What to do now
Mildest step first.
- Harden transplants off for 7 to 10 days, and set tomatoes, peppers and other tender crops out after your last frost date.
- Before planting day, get row cover and hoops or stakes, plus buckets, jugs or walls of water for single plants.
- On a frosty evening, water the bed if it's dry and cover the plants before sundown, edges weighed down.
- Uncover or open the covers the next morning once it's above freezing, so plants don't overheat.
Pick the planting date
The best frost protection is planting tender crops at the right time. Iowa State University Extension advises waiting to plant vegetables such as tomatoes, peppers and green beans until after the typical frost-free date. Your last frost date is an average, so frost after it happens in many years; keep watching the forecast for the first couple of weeks after planting. Harden plants off for 7 to 10 days before they go out, so they're used to sun, wind and cool nights. This page is about getting ready. If a frost is already forecast for tonight, the late-frost cover page has the short version.
Have covers ready
Buy covers before planting day, because frost warnings often come with a day's notice. Floating row cover is the most useful. The University of New Hampshire Extension says a lightweight cover, about 0.5 oz per square yard, might give 2°F of frost protection, while heavier fabric of 1.5 to 2.2 oz can give 6 to 10°F. When transplants are small, UNH says row cover can often be draped right over them. Hoops underneath leave an air gap, which Illinois Extension says gives extra insulation and better frost protection. Weigh the edges down with bricks, stones or garden staples so wind can't lift the cover and heat can't escape. For single small plants, Alabama Extension suggests covering them with a bucket or a gallon milk jug.
Walls of water and warm soil
For tomatoes and peppers set out early, water-filled cones protect the most. Colorado State University Extension describes the Wall O' Water as a cone-shaped ring of connected plastic tubes filled with water that surrounds a single plant, and says these typically protect plants down to the mid-teens, because water gives off heat as it freezes. CSU recommends warming the soil with black plastic mulch when using them, since cold soil limits early growth as much as cold air does. Clear plastic covers also work if they're held up on a frame, off the leaves; in CSU's trials in Fort Collins, a plastic cover on a frame gave 3 to over 6°F of protection.
On a frosty night
On the day of a frost, water the bed if it's dry. Alabama Extension says plants that are short on water are more vulnerable to frost, and that wet soil holds heat longer and releases it slowly during the night. Put covers on before sundown, while the soil is still warm; Iowa State says covers work by trapping heat radiating from the ground, and should be propped on stakes or hoops so they don't touch the foliage. Leaves touching plastic can freeze, Alabama warns. Iowa State adds that once temperatures drop below about 28°F, covering is often not enough to prevent damage, and that damage can occur above 28°F if freezing temperatures last five hours or more.
The next morning
Covers have to come off or open the next day. CSU warns that with only an hour of sun, temperatures under a closed plastic cover can rise to over 130°F, and that sheets and blankets need to come off in the daytime so the soil can soak up heat for the next night. Alabama says leaving a cover on in direct sun can damage new growth, so lift off buckets and jugs once it's above freezing. Row cover fabric lets in much more light than a bedsheet, so crops can keep growing under it for days at a time, but UNH says to take it off when insect-pollinated crops such as squash and cucumbers begin to flower.
- TomatoThe warm-season anchor of the summer garden.
- PepperA tropical perennial grown as an annual — patient, slow, and particular about warmth.
- EggplantA heat-demanding nightshade that needs more warmth than the tomato next to it.
- BasilThe summer companion — to tomatoes, to pasta, and to the gardener with a south-facing window.
- CucumberA thirsty vine that wants warm soil, steady water, and something to climb.
- 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.
- How do I harden off seedlings before transplanting?Gradually expose indoor seedlings to outdoor conditions over 7–10 days, starting with an hour of shade and building up to full sun and overnight temperatures before planting.
- How cold is too cold to plant tomato transplants?Wait until nights stay above 50°F and the soil is about 60°F. Tomatoes survive cooler nights, but below 50°F they grow slowly and their flowers often fail to set fruit. A frost at 32°F can kill an unprotected plant. In many places that means about two weeks after the average last frost date.
- What is a last frost date and how do I actually use it?Your last frost date is the average date of the final freezing night in spring — it's a probability, not a promise, and smart planting adds a buffer of 1–2 weeks beyond it for cold-sensitive crops.
- 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.