Planting & Harvest Dates

Seed Starting Date Calculator

Pick a plant and enter your last frost date to find the exact window to start seeds indoors, plus when to transplant the seedlings outside.

Not sure? Find it with the Frost Date Calculator.

How to use the seed starting calculator

Choose your plant from the dropdown, enter your last spring frost date, and press Calculate Seed Start Date. The calculator counts back the right number of weeks for that plant and shows the window when you should sow seeds indoors, along with the date to move the young plants outdoors. If you do not know your last frost date, our Frost Date Calculator will find it from your ZIP code.

Why timing your seed starting matters

Starting seeds indoors gives heat-loving and slow-growing plants a head start on the season, but the timing has to be right. Sow too late and your plants will not have time to produce a good harvest before fall. Sow too early and the seedlings outgrow their pots, become leggy, and sit stressed on a windowsill for weeks waiting for warm weather. Counting back from your last frost date keeps each plant on schedule so it reaches transplant size right when the garden is ready for it.

How the weeks-before-frost numbers work

Every plant has a typical number of weeks it needs to grow from seed to a transplant-ready seedling. Quick growers such as lettuce and basil only need about four to six weeks, while slow starters like onions, celery, parsley, and many bedding flowers need ten to twelve. Tomatoes sit in the middle at roughly six to eight weeks. The calculator subtracts that number of weeks from your last frost date, so a tomato in a region with a May 1 last frost would be started indoors in early to mid March.

Vegetables, herbs, and flowers in this calculator

This tool covers sixteen popular plants that reward an indoor start. The vegetables include tomato, pepper, eggplant, broccoli, cauliflower, kale, cabbage, lettuce, onion, and celery. It also handles the herbs basil and parsley, plus four classic bedding flowers: petunias, impatiens, marigolds, and snapdragons. Warm-season crops are transplanted after the last frost has passed, while cold-hardy vegetables can go out a little earlier because they tolerate light frost.

Tips for healthy transplants

  • Use a sterile seed-starting mix and clean containers with drainage holes to prevent damping-off disease.
  • Give seedlings 14–16 hours of bright light a day; a south window is rarely enough, so a simple grow light helps.
  • Keep the mix evenly moist but not soggy, and bottom-water when possible to protect tender stems.
  • Pot up seedlings that outgrow their cells before transplant time so they never become root-bound.
  • Harden plants off over 7–10 days before moving them into the garden to avoid transplant shock.

Frequently Asked Questions

How do I find my last frost date?

Your last spring frost is the average date after which freezing temperatures are unlikely in your area. The quickest way to find it is our Frost Date Calculator, which returns the average last frost for your ZIP code using 30-year NOAA climate data. Once you have that date, enter it here to get your seed-starting window for any plant in the list.

Why does the calculator give a range instead of one date?

Most seed packets list a span of weeks, such as 6 to 8 weeks before the last frost, because seedlings tolerate a little flexibility in their age at transplant time. The earliest date counts back the larger number of weeks and the latest date counts back the smaller number. Starting anywhere inside that window works well, and aiming for the middle gives you a comfortable buffer.

What happens if I start seeds too early?

Seedlings started too early can outgrow their containers before it is warm enough to plant out. They become tall and leggy, get root-bound, and may stall or suffer transplant shock once they finally go outside. If your seedlings are ready before the weather is, pot them up into larger containers and keep them under bright light until the transplant window arrives.

When should I move the seedlings outdoors?

Frost-tender plants like tomatoes, peppers, eggplant, and basil should go out about one to two weeks after your last frost, once nights stay reliably warm. Cold-hardy crops such as broccoli, cabbage, kale, and onions can handle cooler conditions and may be transplanted a week or two earlier. Always harden seedlings off over several days before planting them in the garden.

What does hardening off mean?

Hardening off is the process of gradually acclimating indoor-grown seedlings to outdoor conditions. Over 7 to 10 days, set the plants outside for a few hours at first, then progressively longer, increasing their exposure to sun, wind, and temperature swings. This toughens the foliage and prevents the shock that can stunt or kill seedlings moved straight from a windowsill into the garden.

Do all of these plants need to be started indoors?

The plants in this list all benefit from an indoor head start because they are slow to mature or sensitive to cold. Many other crops, including carrots, beans, peas, radishes, and squash, are better sown directly into the garden because they dislike root disturbance. If you are planning a whole garden, our Planting Calendar shows which crops to start indoors and which to direct sow.

How accurate are these seed-starting dates?

The dates are reliable planning guidelines built from typical seed-packet timing and your average last frost, but frost dates are averages rather than guarantees. About half of years will see a slightly later frost, and local microclimates can shift your true date by a week or more. Keep an eye on the forecast and have row cover or a cold frame ready for any late cold snaps.

Related Tools

Frost Date Calculator Open tool → Planting Calendar Open tool → Square Foot Garden Planner Open tool →
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GardenCalc Editorial Team

Horticulture writers & master gardeners

Our calculators and growing guides are written and fact-checked by gardeners with hands-on experience in vegetable production, soil management, and home landscaping.