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Mulch Your Strawberries

Posted: January 7, 2015

The strawberry is an herbaceous perennial plant and it is fairly susceptible to low winter temperatures. Dr. Rich Marini at Penn State tells us an understanding of the cold acclimation process explains why it is important to delay mulch application until the plants have acclimated. But this also explains why it should be completed before plants are exposed to injurious temperatures.

Depending on the variety and time of winter, flower buds may be killed by exposure to 20 degrees F and plants may be killed by 16 degrees F. Fortunately the short stature of the plant allows us to protect plants with mulch or snow.   

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 degrees 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 and this usually occurs by early December.

The best way to prevent winter injury is to cover the plants with some type of mulch in the early winter, but it is important to apply adequate amounts of mulch and at the appropriate time. Dr. Bertie Boyce performed several studies to learn about the acclimation process and winter injury in Vermont. In growth chamber studies he found that strawberry plants don’t acclimate when defoliated, and light is required for acclimation and the development of cold tolerance. In a field experiment he applied mulch at the rate of 5 tons per acre at 4 different times during the fall for 3 years. In 1984, 1985, and 1986 mulch was applied at approximately 14-day intervals starting in mid-October. The coldest air temperatures recorded during October, before the second mulching date was 24 degrees F, the lowest temperature recorded in early November was 17 degrees F and temperatures of 12 to 16 degrees F were recorded during the second half of November. Average yields (pounds/acre) for the 4 mulching dates were 5,215 (Oct. 1), 11,088 (Oct. 15), 14,434 (Nov. 15), and 14,340 (Dec. 1), so applying mulch before November resulted in reductions in yield of 23 to 65% compared to mid-November. 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.

In conclusion strawberries are only moderately able to survive low winter temperatures. The reason strawberries can be successfully grown in the North is because plants are low to the ground and are often covered with snow and can be covered with straw mulch. To ensure high yields, growers should plant varieties that have performed well in their region and plants should be mulched with straw now if you have not already done so. 

(Contributed by Leon J. Ressler, District 17 Director as part of his Now Is The Time Column for January 3, 2015.)