Seed starting

When to start seeds indoors

Every seed-starting date counts back from one number: your average last spring frost. Put yours in below and the calendar gives each crop its start-indoors window and its plant-out date, from extension-service timing.

The short answer. Onions first, 10 to 12 weeks before the last frost. Peppers, lettuce and the cabbage family at 6 to 8 weeks. Tomatoes at 4 to 6. Squash, cucumbers and melons under 4 weeks, or straight into the ground once it is warm.

Your seed-starting calendar

Everything counts back from your average last spring frost. Pick your USDA zone for a typical date, or enter your own: a local date beats a zone average every time.

USDA hardiness zoneOr your own last frost dateYour county extension office or the National Weather Service publishes it.

Working out your dates…

Indoor-start weeks: Cornell Cooperative Extension, Rockland County planting guide (Hort 52). Planting-out weeks: Colorado State University GardenNotes #720. Best germination soil temperature: Alabama Cooperative Extension ANR-1061. Zone frost dates are typical, not local; a late cold snap overrides any calendar.

What seedlings need

A seed-starting mix, not garden soil. Commercial mixes without true soil are sterile, light and free of weed seeds.
Warmth at the roots. Most seeds started indoors germinate sooner and root better in warm mix, and the mix can run up to 5°F cooler than the room. Keep the air above 60°F. The calendar shows each crop's best soil temperature; a heat mat closes the gap for peppers and tomatoes.
Strong light, close. 12 to 16 hours a day, with the lights no more than 4 inches above the seedlings and as close as 2. Raise them as the plants grow.
Even moisture. Keep the mix moist through germination without letting it dry out, and drain standing water from the tray. Damping-off, seedlings dying at the soil line, comes from pathogens at the surface of the mix.

From seed to garden

Sow on the start date from the calendar, in moist mix, at the depth on the packet.
Once seedlings have true leaves, thin to one per cell by cutting the extras at the soil line rather than pulling them.
Hold off on fertilizer until there are several sets of true leaves.
Move plants that outgrow their cells into pots 1 to 2 inches wider.
Harden off over about two weeks: a few hours outside in afternoon shade, out of the wind, then longer and sunnier each day, inside before cold nights.
Plant out on the plant-out date, after the soil has warmed.

Method: University of Minnesota Extension, Starting seeds indoors (reviewed 2026).

Seeds and plants

Open-pollinated and heirloom seed can be saved and replanted, which is the point of most of the techniques in the almanac. Buy for your zone and your water, not the catalog photo.

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Questions

How many weeks before the last frost should I start tomato seeds?: Four to six weeks before your average last frost, per Cornell Cooperative Extension. Set the plants out about two weeks after the last frost, once the soil has warmed.
Which seeds are started earliest?: Onions, ten to twelve weeks before the last frost. Peppers, lettuce, cabbage, broccoli and kale follow at six to eight weeks. Squash, cucumbers and melons need less than four weeks, and many gardeners sow them straight into the ground instead.
How close should grow lights be to seedlings?: No more than 4 inches above the tops of the seedlings, and as close as 2 inches is ideal, for 12 to 16 hours a day (University of Minnesota Extension). Raise the lights as the plants grow.
Can I use garden soil to start seeds?: Extension services recommend a commercial seed-starting mix without true soil: it is sterile, light and free of weed seeds, which cuts the risk of damping-off.
What is hardening off?: Getting indoor seedlings used to sun, wind and cold before planting out. About two weeks before transplanting, set them outside for a few hours in afternoon shade, lengthen the time and the sun each day, and bring them in before cold nights.

When to start seeds indoors

What seedlings need

A seed-starting mix, not garden soil. Commercial mixes without true soil are sterile, light and free of weed seeds.

Warmth at the roots. Most seeds started indoors germinate sooner and root better in warm mix, and the mix can run up to 5°F cooler than the room. Keep the air above 60°F. The calendar shows each crop's best soil temperature; a heat mat closes the gap for peppers and tomatoes.

Strong light, close. 12 to 16 hours a day, with the lights no more than 4 inches above the seedlings and as close as 2. Raise them as the plants grow.

Even moisture. Keep the mix moist through germination without letting it dry out, and drain standing water from the tray. Damping-off, seedlings dying at the soil line, comes from pathogens at the surface of the mix.

From seed to garden

Sow on the start date from the calendar, in moist mix, at the depth on the packet.

Once seedlings have true leaves, thin to one per cell by cutting the extras at the soil line rather than pulling them.

Hold off on fertilizer until there are several sets of true leaves.

Move plants that outgrow their cells into pots 1 to 2 inches wider.

Harden off over about two weeks: a few hours outside in afternoon shade, out of the wind, then longer and sunnier each day, inside before cold nights.

Plant out on the plant-out date, after the soil has warmed.

Method: University of Minnesota Extension, Starting seeds indoors (reviewed 2026).

Questions

How many weeks before the last frost should I start tomato seeds?

Four to six weeks before your average last frost, per Cornell Cooperative Extension. Set the plants out about two weeks after the last frost, once the soil has warmed.

Which seeds are started earliest?

Onions, ten to twelve weeks before the last frost. Peppers, lettuce, cabbage, broccoli and kale follow at six to eight weeks. Squash, cucumbers and melons need less than four weeks, and many gardeners sow them straight into the ground instead.

How close should grow lights be to seedlings?

No more than 4 inches above the tops of the seedlings, and as close as 2 inches is ideal, for 12 to 16 hours a day (University of Minnesota Extension). Raise the lights as the plants grow.

Can I use garden soil to start seeds?

Extension services recommend a commercial seed-starting mix without true soil: it is sterile, light and free of weed seeds, which cuts the risk of damping-off.

What is hardening off?

Getting indoor seedlings used to sun, wind and cold before planting out. About two weeks before transplanting, set them outside for a few hours in afternoon shade, lengthen the time and the sun each day, and bring them in before cold nights.