Open Grow Guide

Frost dates explained

By Open Grow Guide Updated

Frost dates tell you when to start seeds indoors and when to move plants outside. Tender and half-hardy transplants begin indoors about four to six weeks before the last frost date. They should not go outdoors until the danger of frost has passed. Frost also comes in different strengths: a light frost occurs at 32°F (0°C), while a hard frost of 28°F (-2°C) or colder will kill most warm-season crops. This guide walks through the numbers, the planting steps and the protection methods that the sources describe.

When to do it

Your average last frost date is the reference point for spring planting. Pepper seed starts indoors 6-8 weeks before that date, and melon seed can start indoors four to six weeks before it. Hardy annual seeds go outdoors in fall, about six weeks before the first frost. Soil temperature gives you a second signal for timing. Early, cool-season crops can be planted when soil reaches 40-50°F (4-10°C), and moderately warm-season crops do best at 55-60°F (13-16°C). Heat-loving crops should wait until soil reaches 65°F (18°C) or warmer and all danger of frost has passed. A few warm days in early spring can be deceiving, as overnight freezes and cold soil often persist for weeks afterward.

What you need

A soil thermometer lets you measure soil temperature about 4 inches (10 cm) deep. A properly calibrated thermometer should read 32°F (0°C) when placed in an ice bath solution. For cold nights, gather lightweight row covers of 0.5 to 1 oz, which protect down to 28°F (-2°C), or heavier covers for deeper cold snaps. Clear plastic domes can protect seedlings from nighttime cold damage. Mulch or extra soil lets you insulate pots and the roots of balled-and-burlapped plants. Commercial tree wraps or one- to two-foot (0.3-0.6 m) mounds of soil insulate the trunks of young or tender plants before cold weather. A fan placed near seedlings gives them a gentle sway during hardening off, once they have two true leaves. A cold frame is another option, and on sunny days it needs ventilating to keep the temperature below 60°F (16°C).

Step by step

  1. Step 1

    Find your area's average last and first frost dates before you plan the season

    Average last frost dates run from early May in southern regions to late May or early June farther north, such as in Minnesota. First fall frost dates shift by USDA Hardiness Zone too, landing around November 19 in zone 9b, November 9 in zone 8b, and October 22 in zone 7b.

  2. Step 2

    Don't trust a run of warm days in early spring

    Overnight freezes and cold soil often linger for weeks after a mild spell, even when the days feel warm.

  3. Step 3

    Measure your soil temperature before you plant

    Push a soil thermometer about 4 in (10 cm) deep to get an accurate reading.

  4. Step 4

    Match your planting dates to soil temperature, not the calendar

    Plant early, cool-season crops once soil hits 40-50°F (4-10°C). Wait for 55-60°F (13-16°C), usually by mid- to late May, before you plant moderately warm-season crops. Hold heat-loving crops until soil reaches 65°F (18°C) or warmer and all danger of frost has passed.

  5. Step 5

    Use May 21 as your reference frost-free date if you garden in Eastern Oregon

    Regional planting schedules are often built around a single reference date like this one.

  6. Step 6

    Sow hardy annual seed outdoors in fall

    Plant it about six weeks before your area's first frost.

  7. Step 7

    Start tender and half-hardy seed indoors before your last frost date

    Begin four to six weeks ahead of that date.

  8. Step 8

    Start pepper seed indoors ahead of your last frost date

    Give peppers six to eight weeks indoors before that date arrives.

  9. Step 9

    Start or sow melon seed around your last frost date

    Plant melon seed outdoors after the last frost, or start it indoors four to six weeks before your average last frost date.

  10. Step 10

    Begin hardening off seedlings in a sheltered, shady spot

    Set them outside for just a few hours at first, then slowly increase their time outdoors and sun exposure over seven to ten days.

  11. Step 11

    Add a fan near your seedlings once they grow two true leaves

    The moving air makes the seedlings sway gently, which is part of hardening them off.

  12. Step 12

    Continue the hardening-off routine for two to three weeks

    This conditions the seedlings before they move outdoors for good.

  13. Step 13

    Increase sun exposure gradually during hardening off

    Moving plants into more direct sun too fast can injure them.

  14. Step 14

    Keep tender and half-hardy plants indoors until the danger of frost has passed

    For peppers specifically, wait until any possibility of frost is gone before you set seedlings or purchased plants outside.

  15. Step 15

    Transplant on a cool, cloudy day if you can

    This timing helps you avoid transplant shock.

