How do I find my last frost date?

Your average last spring frost date is the anchor for all indoor seed timing. Find it by entering your ZIP code at your regional Cooperative Extension site or the NOAA freeze/frost data, which give a date with a probability attached — the commonly used figure is the date with a 50 percent chance of a later frost. Note it is an average, not a guarantee: roughly half of years see a frost after it. That is why tender transplants are hardened off and set out a week or two past the date, not exactly on it.

Is it possible to start seeds too early?

Yes, and starting too early is more common and more damaging than starting late. Seedlings held indoors past their transplant window become root-bound, leggy and stressed, and they often transplant poorly and get overtaken by later, healthier starts. Fast growers like squash and cucumbers are the worst for this — started more than 3 to 4 weeks before transplanting they outgrow their cells almost immediately. Always count back from your frost date using the weeks on the packet rather than starting everything in January out of enthusiasm.

What vegetables should not be started indoors?

Root crops and fast direct-sowers generally resent transplanting and do better sown straight into the garden: carrots, beets, radishes, turnips and parsnips fork or stall when their taproots are disturbed, and beans, peas, corn and squash grow so fast that direct sowing catches up to indoor starts with none of the transplant shock. Start indoors the crops that need a long warm season your climate cannot fully provide — tomatoes, peppers, eggplant — plus slow growers like onions, leeks and many herbs and flowers.

Do I need grow lights to start seeds indoors?

For strong, stocky seedlings, yes — a sunny windowsill almost never provides enough light in late winter, and the result is pale, stretched, floppy plants that flop over. An inexpensive shop-style LED light held 2 to 4 inches above the seedlings for 14 to 16 hours a day produces far better transplants. A south-facing window can work for a few easy, short-season starts but struggles with tomatoes and peppers begun in February. See our guidance on whether you need grow lights for the full picture.

How long can seedlings stay indoors before planting?

Only as long as the packet timing intends — the whole point of counting back from the frost date is that seedlings reach transplant size right as the weather is ready. Held longer, they become root-bound and set back. If bad weather forces a delay of a week or two, pot seedlings up into larger containers to give the roots room and keep them from stalling, and keep pinching or trimming tall ones. Do not simply leave them in their original small cells for extra weeks.

When to Start Seeds Indoors: Count Back From Your Frost Date

Written & researched by Elena Marsh Gardening & Landscaping Editor

Updated May 2026

Low-angle view of a person harvesting ripe tomatoes into a basket in a backyard vegetable garden, in soft early-morning light

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When to start seeds indoors: count backward from your average last spring frost date by the number of weeks printed on the seed packet. Most warm-season crops are started 6 to 8 weeks before that date — so if your last frost is May 15, you sow tomatoes and peppers indoors in mid-to-late March. Slow growers like onions and leeks go earlier (10 to 12 weeks); fast growers like cucumbers and squash go later (3 to 4 weeks). The frost date is the anchor; the packet gives the offset.

Almost every indoor-sowing mistake is a timing mistake, and the two most common are starting everything far too early and ignoring the packet. The method is genuinely simple once you have your frost date and a calendar. We do not run trial sowings ourselves; this reflects extension-service guidance and aggregated grower reviews, as described in how we rank.

The counting-back method

There are only three steps, and they work for any crop.

Find your last frost date. Enter your ZIP code at your regional Cooperative Extension or the NOAA freeze data. This is the date with roughly a 50 percent chance of a later frost — an average, not a promise.
Read the weeks on the packet. Most seed packets say something like start indoors 6 to 8 weeks before last frost. That number is the crop-specific offset.
Count back on a calendar. Subtract those weeks from your frost date to get your sowing date. Do this for each crop separately, because the offsets differ widely.

For the region-specific spin on this, including how zone shifts the whole schedule, see when to start seeds by zone.

Overhead view of a person harvesting ripe tomatoes into a basket in a backyard vegetable garden, in warm late-afternoon light

Crop-by-crop timing chart

These are the standard offsets. Adjust them to your own packets, which reflect your specific varieties.

