Strawberries, unlike other small fruit such as blueberries, are herbaceous plants without any woody parts, and that makes them less resistant to cold temperatures. If you want to grow strawberries in USDA zone 5 or below, you need to protect the plants during the winter, otherwise, they might die or suffer severe injury from the cold, often resulting in no or poor harvests next season.
Whether you grow your plants in a strawberry patch in your garden, in containers, or in raised beds, here is how and when to protect the plants through the winter.
Winter Injury of Strawberries
Strawberry plants are most prone to winter injury when there is no snow cover. Ideally, strawberry plants should be covered with snow because it not only insulates the plants from freezing temperatures, it also protects them from damaging winds. Winters with temperature fluctuations and freeze-thaw cycles are perilous for unprotected strawberries plants and can cause the plants to heave out of the ground.
How severely strawberry plants are damaged depends on the cultivar as well as the plant’s age and health, the microclimate, growing method (raised beds, conventional garden beds, or containers), lowest winter temperatures as well as temperature fluctuations—and some of these factors are beyond your control. However, you can choose a strawberry variety that is hardy in northern climates, USDA hardiness zones 3 and 4, such as 'Honeyoye', 'Ogallala', or, 'Cavendish'.
Why Strawberry Plants Need Dormancy
Strawberry plants need to go through a period of dormancy. They set flower buds in the fall, then over a period of six to eight weeks use the cooler fall temperatures and decreasing daylight to transition into a state in which they build sugars for next year’s fruiting and stems and stolons that will become next year’s runners. Without dormancy, strawberries would not stop flowering and setting fruit, which would soon exhaust the plant, make it prone to disease, and precipitate its death. That’s also why strawberries cannot be grown as perennials in hot climates; they are usually grown as annuals in USDA hardiness zone 9.
How to Check Strawberry Plants for Dormancy
The start of dormancy depends on the temperatures and amount of daylight. It requires several nights in a row with a strong frost for the strawberry plants to enter dormancy. In USDA hardiness zone 5 and lower, plants have usually gone dormant by the end of November and in USDA hardiness zones 6 through 8 they go dormant in December.
Instead of only relying on the calendar, look for signs of dormancy by checking the crowns to make sure there is no active new growth.
How to Protect In-Ground Strawberry Plants
After the plants have started dormancy, cover them with a four-inch layer of mulch that lets water through and allows good air flow. Clean straw, as free of weed seeds as possible, is the first choice. Pine needles are a suitable alternative. Hay is not recommended because it contains too many weed seeds. Leaves are also not recommended because they clump together and retain moisture, which is an invitation for fungi and bacteria to grow. Row covers and plastic do not provide adequate protection either; they don’t have the same insulating properties as straw.
The colder the zone you’re in, the more mulch you'll need to insulate your strawberries. Keep in mind that straw will compact considerably so be generous. Fluff up the straw and spread it loosely to create air pockets. Raised beds are more exposed than conventional garden beds, so give them an extra layer of mulch. Before you cover the strawberries, remove any dead leaves from the plants.
In USDA hardiness zones 7 and higher where winters are mild, mulching strawberry plants is usually not necessary.
How to Winterize Strawberry Plants in Pots or Containers
The most vulnerable parts of container-grown strawberries are the roots, which are insufficiently insulated against freezing temperatures. The soil in containers freezes faster than garden soil and that can kill strawberry plant roots.
After the plants have entered dormancy and the ground is not yet frozen, you can bury the containers in the ground up to the rim. Mulch them just like you would mulch in-ground strawberries. If the containers are too large and heavy to be moved, mulch them as you would in-ground strawberries but give them an extra layer of mulch. You might also move the containers to an area on your property or against your house that offers protection from the wind and harsh winter conditions, perhaps in a southern-facing direction.
Winter Strawberry Care
Other than winter mulching, in-ground strawberries do not require any care during the winter. Container plants need to be watered well before the onset of winter, as dry soil freezes much faster than wet soil. If you overwinter the plants in a location where they are not exposed to precipitation, for example under an overhang, make sure the soil does not dry out completely. After a snowfall, pack the soil with snow to provide both insulation and moisture.
Transitioning to Spring Care
Strawberries often bloom before the last frost, which can significantly reduce the harvest and damage the fruit. At the same time, you don’t want to leave mulch on the strawberries for too long because it will delay blooming. When the daytime temperatures become warmer in March, remove the mulch while keeping a close eye on the plants as the buds develop. You can leave the straw on the ground around the plant, but uncover the plant's crown. The straw helps retain moisture and suppress weeds around the plants.
If a frost is in the forecast during or just before bloom, it is crucial that you cover your plants again with fresh, clean fluffed-up straw or pine needles but only at night. Promptly remove the mulch in the morning and repeat the process for as many nights as needed until the danger of spring frost is past.
“Growing Strawberries in the Home Garden.” Umn.Edu, https://extension.umn.edu/fruit/growing-strawberries-home-garden
“Growing Strawberries in the Home Garden.” Rutgers.Edu, https://njaes.rutgers.edu/fs097/