Montana AG Live Host Update: Ash Winter Kill Update from Host Jack Riesselman

Written by Jack Riesselman

I've had several people ask me to expand my comments made on the September 12 broadcast of Montana AG Live relative to winter killed ash trees.

First, a bit of background. Years ago, most ash grown in Montana were referred to as the "common Green Ash".  Genetically, they were a diverse group, but overall, they were well adapted to the rigors of the northern great plains.  Early propagation and seedling development general occurred in the north central states.  Eventually, new cultivars were developed including the Marshall seedless ash which gradually replaced the "common Green Ash".  As time passed, other lines were developed including the popular cultivar called Summit.

The Summit green ash grew in popularity and was extensively planted in Montana during the 1970's and early 1980's.  In 1983, Montana experienced an extended Indian Summer that ground to a sudden halt in mid-September when a cold front dropped nighttime temperatures into the mid-teens.  The following spring, thousands of young green ash, primarily the Summit line, failed to leaf out as they had not hardened properly the previous fall.  The young ash trees that did survive were those that had NOT received tender loving care, for example, not watered and not fertilized. Looking at annual growth rings in 1984 showed that trees that had growth rings greater than roughly one quarter inch failed to survive whereas those with growth rings around and eighth inch did survive.  One needs to remember that water and fertilizer during the growing season increases annual growth rate, resulting in larger growth rings.

Following the massive winter kill in 1983, it was noted that an Alberta, Canada, line called the Patmore ash seemed to have survived better than most other lines.  As a result, Montana's nursery industry started replacing the Summit variety with the Patmore ash.  This line survived reasonably well even during the less dramatic winter events in the early 2000's.

Over the past two decades Montana's growing season has been extended up to nearly a month in some locations.  This extended growing season appears to have reduced the ability of the ash to properly harden prior to the normal late September or early October hard freezes.  Following extended fall seasons in 2019 and again in 2020, Montana again experienced massive winter kill in young and also maturing ash trees.  Even trees with 6 to 8-inch calipers were killed or partially damaged.  Ironically, it now appears that the once very hardy Patmore line was more prone to winter injury that other less commonly grown varieties.  While the reason for this is currently unknown, it appears that the extended growing season may have altered the genetic growth habit of this line, along with the fact that most tree deaths and injury again occurred in well-watered and fertilized trees.

Looking forward, remember ash trees are usually the last tree to leaf out in the spring and the first to shed its leaves in the fall.  This growth habit is generally thought to be a good way to avoid winter damage as it allows the tree to harden properly prior to winter.  

By avoiding fertilizer, even under the drip line of the tree, and by eliminating supplemental water after mid-August you can help the ash tree harden properly. Additionally, some relatively new cultivars including the North Dakota line, "Prairie Spire", have performed well in most Montana regions.

On a positive note, replacing dead ash with a mixture of other adapted tree species will lessen the potential devastation should the emerald ash borer arrive in the state.  Contact your County Extension agent or local certified nursery professional for more information and recommendations on adapted replacement trees

Written by Jack Riesselman, Professor Emeritus in Plant Pathology at MSU and Executive Producer and Host of Montana AG Live. Montana AG Live is regular call-in information program dealing with agricultural and gardening issues in Montana. Presented by MontanaPBS in association with Montana State University Extension