When to Start Seeds Indoors: Crop Chart and City Dates

Start seeds indoors by counting back from the day each crop can go outside, not from your last-frost date. Extension guides give about 8 to 10 weeks for onions, 8 for peppers, 6 for tomatoes, 4 to 6 for broccoli and 3 to 4 for cucumbers and squash. Cool crops go out before the last frost and warm crops weeks after it, so in Columbus, Ohio, that means onions in late January, broccoli in early March, peppers in mid-March and tomatoes around March 30.

The shortcut most charts use, starting tomatoes 6 to 8 weeks before your last frost, skips the planting-out step. We tested it against NOAA’s 1991–2020 climate normals for 38,123 ZIP codes. In almost all of them (99.9%) it keeps tomato seedlings indoors for 8 weeks or more, and in 81% for 9 weeks or more. The University of Maryland and the University of Minnesota both say 5 to 6 weeks.^2,3

Seed starting chart: weeks indoors and when each crop goes out

Every number below comes from an extension service or a seed company’s published table, read on October 3, 2026. The “goes outside” column is the step most charts leave out. Oregon State, Johnny’s Selected Seeds and Cornell’s Tompkins County office set it relative to the average last-frost date;^4,13,7 we use NOAA’s median (50%) date for that. Where sources disagree, the table shows the range.

UMD is the University of Maryland Extension, UMN the University of Minnesota, OSU Oregon State’s central Oregon guide, USU Utah State and Cornell its Garden-Based Learning program; Johnny’s is a seed company whose calculator uses the same count-back.^2,3,4,5,6,13 Penn State gives 3 to 4 weeks for cucumbers, melons and squash.⁸

Why the sources disagree

Most disagreement is about two crop groups. The University of Minnesota keeps onions indoors 14 to 15 weeks and broccoli, cabbage and head lettuce 10 to 11. Maryland, Oregon State and Johnny’s give onions 8 to 10 weeks and brassicas 4 to 6.^3,2,4,13 Longer indoor time gives you bigger transplants. It also gives more chance of root-bound, stretched plants, and Maryland warns about exactly that:

A common mistake is to sow seeds too early

If you are new to it, use the low end of each range. Cornell gives the same advice: small plants catch up with overgrown ones.⁶

Some crops are better sown straight into the garden. Root crops such as carrots, beets and turnips dislike having their roots disturbed, and fast, cold-tolerant peas and radishes gain little from an indoor start. The Old Farmer’s Almanac chart also marks beans and corn for sowing outdoors.¹⁷ Cucumbers, squash and melons can go either way; if you start them inside, keep it to 3 or 4 weeks.⁸

Count back from the planting-out date, not the frost date

Extension planners all work in three steps: find your last-frost date, decide when each crop can go outside, then count back the indoor weeks. Oregon State’s worksheet has a column for each step.⁴ Utah State puts the last step this way:

Base this answer on the outside planting date

The middle step is what changes the answer. Onions and broccoli tolerate cold and go out weeks before the median last frost. Tomatoes and peppers go out after it. Cornell says warm-season crops do best planted 2 to 3 weeks after the average last frost, once nights stay above 45°F.⁶ Our tomato planting guide found the gap is usually wider. Planting out works best once frost risk is down to one year in ten and the normal night low reaches 50°F, a median of 24 days after the median last frost across US ZIP codes.¹

That is why the frost-date shortcut runs long for warm crops. Counting 6 to 8 weeks back from the median last frost, as the Old Farmer’s Almanac does for tomatoes,¹⁷ gives a typical tomato seedling 9 to 11 weeks indoors before its real planting day. Charts that give peppers 8 to 10 weeks before the last frost, as Google’s AI Overview did when we checked, work out to 11 to 13 weeks.

Seed starting calendar for 20 US cities

We applied that count-back to the NOAA station nearest each city ZIP code.¹ Method: onions go out 4 weeks before the median last frost after 8 weeks indoors. Broccoli goes out 2 weeks before it after 5 weeks indoors. Tomatoes and peppers go out on the later of two dates: the 10% last-frost date, or the day the normal low reaches 50°F. Tomatoes get 6 weeks indoors and peppers 8.

