Karl Foord, UMN Extension Educator
What are your apples doing right now?
Apple trees have two kinds of buds; vegetative buds that only give rise to leaves and shoots, and mixed buds that give rise to flowers as well as shoots and leaves. The parts of the flower in their primordial state can be identified in the dormant mixed bud as seen in this cross section (Figure 1).
Apple flower buds progress through a series of developmental stages. This begins with the Silver Tip stage where the bud has started to open but no green tissue is visible. The progression runs through to petal fall and the beginning stages of development of the apple fruit (Figures 2 - 10).
Apple varieties can break bud at different times, and buds can be at different stages on the same tree. For example, the buds on my Haralson apple tree are almost all between ½ inch green and tight cluster. Only the leaves are visible but the tips of the leaves are red. My Honeycrisp has a few buds beyond the ½ inch green stage like the Haralson, but many of the buds are between silver tip and green tip.
Bud sensitivity to temperatures
As we move into May and watch our apple flowers develop, what temperatures should we recognize as damaging to apple flowers? The temperature at which flower buds are injured depends to a significant degree on their stage of development. Buds are most hardy during the winter when they are fully dormant. As they begin to swell and expand into blossoms, they become less resistant to freeze injury. The temperature sensitivities of the bud stages noted previously are listed in Table 1.
The month of May can bring some damaging low temperatures. On May 9 of last year a low temperature of 25oF was reached in many locations in Minnesota. The apple flowers of many trees were in full bloom and vulnerable to temperatures below 28°F. In some orchards the frost damage to the flowers was practically complete and the whole apple crop was lost except for some late flowers on Honeycrisp and a significant number of flowers on Sweet 16, a late flowering variety.
At the present developmental stage, apple flower buds in Minnesota could survive temperatures in the low 20's. Let us hope we have seen the last of the temperatures listed in table 1, for this year at least.
Photo credits: http://web3.canr.msu.edu/vanburen/crittemp.htm