By Mark Longstroth
I receive many calls about what warm weather will do to fruit in the wintertime. Often, I will say there is no need to worry, they have not completed their chilling requirement. When perennial plants go dormant in winter, they track the hours just above freezing (between 35 and 45 degrees Fahrenheit) to avoid winter cold. This is the chilling requirement. Temperatures below freezing do not matter. During winter dormancy, the trees can acclimate to very cold temperatures. Once the chilling requirement is met, then the plants can grow with warmer weather.
The fruits we grow in Michigan require about 700 to 1,300 hours chill hours to satisfy their dormancy requirement before they will begin growth in spring. Normally, we get over 1,400 hours in Michigan and chilling is not a problem. Chilling can be a big problem in the southern United States with its warmer winters and earlier springs. Different species have different chilling requirements and varieties differ within the species. Peaches require 700 to 1,000 hours (there are lower chill varieties), cherries require 600 to 1,300 hours (most in Michigan 900 to 1,300) and apples require 800 to over 1,500 hours. In Michigan, I assume most peaches need 800 to 1,000 hours, cherries 900 to 1,000 hours, apples about 1,200 to 1,300 hours and blueberries are about the same chilling as apples. Grapes have a relatively short chilling, but need a warmer temperature (above 50) to start growing.
The North Central Climate Center maintains a Chilling Hours Map (above 35 F and below 45 F) using weather data. On this map, it looks like most of Michigan has 800 to over 1,000 hours of chilling. I also have been tracking chilling hours for several Michigan State University automated weather stations on the Enviroweather system. These stations are in southwest Michigan (Benton Harbor, SWMREC), west central Michigan (Clarksville, CHES) and northwest Michigan (Traverse City, NWMHRS). I get results that indicate the southern portion of Lower Michigan has completed chilling for many varieties with a chilling requirement of less than 1,000 hours and many others are close to completing chilling. The central and northern portion of the state will be close to completing their chilling requirements once this current warm ends.
This does not mean Michigan’s fruit crops will be wiped out by cooler temperatures below freezing. It only means they have lost the ability to acclimate to deep winter cold. As fruit plants begin to grow in spring, we use a table of critical bud temperatures to tell us what temperatures will harm the flower buds. This table gives a temperature that can kill 10 percent and 90 percent of the buds. At the earliest stages of growth, the spread for the critical bud temperatures between a 10 percent kill and a 90 percent kill is very wide. For example, at the earliest bud stages in apples, 15 F will kill 10 percent of the flower buds, but it would need to go to 2 F to kill 90 percent. In peaches, 18 F will kill 10 percent, but it would need to go to 1 F to kill 90 percent. For peaches and apples, the trees have many more flowers and can set more fruit than we want to harvest, so growers need to thin their crop after bloom. Other fruit show the same pattern where temperatures need to fall down to around 10 F to cause significant damage.
I do not expect temperatures down to 10 F any time soon and in fact, I consider them very unlikely. Most of the last month has felt like March and I consider the spring warmup has begun. Some people are worried about snow. I would be happy to see snow. This time of year, when it snows the temperatures are just below freezing in the 20s and this would not cause any damage.
Fruit growers remember the early spring of 2012 when two summer-like weeks in March set the Michigan fruit industries up for widespread losses to spring freezes in April. Growers ask me how far ahead of 2012 we are and that makes no sense to me because we are just starting growth in southwest Michigan. There is nothing to compare yet. We are in the middle of several days of warmer weather with daytime highs near 60. In 2012, we had a week of highs in the 80s and nighttime lows in the 60s.
When I want to compare years, I look at the growth stage of the plant (bud break, bloom, harvest) or the growing degree-days (GDD). For tree fruit and blueberries, I look at the GDD base 42. For grapes, I look at GDD base 50. Last week, we were at the same place for growing degrees where we were in 2012, but in 2012 we had a couple of weeks of summer in March. In 2013, we actually had a warmer February than in 2012. However, in 2013 the spring cooled down, plant growth slowed and we did not have any spring freeze damage in most areas, and actually had a huge crop. It all depends on whether we stay warm or if we cool back down in March and April.
In southwest Michigan, we have warm and cold spells in spring. When it is warm, the plants grow a little. When it is cool, the plants slow down or stop growing. I am always happy in spring if the nightly lows are below freezing, because that makes the plants slow down and stop at night. I expect to see some plant growth down in southwest Michigan during this warm spell.
I am concerned, but not worried about a damaging freeze soon. As the buds swell, I am still only concerned. When growth continues and the buds begin to burst or open, then I will be worried. Generally, as buds burst they can be damaged by temperatures around 20, and the difference between light damage (10 percent) and severe damage (90 percent) is much smaller. Freeze events with lows down to 20 are common in March.
Yes, it looks like we are off to an early start this spring in southern Michigan, but it seems unlikely we will suffer any damage in the near future. Colder weather will slow down plant growth. If warmer than normal temperatures return and are the rule this spring, our fruit crops will be more advanced and will probably bloom early. (See the MSU Extension article “2013 Bloom dates for southwest Michigan tree fruit crops.”)
If we are in a vulnerable growth stage before and during bloom, that will increase the chance of damaging spring freezes. All we can do is watch the weather forecasts and hope for cool weather. If you want to compare 2017 to the last five years, you can easily select the “Degree-day comparisons: last 5 years at this station” from the homepage of any weather station on the Enviroweather website.
Source: msu.edu