*In Seattle and Portland the median last frost comes in late February or March, but nights stay cool into May. A frost-based count-back would put onion seeds in late fall there, so we leave those cells blank; follow your local extension calendar for cool crops. The onion dates assume a spring crop. Where winters are mild, onions are often grown over winter instead: UF/IFAS says bulbing onions in Florida must be planted in fall.¹⁵ Valleys, hilltops and gardens far from a station can be a week or more off. For any ZIP code, look up your frost dates and their odds and count back the same way.

How our dates compare with local extension calendars

We checked the method against six published local calendars:

Minnesota and Nebraska agree within about a week. Illinois runs later for central Illinois because it puts the last frost around early May.⁹ NOAA’s Springfield station puts the median at April 15 and the 10% date at April 30, so Illinois is working from the safer date. Missouri Botanical Garden sows peppers with tomatoes,¹¹ which is later than the 8-week pepper lead most sources give. The February tomato start in Ohio State’s BYGL note would mean about 10 to 14 weeks indoors before Columbus’s May 11 planting-out date.¹² Sources: University of Minnesota,³ Nebraska Extension in Lancaster County,¹⁰ Illinois Extension and Oregon State.⁴

Why your zone doesn’t set your seed-starting dates

Search for “seed starting calendar zone 6” and you get charts by USDA hardiness zone. But the zone map is based on the average coldest winter night, not on spring frost or warm nights.¹⁴ Two gardens in the same zone can reach planting-out weeks apart. We looked up the 2023 zone for 1,934 randomly sampled US ZIP codes¹⁸ and compared their NOAA dates:

Inside zone 6b, the middle 80% of sampled ZIP codes reach the tomato seed date anywhere from Mar 22 to Apr 22, a spread of 31 days. In zone 7a it runs from Mar 16 to Apr 14. Zone 8b takes in both Gulf Coast towns and the cool Oregon and Washington coast, and its spread is 68 days. A zone calendar can only give one date for all of them, so it will be weeks early for some gardens and weeks late for others.

Use your zone to choose perennials, not to set seed dates; cold-loving perennial seeds can go out in winter with winter sowing. For more on what the zones measure, see planting zone vs. frost date and our hardiness zone guides.

Warm-winter areas and seed starting for fall crops

Where frost is rare, broccoli, cabbage and their relatives are fall and winter crops. UF/IFAS lists broccoli planting from September to February in north and central Florida, and October to January in south Florida.¹⁵ Texas A&M AgriLife’s average fall planting dates for broccoli run from July 15 in Region I to November 1 in Region V.¹⁶ Count back 4 to 6 weeks from those dates for the indoor start.

The same works for a northern fall crop. Illinois Extension suggests starting broccoli, cauliflower, cabbage and kale indoors in early July, then planting them out in late July or early August.⁹ In Minnesota, cabbage and broccoli for fall can be started in June or July.³

Light and temperature basics

These are the conditions extension guides give for healthy seedlings. Ranges show where sources differ.

A windowsill is a poor place to start seeds. The University of Minnesota notes that sills can be the coldest spot in the house at night and the hottest by day, and that seedlings grown there tend to be tall with thin, bent stems.³ Lack of light is the main cause of stretched stems, so a fluorescent or LED fixture on a timer usually makes the biggest difference.³ Bottom heat from a seed-starting mat helps germination, but don’t plug the mat into the light timer.³

Potting up and hardening off

1. 1

   Pot up after the first true leaves

   Move seedlings sown in a shared flat into their own containers soon after the first true leaves appear. Maryland moves tomatoes, peppers and eggplant into larger pots 2 to 3 weeks after germination. Handle them by a leaf, never the stem.²

2. 2

   Give roots room

   Minnesota suggests containers 1 to 2 inches wider than the cell packs. Maryland gives tomato, eggplant and pepper 9 to 16 square inches per plant.^3,2

   

3. 3

   Harden off before planting out

   Start a week or two before planting out. Set plants outside in shade, out of the wind, for a few hours, and add time and sun each day. Bring them in when nights turn cold.^3,11

Then plant on a cloudy day or in late afternoon.³ For crop-by-crop timing outside, see our vegetable guides and seed-starting guides, check frost odds in our frost and freeze guides, or read how we process the NOAA frost data. More in the garden section and our climate data.