When to Start Seeds Indoors by Zone (Dated Chart for 24 Crops)

The advice you will find everywhere is “start seeds indoors 6 to 8 weeks before your last frost,” and it is close to useless, because it treats every crop the same and it assumes you already know your last frost date. The real rule has two parts. First, count from your own last frost date, not a calendar month, because that date varies by six weeks between a zone 4 garden and a zone 8 one. Second, the count itself is different for every crop: onions want 10 weeks of lead time, peppers and eggplant 8, tomatoes 7, broccoli, kale and basil 6, lettuce 4, and the cucurbit family, cucumbers, squash and zucchini, only 3.

That last point is where most people go wrong in the other direction. A cucumber started 8 weeks early instead of 3 does not get a head start, it gets leggy, rootbound, and stressed by the time it goes outside, and it usually performs worse than one started on time or even direct sown. Getting the number right for each crop matters as much as getting your last frost date right in the first place, and it is the difference between a seed starting calendar you can actually trust and one you copied from a packet meant for a different plant entirely.

Find your last frost date

Seed starting dates for every USDA zone

The spread between crops in this table is not arbitrary. Onions need 10 weeks because they grow slowly from seed and barely mind being started early, since they spend most of that time as thin, grass-like sprouts that are in no danger of getting leggy. Cucumbers, squash and zucchini need only 3 weeks because the opposite is true: they germinate fast, grow fast, and resent root disturbance, so a longer head start just means a bigger, more stressed plant to transplant. Tomatoes and peppers sit in between, slow enough to need real lead time, fast-growing enough that overdoing it costs you.

Read down to your zone and across to your crop. Dates are calculated backward from the representative last frost date for that zone; your exact ZIP code will usually be within a week of it.

The zones without a frost do not run on a starting date at all. Cool-season crops go in from roughly November through February, and the whole calculation this table does for everyone else does not apply, because there is no last frost to count backward from.

Which seeds should not be started indoors

Not everything belongs on a seed tray, and starting these crops indoors either wastes effort or actively hurts them.

Carrots, beets and radishes are root crops. Their taproot does not tolerate being disturbed, so a transplanted carrot forks, stunts or bolts. All three go directly into the garden, timed from your last frost: radishes about 4 weeks before, beets 3 weeks before, carrots 2 weeks before.

Peas and beans germinate fast enough outdoors, within a week in warm soil, that starting them indoors buys nothing and their roots resent the transplant. Peas go in about 5 weeks before your last frost, since they are frost tolerant. Beans wait until about a week after it, since they are not.

Corn and spinach follow the same logic from opposite ends of the hardiness spectrum. Corn is tender and direct sown roughly a week after your last frost. Spinach is hardy and direct sown about 5 weeks before it, tolerating a fair amount of cold on the way up.

Potatoes are not started from seed at all in the home garden. Seed potato pieces go directly into the ground about 2 weeks before your last frost, since a sprouted seed potato has no business on a windowsill.

The pattern across all eight: crops that either dislike root disturbance or germinate quickly and reliably outdoors gain nothing from an indoor head start, and several of them actively lose from it. A tray of started beans or peas is not a crime, and gardeners in short-season zones sometimes do it anyway to buy a week, but expect a lower success rate on the transplant than you would get from any of the crops actually built for indoor starting, and expect a fussier hardening off process on top of it.

Why starting too early is the most common mistake

Ask any experienced gardener what they would tell a first-year seed starter, and most of them will say the same thing: you started too early, not too late. The instinct makes sense. Spring feels far away in January, the seed packets are sitting right there, and starting early feels like getting ahead. What actually happens is a plant that outgrows its pot before the garden is ready for it.

Leggy growth is the first sign. Seedlings kept indoors past their transplant window stretch toward whatever light they can get, producing thin, pale stems that flop over and never fully recover, even after they go outside and thicken up. A tomato that should be a stocky 6 inches at transplant time can end up a foot tall and floppy if started too early.

Rootbound plants are the second problem. A six-pack cell has a finite amount of soil, and roots that fill it and keep growing start circling. A rootbound transplant struggles to send roots outward into garden soil for weeks after transplanting, which erases whatever head start the early sowing was supposed to buy.

