While winter is not growing season for strawberries, taking care of strawberry plants remains vitally important. Before winter arrives, mulch strawberries to protect them, so they’re ready to grow in the spring.
Strawberries should be mulched in fall to prevent winter injury. Low temperatures and repeated freezing and thawing of the soil through the winter months are the main threats to strawberry plants. Temperatures below 20°F may kill flower buds and damage the roots and crowns of unmulched plants. Repeated freezing and thawing of the soil can heave plants out of the ground, severely damaging or destroying the plants.
When to Mulch Strawberries
Strawberries should be mulched in fall before temperatures drop below 20°F. However, allow the strawberry plants to harden or acclimate to cool fall temperatures before mulching the planting. Plants that are mulched prematurely are more susceptible to winter injury than those that are mulched after they have been properly hardened. In northern Iowa, strawberries are normally mulched in late October to early November. Gardeners in central and southern Iowa should mulch their strawberry plantings in mid-November and mid- to late November, respectively.
Straw is the Best Mulching Material
Excellent mulching materials include clean, weed-free oat, wheat or soybean straw. Chopped cornstalks are another possibility, although often difficult to source. The depth of the mulch should be 3 to 5 inches at application. The material should eventually settle to 2 to 4 inches.
In windy, exposed areas, straw mulches can be kept in place by placing wire or plastic fencing over the area. The fencing can be held in place with bricks or other heavy objects.
Leaves are not a good winter mulch for strawberries. Leaves can mat together in layers, trapping air and creating space for ice to form. The leaf, air and ice layers do not provide adequate protection. Leaf mulch actually may damage plants due to excess moisture trapped under the material.
Remove Mulch Carefully in the Spring
To reduce the chances of crop damage from a late frost or freeze, leave the mulch on as long as possible. Removing the mulch in March may encourage the plants to bloom before the danger of frost is past. A temperature of 32°F or lower may severely damage or destroy open flowers. Since the first flowers produce the largest berries, a late spring frost or freeze can drastically reduce yields.
To determine when to remove the mulch, periodically examine the strawberry plants in spring. Remove the mulch from the strawberry plants when approximately 25 percent of the plants are producing new growth. New growth will be white or yellow in color. (If possible, the winter mulch should remain on strawberries until mid-April in central Iowa. The average date of the last 32°F temperature in spring occurs in late April in central Iowa.) When removing the mulch, rake the material to the aisles between rows or an area next to the planting. If there is a threat of a frost or freeze later in the season during bloom, lightly rake the mulch over the strawberry plants.
Winter Protection for Strawberries Growing in Pyramids or Strawberry Jars
A strawberry pyramid is a type of raised bed. In winter, temperatures in raised beds may be several degrees colder than ground level plantings. Because of colder temperatures, strawberry plants growing in raised beds require more protection that ground level sites. Place 6 to 8 inches of straw or chopped cornstalks on strawberry pyramids or other raised beds in fall.
Strawberry plants growing in a strawberry jar or other container likely will be seriously damaged or destroyed if left outdoors in winter. One option is to place the container in an attached, unheated garage in November. A second option would be to discard the strawberry plants in fall, dump out the potting soil, store the container indoors in winter and replant in spring. Day-neutral and everbearing strawberry varieties perform better in containers than June-bearing strawberries.