Frost Date Planner

Frost date planning is the foundation of every vegetable garden calendar. The last spring frost date determines when tender plants can safely go outdoors; the first fall frost date tells you when to wrap up the season or bring plants inside. Getting these dates wrong can mean losing an entire crop overnight.

Core formulas:

Safe Outdoor Planting Date = Last Spring Frost Date + Weeks After Frost

Indoor Seed Start Date = Safe Outdoor Planting Date − Indoor Growing Weeks

Days to Harvest = Planting Date + Days to Maturity (from seed packet)

Growing Season Length = First Fall Frost Date − Last Spring Frost Date

Variable definitions:

• Last Spring Frost: the average date of the final below-32°F night in spring in your area
• First Fall Frost: the average date of the first below-32°F night in autumn
• Weeks After Frost: buffer time after the last frost before planting tender crops
• Indoor Growing Weeks: time seeds need indoors before they are large enough to transplant

Plant hardiness categories:

These are the figures this planner uses. Days to maturity is a category average, so check your seed packet: a cherry tomato can be picking in 55 days where a big beefsteak takes 85, and that difference decides whether the crop fits a short season.

This page wants your own two dates, which is the right way round if you know them or can look them up.
If you do not, the frost date planting calculator picks both dates for you from a USDA zone, and adds a warm-loving category (melons, okra, sweet potatoes) that goes out two weeks after the frost rather than three. On the three categories both pages share, they return the same dates.

Worked example: Location: Chicago, Illinois. Last frost: May 15. First fall frost: October 10. Growing season = October 10 − May 15 = 148 days

For tomatoes (tender): plant outdoors May 22 (1 week after frost). Start indoors = May 22 − 8 weeks = March 27 First tomatoes around August 5, which leaves about nine weeks of season after them.

For peas (hardy): plant outdoors April 17 (4 weeks before frost). Direct sow outdoors, no indoor start needed. Picking from mid-June.

Frost probability tip: Frost dates are based on 30-year historical averages with a 50% probability of frost. For frost-sensitive crops, use the 10% probability date (usually 2–3 weeks later) as your safe planting target. Local agricultural extension offices publish these tables by zip code.

USDA Hardiness Zone reference

Move two zones north or south and your last frost shifts by about three weeks. The season itself moves further than that, because the autumn date travels the other way at the same time. Zone 6 to Zone 3 costs three weeks in spring and a further month in autumn, which is why the same tomato variety that fits a Zone 6 schedule needs an indoor head-start and a short-season pick to finish in Zone 3.

Frost protection at the margins

When a cold night threatens after you’ve already planted:

• Light frost (29-32°F / -1.5 to 0°C): Cover plants with row covers, cloches, plastic bins, or even old bed sheets overnight. Remove in the morning.
• Hard freeze (below 28°F / -2°C): Bring potted plants inside. Most exposed tender crops won’t survive a hard freeze regardless of covering. Mature tomatoes and peppers can sometimes be salvaged by harvesting everything that’s started to color, even if not fully ripe.
• Microclimates matter. South-facing walls, raised beds, urban areas, and spots near concrete or rocks run 3-5°F warmer than open lawns or low-lying frost pockets. A protected spot can buy you two weeks of extra season at either end.