A number of people have asked us if the warmer weather we’ve had is a problem for the trees. As in, will the trees bloom early if this warmth continues? This is a great question and what follows is a primer on degree days or, What Makes A Fruit Tree Happy.
Most fruit trees need a certain amount of chilling units during the winter before they progress into bloom at spring. I kind of think of this as the “dormancy” time for a tree. They’re just kind of hanging out, soaking up the moisture of the rain or snow, looking at their neighbors and hoping their bark isn’t eaten by errant deer or hungry mice. The trees need to “bank” a certain amount of chilling units. There are models we can follow to tell us how many chilling units peaches and apples have accumulated, but since we can’t control the weather, these models mostly act as a guide. Researchers have shown that the highest (or fastest) temperature to accumulate chilling units is between 40-43 degrees. Ironically, it’s not necessarily the coldest temperature that wins the race –when it drops below the 20’s, the trees stop accumulating chilling units and just hunker down and wait. It is sort of like how the highest risk of hypothermia is in the 40’s. You don’t think it is that cold except that you are thoroughly chilled. When it is colder, you dress for it rather than ignoring how chilly it is. This is why you’ll see peach trees growing in some Mediterranean climates like Turkey where the temperatures will fall to around the 40 degree mark in winter but not below freezing.
So, until trees get enough chilling units, they ignore warm days. It may be warm, even hot, but they are still in the “I need chilly” mode. But, once they have gotten their set number of chilling units in, the gloves come off and they are rarin’ to go for spring. Each day that it is warm, they put “warm units” or degree days in the bank towards bloom. This is a bank where there are no withdrawals. Cold days only mean they don’t accumulate more warm units. You don’t subtract, just delay. Generally our trees finish accumulating their chilling units sometime in late January to mid-February. So, now, every warm day is a day closer to bloom.
Peaches need fewer chilling units than do apples. Where we’ve had a couple of weeks of warmer temperatures (which is typical for this time of year), the peach buds swell slightly. If the temperatures cool towards a more winter-like pattern toward the end of March, this would be ideal since it delays bloom towards later in the season. If it continues to warm as it has and doesn’t cool, the blooms will have a free-for-all and will swell and begin to open, sensing warm weather that really won’t be here until mid-May. Then we will play cat-and-mouse with blooms beginning to open, cold nights (under 30) and running wind machines to mitigate the damage of frost on blooms. Do you ever wonder why Steve has grey hair? It is this cat and mouse game that controls our destiny for the year.
Once the trees have bloomed, we can also use temperatures to calculate degree days that predict harvest and to monitor insect development. The photo shows a device we use to take temperatures and calculate how insects are developing. Insects are cold-blooded and develop based on temperatures. We enter the temperatures the bugs develop between and the device calculates where in their life cycle they are at. Degree days can tell us when codling moth and twig borer will begin to fly, and even worse, mate. That informs us when we need to begin our monitoring programs to see how many of those insects are in our orchards and then if we need to spray to supplement the control supplied by our good insects (lady bugs, lacewings, Dareocris, etc.)
For the time being, we aren’t too concerned about the warmer days. In fact, from a human point of view, the warm sun and mild temperature is most welcome. However, if there isn’t a cool-down of temperatures within the next several weeks, we will be singing a different tune. We have lived in Colorado long enough to know that warm temperatures don’t stay from the middle of March into June. Once those buds move towards bloom, there is no turning back the clock, even when the Weather Service says it is going to cold again. We can’t predict what Mother Nature will do, but we would sure like to know! As always, stay tuned to see what Mother Nature has in store for her next chapter.
