Grapevines, like all woody plants, take cues from the environment. During the long days of summer, the vines are growing and making grapes. They could not go dormant during that period if they had to. But as the days grow shorter toward the end of summer, the vines are able to detect that the change of seasons is coming and respond by storing starch reserves in their roots and trunks. They detect a change in the ratio of red light to far – red light that occurs as the days grow shorter. The green shoots that grew all summer begin to turn brown and woody so they can over - winter and carry next year’s crop. As the shoots turn brown or “lignify” they become tolerant of cold temperatures and can survive a frost. As the crop ripens the plant becomes more and more prepared for the cold winter months ahead. Finally, the leaves fall and the vine becomes dormant. So what happens if there is warm weather during the winter? As it turns out, the vines do not achieve their maximum level of hardiness until there has been sub-freezing temperatures for several days. The vines become more tolerant of deep cold as they head into the coldest part of winter. During this time the vines are in what is called “enforced dormancy”. During this period they cannot begin to grow, no matter what the weather does. If you took a cutting of an over wintering cane into your house and put it in water it would not grow. This protects the plants from responding to warm temperatures and starting to grow in the middle of winter. As winter continues however, the vines are able to measure the passing of time so they “know” when to begin growing. The actual mechanism is not fully understood but the plants need a certain amount of time at temperatures between freezing and 40⁰ F to complete what is called their “chilling” requirement. This mechanism gives the plants a way to measure time during winter and allows them to begin coming out of dormancy once the chilling requirement is met. The period when enforced dormancy ends is called “eco-dormancy” because now the vines depend on cool temperatures to keep them dormant. Once the temperatures warm the vines begin to grow again.
Understanding that plants have a way to measure time while dormant is fascinating and it helps us understand when plants might become vulnerable to environmental damage like winter injury or frost injury in spring. Let’s look at our current weather for an example. The weather turned cold in December and stayed cold for several weeks. When the temperature stays below the freezing point the plant gets no chilling time. Now, in January, we are having a thaw and the temperatures are forecast to be above freezing for a week or more. During this time the vines are fulfilling their chilling requirements and are moving closer to being able to begin their climb out of deep dormancy toward bud burst in spring. What we don’t want right now is too much of this mild weather. If that were to happen the vines might begin to come out of deep dormancy before the coldest part of winter has passed which might lead to winter injury. Additionally, the more chilling the vines get now the sooner they will be likely to come out of dormancy. This sets the stage for early growth and then frost damage in spring. That’s what happened in 2012 when we had 10 days of 80⁰ weather in March. All of the fruit trees started to grow and bloomed. Then there were a series of frosts in April that killed the blossoms and there was virtually no tree fruit in Michigan that year.
So even in the middle of winter, the woody plants are alive and sensing the environment around them. They are measuring the passing of time and getting ready to start growing in spring so they can make another crop and complete another growth cycle. It is truly an amazing process and we have the privilege to watch it happen – if we just take the time to slow down and look.
Cheers! - Dave Miller