Crop	Weeks before last frost	Transplant timing
Onions, leeks	10 to 12	Around last frost (cold-tolerant)
Celery, parsley	10 to 12	After last frost
Peppers, eggplant	8 to 10	1 to 2 weeks after last frost (warm soil)
Tomatoes	6 to 8	1 to 2 weeks after last frost
Broccoli, cabbage, kale	4 to 6	2 to 4 weeks before last frost (hardy)
Basil	4 to 6	After last frost (frost-tender)
Lettuce	4 to 5	2 to 3 weeks before last frost
Cucumbers, squash, melons	3 to 4	After last frost (fast; hate root disturbance)

Notice the transplant column: cold-hardy brassicas and onions go out before the frost date, while tomatoes, peppers, basil and the cucurbits wait until after it. That difference is why you cannot sow everything on the same day.

Why starting too early backfires

Enthusiasm in January is the enemy. A seedling that outgrows its cell before the weather is ready becomes root-bound, leggy and stressed — and a stressed, pot-bound transplant frequently underperforms a smaller, younger one set out at the right time. Fast growers punish early sowing worst: squash and cucumbers started 6 weeks out are a tangled mess by transplant day. The packet weeks exist precisely so the plant hits transplant size as the ground warms. If in doubt, sow a little later rather than earlier.

Wide view of a person harvesting ripe tomatoes into a basket in a backyard vegetable garden, in bright midday sun

What not to start indoors

Not everything belongs in a seed tray. Root crops — carrots, beets, radishes, turnips, parsnips — have taproots that fork or stall when disturbed, so they are direct-sown; see best vegetables to direct sow and the trade-offs in direct sow vs transplant. Beans, peas and corn grow so fast that direct sowing catches up with none of the transplant shock. Reserve indoor sowing for crops that need a longer warm season than your climate gives — tomatoes, peppers, eggplant — and slow starters like onions and leeks.

Give them enough light

Timing right but lighting wrong still produces poor transplants. A late-winter windowsill almost never delivers enough light, and the result is pale, stretched seedlings that flop. Position an inexpensive LED shop light 2 to 4 inches above the seedlings and run it 14 to 16 hours a day, raising it as they grow. This is the fix for the classic leggy-seedling problem — more on that in the leggy seedlings fix and whether you need grow lights.

Over-the-shoulder view of a person harvesting ripe tomatoes into a basket in a backyard vegetable garden, in soft overcast light

The rest of the setup

Alongside timing, three things make or break indoor starts: a sterile, fine seed-starting mix (not garden soil or potting soil, which are too coarse and may carry disease), gentle bottom warmth around 70 to 75F for warm-season crops, and consistent moisture without waterlogging. A humidity dome speeds germination; remove it the moment sprouts appear. Kit and mix options are covered in best seed starting kit and best seed starting mix. The step-by-step sowing method is in how to start seeds indoors.

A worked example

Say your average last frost date is May 15, a common date for zone 5 and 6 gardens. Here is how the counting-back method turns that single date into a sowing calendar. Onions and leeks at 10 to 12 weeks go in around late February to early March. Peppers and eggplant at 8 to 10 weeks are sown in mid-March. Tomatoes at 6 to 8 weeks are sown in late March to early April. Broccoli and cabbage at 4 to 6 weeks go in mid-April, and can transplant out in late April, two to three weeks before the frost date, because they tolerate cold. Basil at 4 to 6 weeks is sown in early April but held indoors until after May 15, since it is frost-tender. Cucumbers and squash at 3 to 4 weeks are sown around April 20 and also wait until after the frost date to go out. Notice how the sowing dates spread across nearly two months and the transplant dates split into before-frost and after-frost groups. Writing these out on a calendar before you buy a single seed packet is the difference between orderly, well-timed transplants and a windowsill of leggy plants that all outgrew their cells in March.

Before they go outside

Seedlings raised in stable indoor conditions cannot go straight into sun and wind. Over 7 to 10 days before transplanting, harden them off by gradually increasing their outdoor exposure, or the leaves scorch and the plants stall — the method is in hardening off seedlings and timing cues in when to move seedlings outside. Build the whole plan back from your frost date and the timing takes care of itself. More at the seeds and vegetable growing hub.

When to Start Seeds Indoors