When to Start Seeds Indoors: Seed Starting Calendar by Crop and Zone

Start seeds indoors by counting backward from the date you plan to set each crop outside, which is tied to your average last spring frost. Peppers, eggplant, onions and celery need 8-12 weeks indoors, tomatoes 6-8 weeks, broccoli and cabbage 5-7 weeks, and cucumbers and squash only 3-4 weeks. For most of the country, that puts the busiest seed-starting weeks between late January and early April.

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How to figure out when to start seeds indoors

Every seed packet says something like “start indoors 6-8 weeks before last frost.” That number only works once you know two things: your average last frost date and when each crop can safely go outside.

1. Find your average last spring frost date. Use our frost dates by zone page as a starting point, then confirm with your local extension office or a NOAA-based frost date lookup for your ZIP code. Your USDA hardiness zone is based on winter low temperatures, not frost dates, so two towns in the same zone can have last frosts weeks apart.
2. Set a transplant date for each crop. Hardy crops such as onions, broccoli and cabbage can go out 2-4 weeks before your last frost. Tender crops such as tomatoes, peppers and squash go out 1-2 weeks after it, once nights stay above about 50°F.
3. Count back the weeks indoors. Subtract the number of weeks each crop needs under lights from its transplant date.
4. Write it down. A simple seed starting schedule on a calendar or spreadsheet keeps you from sowing everything the first warm weekend of February.

Seed starting chart: weeks indoors, germination temperature and days to sprout

The chart below combines guidance from several extension services. Weeks indoors are counted to your transplant date, not to the frost date. Germination temperatures refer to the seed-starting mix, which a heat mat can raise above room temperature. Days to sprout assume the mix is in the ideal range; expect slower germination in a cool room.

Extension recommendations vary by a week or two depending on climate and how big a transplant you want. University of Minnesota Extension, for example, suggests 14-15 weeks for onions and leeks in Minnesota, while New England guidance uses 8-10. If you pot up into larger containers, you can start tomatoes and peppers a little earlier; if you keep them in small cells, stay on the short end.

Worked example: building a seed starting schedule

Say your average last frost is April 20, which is typical of many zone 6 locations. Here is how the math works:

• Transplant dates. Broccoli and cabbage go out around March 30 to April 6, 2-3 weeks before the frost date. Tomatoes and peppers go out around May 1-4, about 10-14 days after it, once nights reliably stay above 50°F.
• Onions and leeks (10-12 weeks before a late March transplant): sow in early to mid-January.
• Peppers and eggplant (8-10 weeks before May 1-4): sow between about February 20 and March 6.
• Broccoli and cabbage (about 6 weeks before March 30 to April 6): sow around February 16-23.
• Tomatoes (6-8 weeks before May 1-4): sow between about March 6 and March 20.
• Cucumbers and squash (3-4 weeks before May 1-4): sow in individual pots around April 6-13.
• Hardening off starts 1-2 weeks before each transplant date.

Swap in your own frost date and the same math gives you your seed starting calendar. If spring arrives late, hold tender crops indoors a little longer, potting them up so roots have room.

Seed starting calendar by USDA zone

This quick-reference table turns the chart into typical sowing months. It is approximate: frost dates within a zone can vary by several weeks, so treat it as a starting point and confirm your local date with your extension office.

Zones 8-10 work differently. Summer heat, not frost, is the main limit on many crops, so fall through spring is the prime growing season for broccoli, cabbage, lettuce and onions. Onions in the South are usually short-day types sown in fall and harvested in late spring. Tomatoes often get two seasons: one started in winter for a spring crop and one started in mid-summer for fall.

Starting seeds for a fall garden

You can start a second round of seeds indoors in summer for fall broccoli, cabbage, cauliflower, kale and lettuce. Indoors is often easier than direct sowing in July and August, because the seeds avoid hot, dry soil and early pests.

The math runs from your first fall frost instead. Add the crop’s days to maturity from transplant, 2 extra weeks for slower growth as days shorten, and its weeks indoors, then count back that many weeks from your first fall frost. For a broccoli variety listed at 60 days from transplant, that is about 8-9 weeks plus 2 plus 6 indoors, or roughly 16-17 weeks before your first frost. Our fall vegetable garden guide covers what to plant and when.

What happens if you start seeds too early or too late?

Too early: seedlings outgrow their containers, become root-bound, stretch under limited indoor light and may even flower before planting. Tomatoes and peppers held too long in small cells often stall for weeks after transplant. If you must hold them, pot up into 4-inch containers and keep them a little cooler.

Too late: transplants are small at planting time, and long-season crops like peppers may not reach full production before fall. A small transplant is still better than none, and fast crops like squash and cucumbers catch up easily. You can also buy nursery transplants for anything you missed.

Common mistakes to avoid

• Using the zone as a frost date. Hardiness zones describe winter lows. Always look up your actual frost dates.
• Counting weeks to the frost date instead of the transplant date. Tomatoes go out after the frost date, so the clock should run to that later day.
• Sowing everything at once. Peppers and squash need very different lead times. Stagger your sowing.
• Ignoring seed packet notes. Days to maturity and recommended weeks vary by variety, especially for peppers and onions.
• Skipping hardening off time in your schedule. Budget 1-2 weeks before each transplant date; see our hardening off seedlings guide.

FAQ

Keep reading

• How to Start Seeds Indoors: A Step-by-Step Guide for Beginners
• Seed Starting Mix: What to Use, DIY Recipe and Potting Mix Differences
• Grow Lights for Seedlings: Types, Hours, Distance and Setup

Sources

• Starting seeds indoors – University of Minnesota Extension
• Seeding and Transplanting, New England Vegetable Management Guide – UMass Amherst and New England land-grant universities
• Bulletin #2751, Starting Seeds at Home – University of Maine Cooperative Extension
• Starting Plants Indoors From Seeds – University of Missouri Extension
• Starting Seeds Indoors – Clemson Cooperative Extension, Home & Garden Information Center
• Soil temperature conditions for vegetable seed germination – Oregon State University Extension
• Starting Vegetable Seeds Indoors: I. Planning – Utah State University Extension