Stress is cumulative. In other words, trees and shrubs can be affected by stresses that happened up to several years in the past. Recent stresses in Kansas include winter damage this last fall as well as the extremely dry winter of 2017-2018. Let’s take these individually.
The extremely dry winter of 2017-2018 damaged many root systems due to a lack of water. This damaged root system may have been further weakened due to too much rain the following spring. Roots need oxygen as much as they need water. Though the roots were able to keep up with moisture demands during the cooler spring weather of 2018, they may not have been able to keep up when the weather turned hot and dry. Some of these trees and shrubs suddenly collapsed and died or sloughed off branches during the summer of 2018. Others appeared to weather this damage well but likely were weakened due to this stress.
So what about the plants that survived but were weakened? Plants under stress often react by setting an abnormal number of fruit buds. This helps insure the survival of the species even if the parent plant dies. So lots of fruit buds were set during 2018. Those buds matured into flower and then fruit (seeds) during 2019. Think back to last year. I have never seen certain plants bloom as well as they did last year. Maturing this much fruit takes a great deal of energy. I think this resulted in such low energy levels in the fall that the plants just didn’t have enough energy to make it through the winter or may have delayed leaf out or didn’t bloom this year (think lilacs). Areas with too much rain last summer had that additional stress added to the mix. Roots need oxygen as well as water and too much rain can damage root systems.
Now there is one more stress we need to discuss. Kansas experienced a sharp drop in temperature during October of last year. Temperatures in some areas of northwest Kansas were near 80 on October 4 and dropped to near 20 the following Friday morning (October 6). Unfortunately, trees were not hardened off before this happened. In other words, they were not ready for these cold temperatures. Though western Kansas had the most severe drop in temperature, it appears other areas in Kansas were affected as well.
The sharp drop in temperature may damage at least a portion of the phloem and the cambium. Remember the phloem carries food made in the leaves to all parts of the plants including the roots. The cambium produces new phloem. If the phloem and cambium are killed, the cambium cannot produce new, living phloem, and, therefore, the roots don’t receive the food needed to survive as food produced in the leaves cannot be transported to the roots. The roots eventually starve and the plant dies suddenly.
Trees so affected will not die immediately. First of all, a healthy root system has stored energy reserves that it can use to keep the tree alive. When those reserves are depleted, the entire tree or a portion of the tree will die very quickly. Usually this occurs during the summer following the year the damage occurred. That is what we are seeing now.
So, is there anything we can do now to help the trees? Those that collapsed are done for. Others that were slow to leaf out or are thin should be given the care needed to prevent further stress. Primarily, that means to water the tree as needed. Keep the soil moist but not waterlogged (Ward Upham)