What to do now
Mildest step first.
- Check the 10-day forecast and plant when nights stay at 50°F or warmer and the soil 4 inches down reads about 60°F.
- Harden plants off outdoors for 1–2 weeks first, and keep them inside on days or nights below 45°F.
- If you plant earlier, lay black plastic to warm the soil and protect plants with row covers or walls of water.
- If frost is forecast after planting, cover the plants before sundown.
Not sure this is it? Check other tomato problems
Nights below 50°F
Nights matter more than days. The University of Maryland says summer crops want overnight temperatures above 50°F, and that nights in the 40s slow plants down and can lead to misshapen fruit later. Utah State notes that tomatoes do not set fruit when nights are below 50°F, and the flowers drop. Colorado State is a little stricter: it suggests nights above 52°F at transplanting, and says a week of days below 55°F stunts growth and reduces the harvest. A plant set out into a cold spell usually sits there without growing until the weather warms.
Frost
Colorado State says tomatoes are easily killed by a light frost, so 32°F is the hard limit for unprotected plants. A cover can save them on a frosty night of about 28 to 32°F. Maryland notes that well-hardened tomato plants can sometimes come through a light, unexpected frost with little damage, but it is not worth counting on.
Soil temperature
Utah State and Maryland both say to transplant tomatoes once the soil is warmer than 60°F, and that is the number this site's planting check uses. Colorado State gives 55°F as the minimum. In cold soil the roots barely grow, and the plant can sit for weeks without getting bigger. Black plastic mulch warms the soil, and Colorado State says it can bring the harvest two to three weeks earlier. Wait to spread straw or grass clippings: Utah State says to hold off on organic mulch until the soil is warmer than 75°F, because it keeps the soil cool.
Hardening off
Plants grown indoors need time to adjust. Maryland suggests starting 1 to 2 weeks before planting: put them in a shaded, sheltered spot on warm days, bring them in at night, and give them a little more sun each day. Keep tender seedlings inside when it is windy or below 45°F, and water a little less often, but don't let them wilt. Once they are used to it, they can stay out overnight when the low will not go below 50°F.
Planting early with covers
Utah State says that with plastic mulch and , tomatoes can be set out several weeks before the last frost. Hot caps, plastic tunnels, fabric covers and walls of water all trap warmth around young plants, and Colorado State notes that a wall of water protects against a mild frost. Open or vent covers when it is warmer than 80°F under them, and take them off once the weather has settled.
Growing it again? When to plant tomato for your ZIP code
- Blossom DropFlowers fall before setting fruit, often during temperature extremes or after weather stress.
- Gray Mold (Botrytis)Gray-brown fuzzy mold on fruit, flowers, or stems — soft, collapsing tissue beneath the coating in cool, wet conditions.
- Fruit CrackingTomatoes crack at the stem end or shoulder, cabbage heads split, and carrots split lengthwise — usually after heavy rain follows dry weather.
- SunscaldBleached, papery patches on sun-exposed fruit — usually appears after the leaf canopy is reduced.
- 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.
- Why do my transplants look stunted and aren't growing?Cold soil, root-bound roots, or fertilizer burn can all stall a transplant — check soil temperature first before diagnosing anything else.
- 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.
- 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.