typically need a 40-foot diameter.  Knowing the mature canopy spread will help you better determine the spacing of your tree. “Many homeowners make this mistake by planting a small sapling close to their house only to cut it down later or try to over prune not allowing the natural shape of the tree,” Allsup says. 

Do not use staking materials unless you must and remove them after one season. The only reason to use staking materials is if wind is an issue at the site or as protection from vandals. Staked trees have decreased trunk diameter, develop poor root systems and usually suffer from rubbing of materials when they are staked.

By following these steps, you can increase the life of your trees and help ensure it will outlive you and not the other way around.

University of Illinois Extension is the flagship outreach effort of the University of Illinois at Urbana-Champaign, offering educational programs to residents of all of Illinois' 102 counties and far beyond. Illinois Extension provides practical education you can trust to help people, businesses, and communities solve problems, develop skills, and build a better future. Through our Agriculture and Natural Resources programs, Illinois Extension supports the economic viability and environmental sustainability of natural and managed landscapes and productive lands in Illinois. Horticulture program educators provide research-based information and training about gardening, fruits and vegetables, flowers, insects and diseases, composting, landscaping, and more.

News source/writer: Kelly Allsup, Horticulture Educator, Illinois Extension