Winter Injury in Strawberry
Posted: December 11, 2013
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.
Most of the research on strawberry acclimation and low temperature injury has been done with June-bearers. Researchers in Quebec however suggested that day-neutral plants may not respond to day length, but declining autumn temperatures is the primary environmental factor triggering acclimation.
By late January the chilling requirement is satisfied and when the temperatures rise above about 50 degrees 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.
Several factors influence cold tolerance of strawberry. Different varieties vary in their cold tolerance. Northern breeding programs generally select for the ability to survive low temperatures. A study was conducted in Minnesotta (Yao et al. 2009), where 9 varieties from the breeding programs at Kentville, Nova Scotia or Geneva, NY were grown under mulch or with no mulch for two seasons. When mulched, plants were less injured than when not mulched and the variety ‘Sable’ tended to have better plant survival than the other varieties. In a Canadian study (Gagnon, et., 1990), the June-bearer ‘Redcoat’ and the day-neutral ‘Tristar’ were killed at 19
degrees F compared to 20.8 degrees F for the day-neutral ‘Hecker’.
Fall fruiting of day-neutral varieties may also influence cold tolerance. Research on day-neutrals in Quebec showed indicated that fall fruiting can suppress the accumulation of nitrogen, starch and total non-structural carbohydrate level in the plants. Fall fruiting also reduced the cold tolerance and plants that were de-blossomed were killed at 21.6 degrees F, whereas plants that fruited continuously all Fall were killed at only 23.2 degrees F.
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 in 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 another experiment Boyce and Linde (1986) grew ‘Midway’ plants in 8”-high raised beds. They used snow making equipment to apply 6” man-made snow as mulch in mid-December for two seasons. They also had treatments that included no mulch or snow, straw mulch, and natural snow. Crown temperatures were recorded at 5:00AM and 2:00PM each day during the winter. Plants with no snow or mulch had the most days with crown temperatures below 23 degrees F and it was the only treatment with crown temperatures below 14 degrees F. Crown temperatures below 23 degrees F were recorded for only 7 and 9 days, respectively for treatments with natural snow or man-made snow. Six inches of snow was a slightly better insulator against low temperatures than straw mulch and man-made snow was as effective as natural snow. Plant survival and yield were related to crown temperatures. They also found that plant survival was better for daughter plants than for the mother plants.
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 during early winter.