by Jan Cashman
This spring, many deciduous trees in Bozeman are not leafing out. We were hoping the trees were just slow because of our extremely late spring. But now it appears some of the green ash, especially the commonly planted cultivar of green ash called ‘Patmore’, and other deciduous trees, including quaking aspens, cottonwoods, maples, plums, and cherry trees, may not survive.
Although we don’t know for sure what happened to these trees, we blame it on last fall’s weather. In late September, 2009, we had 5 days in a row over 80 degrees, some of them over 90. Then, in October it turned bitter cold, down to 9 degrees on October 12, setting a record low. The ground froze quickly, leaving acres of potatoes frozen in the ground and unusable in the Churchill area. Similar weather conditions occurred in 1983, with similar potato and tree damage, mostly of green ash trees. Because of the fast temperature drop in October, the trees were not “hardened off” gradually, as they would be most years. The leaves didn’t go through the normal process of changing color to yellow and orange and reds, but turned brown and hung on the branches.
This tree loss is frustrating because many of the damaged trees were reaching mature sizes with trunks from 3 to 6 inches in diameter. It is also frustrating that green ash was the tree that was injured the most. Why were green ash hurt so badly when it has always been touted as one of our hardiest shade trees, tolerant of cold, drought and alkaline soils? When asked this question, Gary Strobel, doctor of plant pathology, said it is difficult to answer because we are dealing with complex biochemistry and plant physiology, dictated by the environmental factors.
So, there are no simple answers to how we can keep this from happening again, but here are a few suggestions:
- Don’t give up on your trees too soon. If the branches or buds show any life at all, there still might be a chance they will leaf out and survive. The telling factor will be if they are strong enough to make it through next winter.
- After the 1983 tree injury in Bozeman, Orville McCarver, long-time MSU extension horticulturalist, suggested that the trees hurt the worst were overwatered in the late summer and fall. Starting in July, to prepare trees for dormancy, (harden off), decrease watering. This is not always easy to do if your trees are planted in a lawn area where you want the grass to stay green into September. If possible, plant shade trees on the perimeter of your yard where they are not under your sprinkler system. Or, at least, decrease the amount and frequency of water to your grass in mid to late summer. Grass does not need as much water in late summer and fall when the nights are cooler and the days shorter.
- To discourage late summer growth, don’t fertilize trees after July 1.
- Planting only native trees is not the answer; even some quaking aspens around our area show damage this spring. New varieties of trees are not always the answer either. Newly introduced trees are not necessarily bred for their winter hardiness, but for other desirable characteristics like denseness, shape, or leaf color, therefore, the new selection may not be as winter hardy as the original tree. But, the green ash cultivar called ‘Prairie Spire’, discovered in Rugby, North Dakota, appears to be a hardier green ash than the Patmore. And, we recently heard about a new, disease resistant elm, called ‘Lewis and Clark’, discovered in cold, windy, Cooperstown, North Dakota. When it is released to the public, ‘Lewis and Clark’ elm hopefully will prove to be a hardy shade tree for our area.
- City foresters know the danger of planting too many of one type of tree; 40 to 50 years ago, Dutch elm disease almost wiped out the most common shade tree in the Midwest, the American elm. For a uniform look along its streets, many subdivisions in Belgrade and Bozeman required Patmore green ash to be planted on homeowners’ boulevards. No one could predict that so many of these trees, some of them mature, would be injured or killed.
So the answer is to plant a diversity of trees, to prevent death of one kind tree from a specific disease, insect, or bad winter. Depending on where you live, chose from elms, lindens, honeylocust, maples, oaks, cottonless cottonwoods, mountain ash, or Ohio buckeye, and don’t discount hardy green ash selections like ‘Prairie Spire’, for a variety of beautiful shade trees in your yard