Straw mulch for winter protection in strawberries
Bruce Bordelon, Professor, Department of Horticulture and Landscape Architecture, Purdue University
The strawberry plant is an herbaceous perennial. While it can withstand freezing temperatures, the crowns can be damaged if temperatures drop below 10°F. Crowns exposed to such low temperatures usually fail to bloom the following spring.
Applying straw mulch over the top of strawberries is the most common method of protecting the tender crowns from winter cold. In addition to insulation from severe cold, mulch prevents frost heaving of crowns, and desiccation from winds. Another reason to mulch is to prevent development of black root rot. Plants exposed to cold temperatures, or that have suffered heaving are especially prone to this disease.
Clean wheat, oat or rye straw is the best material to mulch with. It is lightweight and provides a loose covering that prevents smothering the strawberry plants. Leaf mulch is generally too dense, forming a heavy wet mat over the strawberry plants. Straw has the added advantage of being mostly free of noxious weeds, disease causing organisms, or insect pests. Apply the straw 3 to 6 inches deep over the top of the plants (See Figure 1). Loosen the straw completely rather than laying flakes of bale on top. It is best to also spread mulch between the rows to cover bare soil in the planting. One bale of straw will cover about 30 to 40 feet of row out 4 feet wide (See Figures 2 and 3).
Strawberry plants should be fully dormant before mulch is applied to prevent smothering the plants. This generally occurs after the plants have been exposed to 3 or 4 days of temperatures in the mid to upper 20s, which generally occurs in late November to mid December in Indiana. The leaves will change from bright green to a dull green or gray as the plant goes dormant (See Figures 4 and 5). Waiting for the ground to freeze before applying mulch is not safe as plant injury may already have occurred. If an herbicide is to be used, it should be applied before the mulch.
Remove the winter mulch early next spring when the soil temperature in the top 2-4 inches reaches about 40°F. Don’t wait until the plants start to grow under the mulch. Rake the straw off the top of the rows and into the aisles to protect berries from splashing soil. Leave a thin layer of straw on top of the rows. The strawberry plants will grow up through the straw and it will provide a clean surface for fruit.
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Figure 1. Proper mulch depth over a strawberry row; 3-6 inches.
Figure 2. One bale of straw will cover about 40 to 50 ft of row.
Figure 3. Strawberry row after mulching.
Figure 4. Strawberry row before mulching.
Figure 5. Dormant strawberry plants after exposure to mid-20°F temperatures. Note discoloration of older leaves.