What to do now
Mildest step first.
- Check tonight's forecast low and compare it with the three groups below.
- Leave hardy crops uncovered for a light frost; cover lettuce, carrots and other semi-hardy crops if the low is forecast below 28°F.
- Cover tomatoes, peppers, squash and other tender plants before sundown whenever 32°F or lower is forecast, and bring potted plants inside.
Hardy: about 25°F
The University of Maryland lists these as hardy, taking hard frosts of usually 25 to 28°F: asparagus, broccoli, Brussels sprouts, cabbage, Chinese cabbage, collards, garlic, kale, kohlrabi, leeks, mustard greens, onions, parsley, peas, radishes, spinach and turnips. South Dakota State puts cabbage, kale, broccoli, kohlrabi and turnips in a group that takes moderate freezes of about 24 to 28°F, and notes that parsnips and Brussels sprouts are best harvested after several freezes. Horseradish is the toughest of all and takes freezes below 24°F.
Semi-hardy: light frost
Semi-hardy crops take a light frost, which Maryland puts at about 29 to 32°F: beets, carrots, cauliflower, celery, endive, lettuce, potatoes, salsify, shallots and Swiss chard. South Dakota State gives a similar light-frost group at about 28 to 32°F. These plants come through a frosty night with little harm, but a colder freeze can damage leaves, so cover them if the forecast drops below about 28°F.
Tender: damaged at 32°F
Tender crops are damaged or killed by even a light frost: beans, corn, cucumbers, eggplant, melons, peppers, pumpkins, squash, sweet potatoes and tomatoes. Colorado State calls tomatoes, peppers, eggplant, melons and winter squash very tender, because they also dislike cool spring winds and stall when days stay below 55°F. Plant these after your , and cover them whenever a frost is forecast. fabric or old sheets put on before sundown are the usual protection.
Where lists differ
The lists do not agree on every crop. Maryland and Purdue call peas and spinach hardy, while South Dakota State puts them with the light-frost crops. Colorado State lists lettuce as hardy, but most others call it semi-hardy. Potatoes are semi-hardy on the Maryland, Purdue and Colorado State lists, but South Dakota State says their leaves have no frost tolerance. These numbers also assume plants that have toughened in cool weather. Maryland warns that even cold-hardy plants are hurt by freezing temperatures if they go outside before they have been .
- KaleThe cold-weather workhorse that improves when everything else quits.
- CabbageA long-maturing head that splits if you water it wrong at the wrong time.
- Collard GreensThe heat-tolerant brassica that gets Southern gardens through the summer.
- SpinachA cold-weather green that gives you leaves when almost nothing else grows.
- PeaA cool-season crop that rewards early sowing and quits when summer arrives.
- 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.
- 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.
- 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 can I extend my growing season in fall?Row cover fabric, cold frames, and switching to cold-hardy crops are the three most reliable tools for keeping a garden producing several weeks past your first fall frost.
- 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.