Dry, Hot Conditions Contribute to Summer Stress for New Trees

by Bert Cregg, Horticulture and Forestry

From Landscape Crop Advisory Team Alert, August 13, 2010

The warm temperatures of the past couple months coupled with low precipitation in many areas have caused many trees in Lower Michigan to begin to shut down. As I've noted before irrigation during the summer is critical for newly established trees and shrubs. By "newly established," I'm referring to trees that have been planted in the past two years. Even with the best of circumstances, newly planted trees should be irrigated every seven to 10 days during the summer. This year, however, is not the best of circumstances and even well-established trees are showing signs of stress.

What are the signs? For some trees, such as maples, we're seeing early fall color. In some cases, we'll get some red, but more typically we'll just get some yellow. We're also seeing some leaf folding or leaf rolling. I have noticed a lot of cottonwoods, tulip-poplars, and sycamore that are shedding leaves. It's important to remember that conifers are suffering in the heat as well, even though the appearance may not be as dramatic as some shade trees. When the weather gets as hot as it has and rainfall is lacking, it's important to get landscape trees some water. If our current patterns continue, we may see effects into this fall and beyond. For maples, although we're seeing some early fall color, we may lose a lot of leaves, so there are fewer around for the regular fall show. Also, as trees are stressed in the late summer and fall, they are less able to acclimate going into winter. Therefore, we may see more evidence of winter injury come next spring.

What to do? To the extent practical, try to irrigate trees showing signs of stress as soon as possible. If you can't get to all your trees; prioritize .Water newly established trees first. A lawn covered with leaves from a large old tulip-poplar may be hard to ignore, but it will likely survive and recover; whereas the next few weeks could be make or break for trees planted this year or last year. Remember: don't kill with kindness. One good soaking once a week is better than drowning the tree every day. Make sure to avoid surface run-off. Dry soils often crust over and resist infiltration until they are wetted. If your watering by hand, go from tree to tree, giving each a little water and cycle through a few times to make sure water is infiltrating. And, of course, mulch, mulch, mulch.