Transplant shock compounds both problems. A leggy, rootbound plant already has less reserve to draw on, and moving it outdoors, even carefully, sets it back further than a smaller, properly timed plant that was never stressed in the first place.

There is also a light problem specific to early starts in cold zones. If you are in zone 3 through 6 and you start seeds in January or early February, the sun is weak and the days are short at exactly the time your seedlings need the most light to stay compact. A bright windowsill in February in zone 5 delivers a fraction of the light intensity the same window gets in April, which is the single biggest reason indoor-started seedlings get leggy that far north. A proper grow light setup fixes this directly, and it matters more the earlier in the year and the further north you are starting. If you are setting up a dedicated space for the first time, what you actually need for seed starting covers the difference between what the marketing wants to sell you and what a seedling actually requires.

Overcrowding and damping off follow the same pattern of “started too early, sat too long.” A tray of seedlings kept indoors past its window gets potted up, or it does not, and either way the plants are packed together longer than intended, with less air movement around each stem. Damping off, the fungal collapse that kills seedlings at the soil line almost overnight, thrives in exactly those conditions: still air, damp soil, and crowded stems touching each other. A small fan running a few hours a day and sowing on schedule rather than early solves most of it without any fungicide.

Paper towel or soil

The paper towel method, sometimes called the baggie method, involves folding seed between damp paper towels inside a sealed bag and checking daily for a sprouted root. It has a real use: testing whether old seed is still viable before you commit a whole tray to it, and germinating slow, expensive, or fussy seed where you want to see results before waiting weeks in soil not knowing if anything is happening.

It is not, in general, a better way to start the seed you plan to grow on. Moving a sprouted seed from a paper towel into soil means handling a root that has never touched soil and can snap or dry out in the transfer, and most seed germinates perfectly well sown directly into a moist seed starting mix. Reserve paper towel germination for testing viability or for the handful of crops, parsley among them, that are slow enough that seeing a sprout before committing tray space is worth the extra step.

Germination temperature

Soil temperature, not air temperature, is what triggers germination, and the two are not the same number. A house kept at 68 degrees can have soil in an unheated tray sitting closer to 60, which is fine for lettuce and onions but too cold for tomatoes and peppers to germinate reliably. Warm-season crops want 75 to 85 degrees at the soil line, which in most houses means a seedling heat mat under the tray. Cool-season crops, lettuce, broccoli, kale and onions among them, germinate fine at 60 to 70 degrees and generally do not need one. A soil thermometer removes the guesswork, both indoors on a tray and later outdoors when you are deciding whether the garden soil is warm enough for direct sowing.

Counting backward: how to do this for any crop

Every date in the table above comes from one piece of arithmetic, and you can run it for any crop the table does not cover.

Start date = your last frost date, minus the crop’s recommended weeks, times 7 days.

Look up the number of weeks on the seed packet, which almost always says something like “start indoors 6 to 8 weeks before last frost.” Take your last frost date from the ZIP lookup or the frost dates by city page above, subtract that many weeks, and that is your sowing date. If the packet gives a range, use the longer lead time for slow, fussy germinators like peppers and parsley, and the shorter one for anything that germinates fast and grows quickly, like most brassicas.

The formula also works forward, which is the check worth doing once you have a start date: add the weeks back and confirm you land on a date reasonably close to your actual last frost, then add another week or two of hardening off before the plant actually goes in the ground. The hardening off schedule covers that last step, which is where a lot of otherwise well-timed seedlings get damaged right before transplant.

Here it is worked through for a crop not in the master table. Say your last frost is April 15, zone 6b, and you are growing celery, which most packets list as 10 to 12 weeks before last frost. Twelve weeks back from April 15 is January 22. That is an early, slow, fussy start, which matches celery’s reputation as one of the harder crops to raise from seed indoors, and it is exactly why the formula, not a memorized date, is the thing worth keeping.

Where to go next

• Hardening off seedlings: a 7 day schedule
• Last frost date: what to plant before it
• USDA planting zones explained
• Fall planting calendar by zone