Winter Mulch Protects Strawberries
Tony Bratsch, horticulture educator

If you have a strawberry bed, you’ll want to take steps to protect the plants this winter. Although applying a layer of straw seems like a simple task, timing and the amount of straw used are important.

Depending on variety, strawberries have varying tolerance to cold. Lack of acclimation or “hardening” in the fall, as well as wide temperature swings, can decrease cold tolerance and increase winter injury to crowns. Mulching helps insulate plants from low temperatures and quick changes. Temperatures in the low teens can kill a high percentage of uncovered or poorly hardened plants.

Mulching also protects plants from winter winds, which dessicate or dry out the plants, especially when soil moisture becomes low and/or less available in frozen soils. In addition, strawberries are shallow and somewhat brittle-rooted. Alternate freezing and thawing (frost heaving) of the soil in winter and early spring can damage roots. A layer of straw on the surface helps moderate this soil movement.

Strawberries grow and develop late into the fall months, and should not be covered too early. They respond to low temperatures and moderate frost by gradually becoming more hardy and tolerant of cold. Signs of this acclimation include a “flattened” appearance, lack of any new leaves, and red coloration of older leaves.

Apply mulch after plants have had time to acclimate and develop hardiness. Based on various studies, data indicates that straw should be applied when soil temperatures at a 4-inch depth have reached and stay at 40 degrees F. Soil temperatures can be checked with a simple thermometer.

Another rule of thumb is to apply straw after several 20-degree freezes have occurred. For most of Illinois, this usually happens between mid-November and mid-December, depending on location.

About any type of loose organic material can be applied as a mulch, but straw is the most readily accessible, and it has good insulation ability. Avoid materials such as hay that will likely contain weed seeds; also avoid sawdust or chips that are too heavy and dense. Apply a 2- to 3-inch straw layer, and for raised beds with greater exposure, apply several more inches. As the winter progresses, snow helps add even greater insulation—so don’t shovel it off the plants.

Plants resume growth early in the spring and should be uncovered in a timely manner. Typically, straw is removed when plants show signs of growth and new leaves begin to emerge under the mulch.

Timing of mulch removal can help delay or advance early spring growth and flowering. Earlier removal will allow the soil to warm more quickly, advancing growth and bloom; but, early removal increases the risk of spring frost injury to new blossoms. A delay in removal will keep soils cooler and delay growth and flowering, which can be an advantage in sites where spring frosts are a concern. Just don’t leave the straw on for an extended period of time.

A leaf rake works well to remove straw. Rake the straw into walkways, along the row edge, and between rows to help keep weeds down and provide a clean surface for berries to rest on. Some straw should be left on top of plants to filter down and provide a cushion for berries forming in the row middle.

More information on growing and caring for strawberries can be found in the Small Fruit in the Home Garden publication, C-1343. The book is available through your local U of I Extension office.

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