Frost Date and Planting Calculator

You enter your average last spring frost, your average first fall frost, the crop's days to maturity, how many weeks you start the seeds indoors, and the crop's frost tolerance. From the last spring frost it sets a transplant date (before, on, or after the frost depending on tolerance), counts back the indoor weeks to a seed-start date, and adds days to maturity for a spring harvest. From the first fall frost it counts back to the last safe fall planting date. Everything runs in your browser, so there is no signup.

Count back from your first fall frost. The standard method adds the crop's days to maturity, a 14 day fall factor for the slower growth of shorter, cooler days, and for frost-sensitive crops another 14 day buffer so the plant matures at least two weeks before frost. Subtract that total number of days from the first fall frost date to get the drop-dead planting date. For example, a 68 day cucumber works out to 68 plus 14 plus 14, or 96 days before the frost.

Tender crops are frost-sensitive, so you set them out after the last frost (this calculator uses 14 days after). Examples are tomatoes, peppers, basil, and squash. Half-hardy crops tolerate a light frost and go out on the last frost date, such as beets, carrots, and chard. Hardy crops are frost-tolerant and can go in before the last frost (here 14 days before), like peas, lettuce, kale, and broccoli.

Average frost dates come from your local extension office, a national weather service climate normals lookup, or an almanac by ZIP code. They are averages, so frost can still arrive a week or two on either side. Days to maturity is printed on the seed packet or in the variety description, usually counted from transplant for started crops and from sowing for direct-sown crops.

If you enter a number of weeks to start indoors, the calculator counts that many weeks back from the transplant date to give you an indoor sowing date. If you set the indoor weeks to 0, the crop is sown directly outdoors, so the seed-start date and the transplant date are the same. Use 0 for crops like beans, carrots, and radishes that resent transplanting.

Treat them as planning estimates, not guarantees. They are built from average frost dates and typical days to maturity, but your real dates shift with the season, your microclimate, soil temperature, and the specific variety. Use the output to schedule, then watch your local forecast and harden off seedlings before moving them outdoors.