While winter is not a 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 the 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 the fall before temperatures drop below 20°F. However, allow the strawberry plants to harden or acclimate to cool fall temperatures before mulching the planting. In northern Iowa, strawberries are normally mulched in early November. Gardeners in central and southern Iowa should mulch their strawberry plantings in mid-November and mid- to late November, respectively.
Don't Apply the Mulch Too Early
Plants that are mulched prematurely are more susceptible to winter injury than those that are mulched after they have been properly hardened.
While November is typically the time, it's more about the weather conditions than the date on the calendar. Look for these indicators to determine when it is time to apply mulch.
- Indicators that the plants are dormant
- Cover: You can cover a small section of your strawberry bed with a board or mulch for several days. If the leaves have not etiolated (turned a yellow color) due to a lack of light, they are approaching dormancy.
- Color: leaves on dormant strawberry plants will not have the vibrant green they have during the growing season (Figure 1), but instead will have a grayish or sometimes reddish cast to them that indicates that they are dormant.
- Several consecutive nights of around 20°F.
- Soil temperatures consistently below 40°F for several consecutive days.
- Most of us don’t have a soil thermometer, but you can find the soil temperature in your county on this website: County 4-inch Soil Temperature Maps for Iowa.
Straw is the Best Mulching Material
Excellent mulching materials include clean, weed-free oat, wheat, or soybean straw. Chopped cornstalks are another option, although they are often difficult to source. The depth of the mulch should be 3 to 5 inches at the time of application. The material should eventually settle to a depth of 2 to 4 inches.
In windy, exposed areas, straw mulch can be kept in place by placing wire or plastic fencing over the area to prevent it from blowing away. 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 may actually damage plants due to the 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% of the plants have started 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 can be several degrees colder than those in ground-level plantings. Due to colder temperatures, strawberry plants growing in raised beds require more protection than those at ground level. 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 the 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.