About the Seed Starting Calculator
This seed starting calculator builds an indoor sowing schedule from your average last spring frost date. Pick a crop and enter the frost date for your area, and it tells you the window for sowing seeds indoors, when to begin hardening off seedlings and when to transplant them into the garden. A table lists the same dates for every common crop so you can plan the whole season at once.
It is meant for home gardeners raising their own transplants under lights or on a windowsill. Warm-season crops such as tomatoes, peppers and basil are started 6–10 weeks ahead and planted out after the frost danger has passed; cool-season crops such as broccoli, cabbage, lettuce and onions go out several weeks before the last frost.
Timings are typical guidance used by seed companies and extension services. Your local frost date is an average — there is still a chance of frost after it — so watch the forecast and wait for warm soil before planting tender crops.
How to use the seed starting calculator
- 1Look up your average last spring frost date.
- 2Enter it and choose the crop you want to grow.
- 3Sow seeds indoors between the two start dates.
- 4Begin hardening off on the date shown.
- 5Transplant on or after the transplant date if the forecast is favorable.
Formula and method
Each crop has a typical number of weeks it needs indoors to reach transplant size, given as a range before the last frost date. The earliest sowing date subtracts the longer figure and the latest sowing date subtracts the shorter one. Transplant dates are offset from the frost date: after it for frost-tender crops, before it for hardy cool-season crops.
Hardening off — gradually exposing seedlings to sun, wind and cooler nights — begins about a week before transplanting. Dates are calendar arithmetic in whole days; the crop timings are typical guidance and vary by variety and growing conditions.
- Last frost
- Average date of the last spring frost for your location
- Offset weeks
- Weeks after (+) or before (−) last frost to transplant
Worked examples
Tomatoes with an April 15 frost date
Tomatoes are started 6–8 weeks before the last frost, so sow between February 18 and March 4. They go out about a week after the frost date, on April 22, after hardening off from April 15.
Peppers with a May 10 frost date
Peppers need 8–10 weeks indoors, so sow between March 1 and March 15. They are transplanted two weeks after the last frost, on May 24.
Broccoli with an April 1 frost date
Broccoli is started 5–7 weeks before the frost date (February 11–25) and, being frost-tolerant, is planted out about two weeks before it, on March 18.
Frequently asked questions
When should I start tomato seeds indoors?+
Start tomatoes about 6–8 weeks before your average last frost date, and transplant them outdoors once frost danger has passed and nights stay above about 50°F.
How do I find my last frost date?+
Your local cooperative extension service publishes average frost dates, and many online lookups use NOAA climate data by ZIP code. An average date still carries about a 50% chance of a later frost, so cautious gardeners plan tender transplants a week or two after it and watch the forecast.
What does hardening off mean?+
Hardening off is gradually acclimatizing indoor seedlings to outdoor sun, wind and temperature over 7–10 days, starting with an hour or two in shade and increasing each day. It prevents transplant shock and sunscald.
Can I start seeds too early?+
Yes. Seedlings kept indoors too long become leggy and root-bound and often establish worse than younger transplants. Stick to the recommended window and pot up if planting is delayed.
Which vegetables should not be started indoors?+
Root crops such as carrots, radishes and beets, and large-seeded crops such as beans, peas and corn, are usually sown directly in the garden because they dislike root disturbance or grow quickly outdoors.