Critical Temperatures for Frost Damage!
With the burst of warm weather we had over the past several weeks, our apple trees are about three to four weeks ahead of schedule. And now we have a frost/freeze warning for tonight and tomorrow night. Normally we would be at or near green tip, but those 80+ degree days we had have placed the trees at tight cluster/pink. The real problem is that we have accumulated about 230 - 240 growing degree days thus far this spring. Last year at the exact same time we had accumulated around 20! So we are at least a month ahead for degree days and more than three weeks ahead for tree growth stages, compared to 35-year averages.
This spring degree days have been accumulating faster than tree growth can keep up. The degree days are way ahead of the tree growth stages this year and may continue that way for the rest of the growing season. As the trees develop in the spring and buds start to swell, they lose the ability to withstand the cold winter temperatures that they could withstand in dormancy during the cold winter months. The young, actively growing tissue can then be damaged or even killed. Swollen fruit buds can better withstand temperatures in the teens without any damage. As the buds open, temperatures in the low 20s can cause harm, but sometimes leave other buds undamaged. As growth moves from green tip to 1/4” green to 1/2” green to tight cluster to pink in apple trees, temperatures in the upper 20s can cause considerable harm to an early blooming tree. Near bloom, the range between slight and severe damage can be very small. So the stage of bud development determines how susceptible any given fruit crop is when freezes occur. For more information on what those critical temperatures are that can cause freeze damage to trees during development, I have uploaded two charts from Utah State University below that you can download by clicking on either chart below.
Given the weather patterns we have experienced so far this spring, and the fact that we have gotten snow as late as mid April, a spring frost is highly possible within the next few weeks. We will need to constantly assess the stage of development our trees are at and their susceptibility to freeze injury. If we continue in this spell of warm weather, apple trees will continue to develop quickly and the critical temperature will rise from the low 20’s to the high 20s, to levels just below freezing at bloom time.