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:

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 22–29 (1–2 weeks after frost). Start indoors = May 22 − 8 weeks = March 27

For peas (hardy): Plant outdoors April 17–May 1 (2–4 weeks before frost). Direct sow outdoors — no indoor start needed.

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 a month. The same tomato variety that fits a Zone 6 schedule will need an indoor head-start of 8-10 weeks in Zone 3 to mature before the short season runs out.

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.