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ANSWERS TO SPRINGTIME QUESTIONS ABOUT TREES
My phone's been ringing with spring gardening questions. Homeowners are taking their early spring walks around their yards and discovering problems and questions. The questions don't all have easy answers.
"Why are the squirrels stripping bark off tree branches? This seems like peculiar behavior."
Before moving to Chaska, I watched squirrels strip bark off branches of a 20-year-old sugar maple in my Minneapolis yard. Apparently not even the experts can explain the squirrels' motivation. Best answer I've read is that squirrels tend to get squirrely (i.e., rambunctious) in the spring, much like some humans. The bark stripping will likely cause the affected branches to die but shouldn't cause long-term damage to the tree.
"Why are the needles on my arborvitae and yews so brown this spring?" Unlike deciduous trees that drop their leaves and go dormant during the winter, evergreen trees and shrubs (conifers) retain their leaves (needles) and, as a result, give off moisture throughout the winter. Since the ground is frozen in winter, evergreens are unable to replenish their moisture reserves from their roots. As the needles release moisture, they turn brown.
Several weeks ago the yews in my yard were nearly 50% brown but they have greened up nicely since then. If you still have brown foliage in late May or early June, it will probably not green up and can be pruned back to green tissue.
"What trees should not be pruned at this time?"
The best time to prune most trees and shrubs is during their dormant period in the winter. Oaks must not be pruned during April, May or June because pruning during these months puts the trees at risk of oak wilt disease. Maples and birch trees should not be pruned until they are fully leafed out because pruning right now will cause them to bleed sap. Early blooming shrubs such as lilacs and forsythia can be pruned immediately after blooming. In general, hold off pruning all trees until mid-summer or, better yet, next winter.
"What can be done to prevent sawfly damage to spruce trees?"
Sawfly larvae are 1-inch green worms that show up just as Colorado blue spruce and other spruce trees send forth their tender new needle candles. I inspect my spruce every day during the spring growth flush and apply an insecticide labeled for sawfly when I first detect the presence of sawfly larvae. Sawfly larvae seem to be cyclical; infestations are worse some years than others. So, pay close attention to your spruce during May and early June.
"What's the best way and best time to fertilize trees?"
The best indicator of whether fertilization is necessary is a soil test. Ideally, a soil sample should be taken before trees are planted. Additional samples can be taken every 3 to 5 years thereafter to determine whether any nutrients are lacking. A soil test kit may be obtained from your county extension office.
In the absence of a soil test, the best indicator of the need for additional fertilization of established trees is shoot growth. If new shoot growth (growth occurring in the present year) is in excess of 6 inches, then fertilization is probably unnecessary. If shoot growth is between 2 and 6 inches then fertilizer may be needed. If shoot growth is under 2 inches, fertilizer application is advised.
Most trees experience a single flush of growth during spring followed by slower growth throughout the summer and fall. Because of this single flush of growth, it is desirable to have nutrients available as this growth is about to occur. The most beneficial time to apply fertilizer is from when the ground is workable in the spring until just before trees start growing in early May. On sandy soils, applications should be split, half in early spring and half in mid- to late-May.
High rates of phosphorus (P) fertilizer should not be used unless a need is indicated by a soil test. If soil test indicates adequate levels of P, then it is best to use fertilizers such as 24-0-15, 30-0-10, 32-3-10, 18-5-9, 27-3-3, or 16-4-8 with a high rate of N and a low or zero rate of P. High rates of P can negatively affect the environment by causing excessive algae to grow in nearby lakes and streams which will, in time, kill fish and other aquatic life.
Never use a fertilizer that includes any kind of herbicide around a tree. These fertilizers may be beneficial to turf, but can damage trees.
An excellent tree fertilization guide is available online in the gardening section of the University of Minnesota Extension Service website (www.extension.umn.edu).
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