  16. Step 16

    Water landscape plants before a predicted freeze

    Don't overdo it, though, since soaked soil for too long damages the root systems of most plants. You don't need to wet the soil deeper than 1 ft (0.3 m) for frost protection to work.

  17. Step 17

    Keep your plants healthy going into cold weather

    Healthy plants resist cold better than plants already weakened by disease, insects, or nematodes.

  18. Step 18

    Site new plants to avoid frost pockets and low spots

    Also avoid locations known for rapid temperature swings.

  19. Step 19

    Cover plants with fabric that reaches all the way to the ground

    A cover that stops short lets the trapped heat escape. Choose a lightweight cover of 0.5 to 1 oz for protection down to 28°F (-2°C), and a heavier cover for deeper cold.

  20. Step 20

    Place clear plastic domes over seedlings on cold nights

    These domes can protect seedlings from nighttime cold damage.

  21. Step 21

    Mulch or mound soil around pots and balled-and-burlapped plants

    This insulates the roots against cold.

  22. Step 22

    Remove or vent plastic covers once the sun comes out

    Vent a cold frame on sunny days too, to keep its inside below 60°F (16°C).

  23. Step 23

    Mow cover crops and weeds short before a frost

    A tall, dense stand can drop nighttime temperatures near your plants by up to 10°F (5°C) compared with bare soil, while a closely mowed stand is only about 2°F (1°C) colder than bare soil.

  24. Step 24

    Run sprinklers for frost protection through the coldest hours

    Turn them on around 32°F (0°C), or earlier if the dew point is low. Keep sprinkling continuously once a freeze starts, since this keeps leaf surface temperatures near 32°F (0°C), and continue until the ice has fully thawed. Overhead irrigation can only protect plants down to about 24°F (-4°C) at best.

  25. Step 25

    Calibrate your thermometer before you trust its readings

    A properly calibrated thermometer should read 32°F (0°C) when placed in an ice bath.

  26. Step 26

    Turn off in-ground irrigation systems before freezing temperatures arrive

    Do this ahead of the cold snap, not once it has already set in.

  27. Step 27

    Give container plants extra protection before a freeze

    Soil in small containers can freeze solid for several hours during a freeze, so push pots together and add mulch around them.

  28. Step 28

    Wrap or mound soil around young, tender trunks before cold weather arrives

    Use a commercial tree wrap, or mound soil one to two feet high around the base. Remove the wrap or mounded soil again each spring.

  29. Step 29

    Water plants through late summer and autumn

    This keeps them from heading into winter already stressed by drought.

  30. Step 30

    Give plants enough time to harden off before fall's freezing temperatures arrive

    They need this adjustment period to handle outdoor cold once it comes.

  31. Step 31

    Don't rush to prune or remove plants damaged by frost or freeze

    Cold injury to foliage and tender shoots usually shows within a few days. Wait to see if live green foliage reappears before you act further.

  32. Step 32

    Remove dead wood once you can tell how far the cold damage goes

    Wait for this clear picture before you cut anything back.

  33. Step 33

    Shade plants from early morning sun after a freeze

    This may decrease bark splitting on some woody plants.

  34. Step 34

    Water plants after a freeze has passed

    This allows thawing, helps plants rehydrate, and dilutes fertilizer salts in the soil.

  35. Step 35

    Re-space plants as soon as possible once the freeze has passed

    Don't leave this task for later in the season.

Mistakes to avoid

Setting tender and half-hardy annuals out too early is the clearest mistake. A frost will likely kill them if the last frost date has not passed. A run of warm days is a related trap, because overnight freezes and cold soil often persist for weeks afterward.

Covers are better at protecting plants from frost than from extreme cold, so do not rely on them in a hard freeze. When a row cover directly touches plant leaves, frost can still form on them and freezing can still happen. Overhead irrigation, at best, can only protect plants down to 24°F (-4°C).

Watering before a freeze should not be overdone, since prolonged saturated soil conditions damage the root systems of most plants. Watering cold-damaged plants that appear wilted will not help revive them. Do not be in a hurry to prune or remove plants damaged by frost or freeze.

Do not stimulate new growth by applying excessive nitrogen or pruning in early autumn, ahead of the first frost. Late fall fertilization can result in growth flushes that are more vulnerable to cold injury.

How this page was made

  • Written with AI assistance from the sources listed on this page. AI is never the source of a fact.

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