Free Growli tool
Find your last and first frost
— by ZIP or postcode.
This free calculator returns your average last spring frost and first fall frost dates from a US ZIP or UK postcode. It also suggests six tender crops you can sow after the last frost and tells you when to start seeds indoors (typically 6 to 8 weeks earlier). No signup, no email — calibrated to NOAA and Met Office climate normals.
We cover the top US metros and roughly 100 UK postcode areas today.
Type a 5-digit US ZIP code (e.g. 10001, 60601, 90210) or a UK postcode (e.g. SW1A 1AA, M1, EH1) to get your average last spring frost, first fall frost, and the crops you can sow after the last frost date.
How frost dates work
US frost dates come from the NOAA 1991-2020 climate normals. UK frost dates come from Met Office regional climate averages over the same 30-year window. Both are 50% probability dates — meaning frost has already occurred (or will occur) on or before that date in half of recent years. Treat them as a planning baseline, not a guarantee.
Why your microclimate matters. Urban heat islands push last frost a week earlier and first frost a week later. South-facing slopes warm faster than valleys. Frost pockets at the bottom of a slope run a full zone colder than the regional figure. Coastal gardens stay milder thanks to ocean thermal inertia. Always check soil temperature with a probe before planting tender crops.
What this tool does not capture.Current weather, late cold snaps, your specific elevation, or your soil's actual readiness. For continuous adjustments based on real conditions, the Growli app updates daily and reminds you when it is genuinely safe to sow.
Frequently asked questions
What is the difference between last frost and first frost?
Your last frost is the average final spring freeze, after which it is generally safe to plant tender crops outdoors. Your first frost is the average earliest autumn freeze, which signals when summer crops will stop producing and when garlic and other fall-planted crops should go in. The window between them is your growing season.
How accurate is the frost date the calculator returns?
The dates are 50% probability averages drawn from the NOAA 1991-2020 climate normals for the US and Met Office regional averages for the UK. Real local variability is roughly plus or minus 10 days. Microclimates (urban heat islands, south-facing slopes, lakeside warmth) can shift dates a further week in either direction.
When should I start seeds indoors based on my last frost date?
Most warm-season crops (tomatoes, peppers, basil, cucumbers, squash) start indoors 6 to 8 weeks before your last frost. Slow growers like peppers and eggplant prefer 8 to 10 weeks. The calculator surfaces the indoor-start window for your zone so you do not transplant leggy seedlings or wait too long.
Can I plant before my last frost date?
Hardy cool-season crops can. Peas, spinach, lettuce, kale, broccoli, and onions tolerate light frost and go in 2 to 6 weeks before the last spring frost date. Tender crops (tomatoes, peppers, basil, beans, cucumbers, squash) need soil temperatures above 16 degrees C / 60 degrees F and should wait until 1 to 2 weeks after the last frost.
My postcode is not in the lookup — what should I do?
For v1 we curated the top US metros and roughly 100 UK postcode areas. If your area is not listed, the closest match by region is usually within a few days. US gardeners can cross-check the USDA Plant Hardiness Zone Map; UK gardeners can use the RHS hardiness rating guide linked below.
How does this compare to the Growli app?
The calculator gives you a single snapshot. The Growli app tracks current weather, late cold snaps, soil temperature, and your specific planted crops — so reminders adjust as conditions change. Use the calculator for planning; use the app for ongoing care.