What to do now
Mildest step first.
- Check the rest of the plants; flowers that open after the frost aren't affected.
- Before the next frosty night, cover the bed in the afternoon with heavy row cover, straw or old sheets.
- Take covers off the next day once it's above freezing, so bees can reach the flowers.
- Expect a few misshapen berries from flowers that were only partly hurt; they're fine to eat.
Not sure this is it? Check other strawberry problems
What a black center means
A healthy strawberry flower has a yellow center. Cornell's Berry Diagnostic Tool explains that frost injury turns the center, called the receptacle, from yellow to black, and that fruit won't develop from frost-damaged flowers. The University of Illinois Extension calls this black eye and says affected flowers will not bear fruit. The petals often stay white, so the damage can go unnoticed until you look into the flower. Strawberries often bloom before the , and Cornell's Marvin Pritts notes that because the flower opens toward the sky, it loses heat quickly on clear nights.
How cold is too cold
How cold it takes depends on the stage. Penn State Extension lists the temperatures at which damage starts: about 22°F for blossoms that have emerged but are still tightly closed, 26°F at the popcorn stage when petals show, 30°F for open blossoms and 28°F for young green fruit. Cornell's frost guide puts the line for open flowers a little lower, at flower temperatures below 28°F, but adds that on a clear night a flower can be colder than the air around it. Penn State notes that air near the ground is often as much as 5°F colder than an eye-level thermometer shows. Cornell adds that a freeze after a warm spell does the most harm, because plants that have had cool weather first are more tolerant.
Which berries you lose
Frost tends to take the first, biggest berries. The University of Minnesota Extension notes that the first and largest berry is usually the one affected, and that partly injured flowers can grow into deformed berries with a seam-like mark, next to normal fruit in the same cluster. Cornell's diagnostic tool also says partial frost injury leaves deformed fruit. The plant itself is fine. Illinois says one frost won't affect new blossoms, as long as no more frosts follow, so later flowers can still give you a crop.
Covering the plants
For home plantings, coverings are the practical defense. Iowa State University Extension calls them the easiest and best bet for backyard growers. Illinois suggests covering plants with a layer of straw or other organic mulch, or even bed sheets, when a freeze is forecast, and removing it as soon as it warms above freezing. Penn State advises pulling over the planting in the afternoon, and says they work best on still nights, giving about 3 to 4°F of protection; wind above about 5 mph cuts their effect, and layering an old cover underneath adds some. Sources differ on how much a cover adds. Cornell describes row covers as buying time, slowing how fast flowers cool, rather than adding degrees, and Iowa State says the same and recommends heavy-weight fabric.
Before the next cold night
Know how cold it gets at the plants, not at the house. Cornell suggests placing a thermometer at flower level in the lowest spot of the planting, exposed to the sky, since cold air settles there. Penn State says even a thermometer propped at the end of a row, open to the air on all sides, tracks blossom damage closely. Cornell also cautions against spreading extra straw between rows right before bloom: it keeps the soil from soaking up the day's heat and giving it back at night, which can make frost injury worse. If the aisles need more straw, Cornell suggests waiting as long as you can before fruit set, and notes that wet, dark soil holds heat better than dry, light-colored soil.
Growing it again? When to plant strawberry for your ZIP code
- Why are my strawberries small or deformed?Check the seeds and the cluster: a hard, seedy tip on every berry points to tarnished plant bugs, uneven seed sizes to poor pollination, one bad berry beside good ones to frost. Small berries often mean an old, crowded bed.
- There's a surprise frost forecast — what should I cover and with what?Cover warm-season transplants and seedlings with row cover fabric or old bedsheets — not clear plastic sheeting directly on leaves — and place covers before sundown to trap daytime heat.
- Why are my strawberries rotting on the plant?Gray fuzz means gray mold, the most common strawberry fruit rot. Dark, leathery berries point to leather rot, sunken spots to anthracnose, and holes with slime to slugs. Pick often, remove bad berries, and mulch.
- Which vegetables can survive a frost?Hardy vegetables such as kale, cabbage, collards, Brussels sprouts, spinach, peas and onions can take a hard frost down to about 25°F. Semi-hardy crops such as lettuce, carrots, beets, Swiss chard and cauliflower handle a light frost of about 28–32°F. Tender crops such as tomatoes, peppers, beans, corn, cucumbers, squash and melons are damaged at 32°F.