How Does Warm Winter Weather Affect Plants?

Posted by

This weekend has brought about a long stretch of warm weather, which will last for much of the following week. Winter temperatures in the high 60s and even above 70 degrees Fahrenheit for some of our readers, may seem like spring has arrived. We must all keep in mind; it's only mid-February!

Life in Central Illinois is accustomed to swings in winter temperatures, but 60 plus degrees for a week straight is rare indeed. Many gardeners may wonder how these warm temperatures will affect their plants. Time for some Botany 101.

Plants in like those that survive our temperate Midwestern climate undergo various types of dormancy beginning in the waning days of fall. By the time we hit winter most plant processes have slowed or suspended. So will the warm weather bring our plants out of dormancy? The answer is more than a simple yes or no.

Mother Nature has been at this game for a long time. Over the eons, plants have encountered warm winter weather, triggering growth, which then is killed off when the winter weather returns. To counter the issue of plant tissue being killed by late winter freezes, many of our plants developed a chilling requirement. Chilling requirements, often measured in hours, are a period of time during which the plant must be exposed to cold weather (typically below 45 degrees Fahrenheit) to bring the plant out of dormancy.

Many of our native prairie seeds have a chilling requirement known as stratification built into their genetics, where exposure to cold weather and wet soils initiates the chemical breakdown of their seed coat allowing for germination.

Many trees and shrubs also must experience a certain amount of chilling hours to break dormancy. Retaining dormancy is necessary with spring flowering crops like apples and peaches, whose winter flower emergence could reduce or wipe out a current year's crop.

The next question we must answer is- How many chilling hours have we experienced? Fortunately, this information is available online. Go to enter your weather station identification, adjust the range of dates beginning September 1, 2016, and ending with our current date. Upon clicking on "Calculate Chill," you then are given the amount of chilling time, in hours, for your area. In Macomb, our chill time is at 2,126 hours (as of March 2, 2017). With that number in mind, we can then look up the chill requirements for our plants. It seems most of our crops have met their chilling requirement, therefore, you may want to invest in some winter protection for the possible emergence of flowers.

Even if emerging flower and leaf buds are killed by a frost later on this winter, the plants will survive. However, it may be harder to find a good locally grown peach this summer.

Listed below are selected fruit crops and their chilling requirements. (Adapted from Wahle, Feb 2016)

Weather station ID for Macomb: KILMACOM13

Weather station ID for Quincy: KILQUINC13

Approximate Hours of Chilling (<45°F)

  • Apple 800 to 1750
  • Raspberry 800 to 1700
  • Pear and sour cherry 600 to 1500
  • Cherry (sweet) 500 to 1450
  • Blueberry (highbush) 800 to 1250
  • Peach 375 to 1200
  • Apricot 300 to 1000
  • Blackberry 350 to 600
  • Grape 50 to 400 (adequate growth) 400 to 1650 (better and faster growth)
  • Strawberry 50 to 300