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Hort in the Home Landscape

A blog devoted to sharing timely horticulture topics and answering the questions of gardeners and homeowners.
2014-08-14 13 06 57

Urban Tree Decline

Posted by Candice Hart - Trees

The vast majority of calls into out help desks and hotlines this summer have been tree questions, many of them in urban settings. And that's for good reason. Urban trees have to tolerate a lesser than ideal location in many cases.

Many people are aware of the environmental and psychological benefits that these urban street trees impart on our small towns and large cities, but it's not until a tree is gone or problems arise that many realize the benefits that the tree provided.

This is certainly the case with the Emerald Ash Borer infestation that is killing Ash trees across Illinois. The emerald ash borer (EAB) is a small, metallic green, non-native invasive pest whose larvae feed on the trunks of ash trees thereby cutting off their ability to transport nutrients and ultimately causing the tree's decline.

Many communities and subdivisions are heavily planted with Ash because of its use as a tough urban street tree. It's tolerant of many soil types and before EAB came along, it had few disease or insect issues, but now many Ash are already dead or declining and it's time to start thinking about what to replace those trees with.

Ash is not the only tree showing signs of decline in urban conditions though. We have gotten questions about many other tree issues this year and many of these issues are likely related to the drought of 2012. When trees are severely stressed like they were that summer, it opens up the tree to other issues. Some of these issues might include other diseases moving in or insect problems, borers in particular.

Borers are attracted to stressed and dying trees and borer damage takes a few years before symptoms, like sections of the tree canopy dying, to show up. We are now 3 years out from the drought and people are starting to notice problems.

In late summer, Maples are now showing signs of Maple Tar Spot, which luckily is of no harm to the tree. Maples are also showing other signs of decline, likely due to borer damage or other environmental conditions. Oaks can also have similar borer damage, as well as a few other disease issues like Oak Wilt or Bur Oak Blight. Trees also have a certain age that they live naturally. Sometimes trees may simply be dying of old age.

With diseases and pests such as oak wilt and emerald ash borer posing a threat to some of our favorite trees, it would be wise for homeowners to diversify their plantings. Hopefully we've learned from the Dutch Elm Disease outbreak of the 1970's and the recent Emerald Ash Borer problems, that communities need to plant a diverse grouping of tree types.

Ideally an urban tree population should contain no more than 10 percent of any single tree species, no more than 20 percent of species in any tree genus, and no more than 30 percent of species in any tree family.

Trees are selected for use in an urban setting for particular reason. Not every tree species will tolerate the conditions in an urban setting like compacted soil, minimal planting area, extreme heat and cold, salt damage, and others unfavorable conditions.

No single species is suited for all sites, and consideration should be given to soil conditions, local occurrence of diseases and insects, microclimate, hardiness zone, and mature tree size when selecting any plant. Not all sites are appropriate for trees. Before planting, envision how the mature tree will fit the site. Ask yourself the following questions: Will the tree interfere with overhead utility lines, underground sewers, lighting, street traffic, or parking? Will the street be widened or disrupted in the future, and what problems will this pose for the tree? Will the tree create unusual maintenance problems (messy fruit, leaf litter, roosting site for birds, etc.)?

More information about selecting a tree can be found using the University of Illinois Extension Tree Selector Tool:

Your local Extension office can offer assistance in answering any of your tree questions.

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