Submitted by: Rich Marini, Department of Horticulture, Penn State University
Mulching strawberries is an old practice that helps protect the plants from low temperature injury during the winter and keeps the ripening fruit clean. This summer Kathy Demchak and I observed winter injury in the crowns of plants in strawberry fields that were not mulched until mid-winter. Although few plants were killed, the injury appeared severe enough in some plants that yield was probably reduced. For strawberries grown on raised beds, the potential for cold injury is high because soil heat may quickly dissipate from the increased surface area of the beds relative to the soil volume. Covering raised beds with plastic or row covers likely retards heat loss, but I am not aware of soil temperature data for raised beds with different types of covers. This article is intended as a review of the information on mulching strawberries and on low temperature injury, so growers understand how and when to effectively mulch their plantings.
In the late summer and early fall, strawberry plants enter a physiological stage referred as “dormancy.” There are different phases of dormancy, but that discussion is beyond the scope of this article. Although dormant plants do not appear to be growing, the buds continue to develop throughout the winter. The initial stages of dormancy are triggered by decreasing day length and declining temperatures, but strawberry plants do not become hardy until November. The term “hardiness” refers to the plant’s ability to resist low temperatures. As strawberry plants become dormant, new leaf development ceases, the leaf petioles become more horizontal, resulting in the “flattened” appearance of dormant plants, and older leaves turn red. Plants become hardy upon exposure to freezing temperatures, and strawberry plants continue to increase in hardiness until January. In late winter, after being exposed to sufficient chilling, the plants start to lose cold hardiness in response to warming temperatures. Upon exposure to sufficient heat, the plants begin to grow.
Mulch should be applied after the plants have attained substantial cold hardiness, but before low temperatures injure the plants. A rule of thumb, supported by research data from several locations, is to apply mulch after three consecutive days when the soil temperature is 40°F or lower at a 4-inch depth. This usually occurs after several hard frosts in the low 20s, and in Pennsylvania this usually occurs between mid-November and mid-December, depending on location.
Strawberry plants are covered with straw to insulate plants from low temperatures, to prevent temperature fluctuations that can lead to frost heaving, and to minimize plant desiccation. Mulch also delays soil warming in the spring and minimizes exposure to spring frost by delaying bloom. Following bloom, mulch helps with weed control, conserves soil moisture, and helps keep fruit clean. Several types of loose materials have been successfully used as mulch, but straw is most common in the northeastern U.S. Hay should be avoided because it contains weed seeds. For matted rows, about 2.5 to 3 tons of mulch per acre, providing a 2- or 3-inch-layer, is typically applied on top of the plants. Doubling this amount of mulch is typically suggested for raised beds. Snow is an excellent insulator, and snow combined with mulch is even better. My Master’s research at the University of Vermont involved laboratory experiments where plants were exposed to various temperatures to determine critical temperatures for plant growth, as well as survival of plants and flower buds. In a field experiment, non-mulched strawberry plants were compared with mulched plants. When the air temperature was -4°F, the temperature of non-mulched crowns was 1.5°F, but the temperature of crowns under straw mulch plus 8 inches of snow was 30°F.
Mulch is typically removed in early spring when plants begin to show signs of growth or new leaf emergence under the mulch. Earlier mulch removal will allow the soil to warm, resulting in earlier plant growth and bloom, which is susceptible to spring frost. The mulch should be removed with rakes or pitchforks in small plantings or with various types of mechanical rakes in larger plantings. A little mulch should remain on the plants, and this will work its way to the soil surface to help keep fruit dry and clean, but most of the mulch is pulled to the row middles for weed control.