When talking about flower buds on our fruit trees and flowering ornamental plants, a couple of plants come to mind.
The first is the peach tree. Like other fruit trees, peaches produce flower buds every year. It is just a gamble whether or not we have severe winter temperatures during the winter. Cold weather has already given peach trees in the home orchard a knock-down punch for 2019. When temperatures reach -10 degrees, peach flower buds start to die. For every degree below -10 degrees, we lose another 10 percent of what was left until all the peach flower buds have been killed. The foliage buds are able to withstand these colder temperatures so the peach trees live and will fruit another time. In our area, it is common to have peaches every three to five years. Those home orchardists that have great locations for their peaches get a crop more frequently. Let us wait and see what the Polar Vortex left us.
The second plant represents the home landscape side of things. Forsythia will often have a "snow skirt" of blooms in this spring. Flower buds protected by deeper snow will survive to bloom, but every flower bud above the snow line likely has been killed by the very cold weather. Just like the peach, we will always have next year. Flower buds are formed from the very bottom to the very top of your forsythia, so if you have been lucky enough to see it in full bloom, you know it is capable of quite a show.
What about other plants? Well, a couple of years ago in this column, I talked about plants coming out of dormancy in the spring and discussed a newer phrase – chill hours, or chilling requirement. This is all about our temperate plants getting enough cold weather to know it is time to come out of winter dormancy and begin to grow come spring. It's not so much about the long cold winter temperatures, but instead about those fluctuating spring temperatures. Chill hours are recorded for temperatures between 32 and 45 degrees. Temperatures below 32 degrees are not counted, as plants are completely dormant. Any wintertime temperatures above 60 degrees are subtracted from the totals.
Our temperate landscape plants, having evolved over the millennia, have their own way of keeping track and know when it is safe to break winter dormancy. This explains why some stone fruits (peaches and apricots) are damaged by a late frost or freeze; they have already received enough chill hours and are attempting to grow. Fortunately for the plants native to our area, their chilling requirements are higher so they break dormancy later, with much less risk of having flower buds or leaf buds damaged by late frosts.
For example, fruit trees depending on varieties require the following chill hours to break dormancy, using the 45oF model:
- Apple 800 to 1750
- Pear and sour cherry 600 to 1500
- Plum (Japanese) 600 to 1600
- Cherry (sweet) 500 to 1450
- Peach 375 to 1200
- Apricot 300 to 1000
This can be helpful information for planning new plantings in the home landscape or orchard.