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Winter injury to ash trees in LeSueur

The City of LeSueur is rocked with winter injury on upwards of fifty of it's large green ash trees!

It's after July 4th, and although it has been very wet, a good to excellent growing season is progressing. It's summer, and the topic is winter injury? We must be kidding. Not.

In fact, it's typical to talk about winter injury this time of the year in southern Minnesota. Here a wide variety of tree species are planted, soil/ site conditions are variable, topographical features favor damage and dramatically variable weather conditions are the norm. Additionally, patterns of winter injury needed for field diagnosis only reveal themselves as the summer gets well underway.

How to make a field diagnosis for winter injury:
1. First, evaluate the existing signs and symptoms of the problem. Signs are the physical presence of the problem (caterpillars, fungal fruiting bodies, etc.) causing the injury and the symptoms are the trees response to the problem (dieback, foliage discoloration, swelling, etc.). When did the symptoms first appear? What parts of the tree are affected? What are the patterns of damage, on individual and all the affected trees? Are they associated with topography, site or tree age?

2. Next, rule out by process of elimination, any common pest problems that are usually associated with that particular species. Then rule out other possible problems by careful review of the signs, symptoms, and patterns of damage.

3. Finally, look for a pattern of winter injury. In hardwooods, the pattern of winter injury might include dieback, slow leaf out, undersized and malformed leaves and epicormic shoot formation. Trees affected by winter injury are generally healthy the previous year. The injury is widespread and has a sudden impact in most instances. It is this pattern that is the most important evidence needed to make a field diagnosis.

4. There should be similar injury on more than one of the neighboring woody species, in similar landscape positions and sites, and generally, over most of the greater landscape.

In the case of the recent losses in the City of LeSueur the answer lies not in the injury to the ash trees but in the same pattern of injury to nearby silver maple trees. A few of the silver maple trees have the exact same pattern of winter injury. In fact the pattern of injury to the branches in these injured trees is very similar to the classic winter injury that has been observed on black walnut trees over several years due to low temperatures.

Treatment: The result of winter injury to hardwoods is dead twigs, branches, and portions of entire crowns. Allow the injured trees to express their dieback throughout the summer. New branches (from epicormic buds) will appear where the tree is still alive. Begin removing the dead wood later this summer or wait until next summer after August. Trees that are heavily damaged, over one half of the crown is dead, can be removed and replaced.

Why was there so much damage this past winter especially in just a few limited areas like LeSueur? In most years in this region, extremely low temperatures are associated with resulting winter injury to hardwoods. This year had a very dry fall, a mild winter, a very mild March, followed by frost in April. The exact reason will remain a mystery. These injuries revolve around loss of moisture, acclimation to seasonal temperatures relating to dormancy and intercellular freezing of water.

Elsewhere in this region, there are several instances of classic winter drying in young conifers. With above average temperatures in March, it's possible that these trees began to transpire and lost moisture while their root systems remained non-active in frozen soil.