Strawberry Plants and Winter – Ready or Not?

When talking about this topic, some commonly used terms could use a little explanation.  Those terms are "dormant" or "dormancy" and "chill" or "chilling." For growers who work with woody plants like apples and blueberries, the term "dormant" means that plants have stopped growing and won't be able to resume growth again until they accumulate enough "chilling" or "chill-hours," which is a certain number of hours in a certain temperature range, usually between 32 and 45 degrees. This, along with cold temperatures over the winter, keeps plants dormant.

With strawberries, the plants don't lose their leaves and are not in a state where they can't resume growth. The term "dormant" still means that growth has stopped or at least slowed to the extent that if herbicides are applied, they are unlikely to have any activity in the plant or do damage at this time.  Though the plants can resume growth, further cold exposure results in much better growth and fruit production. This further cold exposure, or "chilling" may occur in the field under straw mulch or row covers, or in cold storage at a nursery.  The minimum additional amount of chilling needed varies widely with the variety, depending on the region it was developed for. The amount affects how much vegetative and floral vigor the plant will have.

Here are some paragraphs and key sentences excerpted from the article "Winter Injury in Strawberry" by Rich Marini that appeared in a 2013 issue of the Vegetable and Small Fruit Gazette. It is hard to improve upon.  It describes what is happening in the plant during the fall, what happens to the plants when temperatures fluctuate during the winter, and why it is a problem to apply straw mulch too early:

"An understanding of the cold acclimation process is important to delay mulch application until the plants have acclimated but before plants are exposed to injurious temperatures.

During the late summer, upon exposure to short days, strawberry plants start to acclimate. A pigment in the leaves, called phytochrome, perceives day length and is responsible for producing compounds that move in the plant to cause the plant to become dormant and to develop some cold tolerance. Short days alone will cause strawberry plants to develop tolerance to about 25°F. Declining non-freezing temperatures will cause further acclimation, but exposure to a frost triggers rapid additional cold tolerance. Characteristics of acclimated plants include leaves with wide angles, so the leaves look flat, and older leaves turn red. Continued exposure to below-freezing temperatures results in maximum cold tolerance, which usually occurs by early December.    

By late January, the chilling requirement is satisfied, and when the temperatures rise above about 50˚F, the plants begin to de-acclimate and lose cold tolerance. When exposed to non-injurious low temperatures, the plants will re-acclimate and regain some of their cold tolerance, but they will not regain maximum levels.

Different varieties vary in their cold tolerance. Northern breeding programs generally select for the ability to survive low temperatures.

The reason for reduced yields following early mulching is probably due to inadequate light reaching the leaves to induce early acclimation and because plants were not exposed to temperatures low enough to induce development of maximum cold tolerance. It is important to apply mulch after the plants are fully acclimated, which usually occurs in early December, but before the occurrence of temperatures low enough to injure the plants."

It is reassuring to know that daylength, which doesn't vary from year-to-year like temperatures can, triggers a fair amount of cold tolerance in the first place.

So, watching the plants to determine whether they have taken on a flattened appearance is a good indicator that the plants are ready to be mulched with straw and that it is safe to apply herbicides. If you would like another indicator for peace of mind, we've found that the soil temperature being at 40 degrees or less at a 4-inch depth has correlated well with flattened leaves in our research plots. It will fluctuate over the course of the day by a few degrees in either direction on clear days and less on cloudy days.

Beyond watching the timing of the mulch application, is there anything else under a grower's control to help with maximizing cold tolerance?  In general, dry conditions and low nitrogen have resulted in greater winter hardiness, while wet conditions and extra nitrogen have reduced it. This is important for growers using plasticulture who have questioned whether they should give their plants an extra dose of nitrogen in the fall, especially since the soil (and thus strawberry crown) temperature in raised beds is typically lower over the winter than the temperature in flat beds, all else being equal.

Thanks to Rich Marini for granting permission to use parts of his previous article in this one.