What Is First Frost and How to Use It

First frost ends warm-season crops. Use your local date in My Garden to count weeks backward for last sowings.

Published 2026-09-02 by ThriveThyme

first frost fall planting zones frost dates

What Is First Frost and How to Use It

Your average first frost is the typical date of the first light freeze in fall. It is a planning tool, not a guarantee for your yard.

First Frost vs Hard Freeze

Light frost (about 32 F / 0 C): tender crops like tomatoes, basil, and beans often stall or get damaged.
Hard freeze (about 28 F / -2 C or colder for several hours): more plants die back; soil starts to shut down warm-season growth.

Yards vary. A low spot, a hilltop, or a wall can shift your date by a week or more.

Do Not Use a National Date

USDA zones describe winter cold hardiness. They do not give you one first-frost day for the whole zone. Set your own date in My Garden frost settings, or use a nearby weather station / Extension average, then keep refining it.

ThriveThyme stores last frost and first frost for your garden. Use those when you plan September sowings.

Count Backward From Your Date

Pick the crop, then subtract the weeks it needs before frost:

Fast radishes: about 3 to 5 weeks
Spinach, lettuce, arugula: about 4 to 6 weeks
Kale, collards: about 6 to 8 weeks
Beets, turnips, carrots: about 8 to 10 weeks
Broccoli / cabbage transplants: about 10 to 12 weeks
Cover crops: enough time to establish before a hard freeze (species vary)

Add a small weather buffer (about a week) if your fall turns cold early.

What to Do This Week

Confirm or set your first frost date in My Garden.
Open What to plant now and your zone September guide.
Sow only what still fits the weeks you have left.
For spent beds, see When to sow cover crops by zone.