As temperatures move closer to freezing and below, remember the sweet, delicious June strawberries from your garden and take time to protect the plants – and next season’s crop. Just like many people, strawberry plants don’t like to feel those colder temperatures. Iowa State University Extension horticulturists tell how to protect plants through the winter. Gardeners with additional questions should contact the ISU Extension Hortline at firstname.lastname@example.org or call 515-294-3108.
Cold winter 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. Plants also can be destroyed by repeated freezing and thawing of the soil, which can heave unmulched plants out of the ground.
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.
Excellent mulching materials include clean, weed-free oat, wheat or soybean straw. Chopped cornstalks are another possibility. 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.
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.