Guide — updated 2026-07

When to Start Seeds Indoors

Wooden seed tray of young seedlings at a window with clay pots and a dibber
Plate II — seedlings under glass

Every indoor sowing date is a count-back from your last frost. Slow crops need the most lead time: celery, onions and leeks at 10–12 weeks, peppers and eggplant at 8, tomatoes at 6–8, brassicas at 5–6, and cucumbers and squash at just 3–4. Find your last frost date, subtract the weeks, and that's your sowing date — no calendar month applies everywhere.

This is the part most guides get vague about, so here it is plainly: the month you should sow is a function of where you live, and the same crop can be a January sowing in Florida and an April sowing in Minnesota. The only number that transfers between gardens is the number of weeks before frost.

Look up your last frost date →

How many weeks before last frost, by crop

Dates in the last column are worked for zone 6b (last frost around April 10) as an illustration — get your own dates by ZIP code.

Indoor sowing lead times — 35 crops, longest lead first
Crop Weeks before last frost Germination soil temp Zone 6b sowing window
Celery 10–12 weeks 60–70°F Jan 16 – Jan 30
Leek 8–12 weeks 65–85°F Jan 16 – Feb 13
Onion 10–12 weeks 60–85°F Jan 16 – Jan 30
Rosemary 10–12 weeks 65–70°F Jan 16 – Jan 30
Kale 8–11 weeks 60–85°F Jan 23 – Feb 13
Broccoli 7–10 weeks 60–85°F Jan 30 – Feb 20
Cabbage 7–10 weeks 60–85°F Jan 30 – Feb 20
Pepper (sweet/bell) 6–10 weeks 70–90°F Jan 30 – Feb 27
Chives 8–10 weeks 65–70°F Jan 30 – Feb 13
Common Thyme 8–10 weeks 65–70°F Jan 30 – Feb 13
Greek Oregano 8–10 weeks 65–70°F Jan 30 – Feb 13
Cauliflower 6–9 weeks 65–85°F Feb 6 – Feb 27
Eggplant 6–8 weeks 75–90°F Feb 13 – Feb 27
Sweet Potato 6–8 weeks 65–70°F Feb 13 – Feb 27
Basil 6–8 weeks 70–85°F Feb 13 – Feb 27
Garden Sage 6–8 weeks 65–70°F Feb 13 – Feb 27
Parsley 6–8 weeks 65–70°F Feb 13 – Feb 27
Spearmint 6–8 weeks 72–75°F Feb 13 – Feb 27
Calendula / Pot Marigold 6–8 weeks 60–70°F Feb 13 – Feb 27
French Marigold 4–8 weeks 70–80°F Feb 13 – Mar 13
Sweet Alyssum 6–8 weeks 65–70°F Feb 13 – Feb 27
Tomato 5–7 weeks 65–85°F Feb 20 – Mar 6
Bok Choy 4–6 weeks 50–70°F Feb 27 – Mar 13
Collard Greens 4–6 weeks 50–70°F Feb 27 – Mar 13
Kohlrabi 4–6 weeks 50–75°F Feb 27 – Mar 13
Lettuce 4–6 weeks 45–80°F Feb 27 – Mar 13
Spinach 5–6 weeks 45–68°F Feb 27 – Mar 6
Swiss Chard 4–6 weeks 55–75°F Feb 27 – Mar 13
Watermelon 2–5 weeks 70–95°F Mar 6 – Mar 27
Cucumber 2–4 weeks 70–90°F Mar 13 – Mar 27
Pumpkin 3–4 weeks 70–90°F Mar 13 – Mar 20
Nasturtium 3–4 weeks 60–65°F Mar 13 – Mar 20
Sunflower 2–3 weeks 70–75°F Mar 20 – Mar 27
Winter Squash (Butternut) 1–2 weeks 65–75°F Mar 27 – Apr 3
Cantaloupe (Muskmelon) 1–2 weeks 65–75°F Mar 27 – Apr 3

Crops to sow direct instead

Some crops are actively harmed by transplanting. Root vegetables fork or stall when their taproot is disturbed, and large-seeded crops germinate so fast outdoors that starting them inside buys you nothing.

ArugulaBeetBush beanCarrotOkraParsnipPeaPole beanPotatoRadishSummer squash / zucchiniSweet cornTurnipBorageCilantro / CorianderDill

The five mistakes that cost a season

  1. Starting too early. Enthusiasm in February produces a leggy tomato in a 4-inch pot by April with nowhere to go. A seedling started at the right time overtakes an overgrown one within a fortnight of planting out.
  2. Not enough light. A windowsill is not enough light for a vegetable seedling. If your seedlings are stretching, leaning, or pale, that's the diagnosis. Hang an LED shop light 2–4 inches above the leaves and raise it as they grow.
  3. Sowing into garden soil or potting compost. Use a fine seed-starting mix. Garden soil compacts in a tray, drains badly, and brings in damping-off fungus that will flatten a tray of seedlings overnight.
  4. Skipping hardening off. Seedlings raised indoors have no wind conditioning and no UV tolerance. Move them outside for progressively longer spells over 7–10 days before planting; skip it and you can lose the lot to one bright, breezy afternoon.
  5. Planting out on the last frost date. That date is a median — half of years the last freeze comes later. Wait one to two weeks past it for tender crops, or keep row cover to hand.

Soil temperature, not air temperature. Seeds germinate on the temperature of the medium they're in. A 68°F room can hold a seed tray at 62°F, which is enough to make peppers sit dormant for three weeks. A cheap soil thermometer removes all the guesswork here.

Next

Once your seedlings are ready, work out how many will fit the bed, check what should sit next to what, and lay the bed out square by square.

What seed starting actually needs

Requisition

Four of the five mistakes on this page are solved by equipment rather than technique. These are the ones that matter.

LED grow light (shop-light style)

The single biggest cause of leggy, flopping seedlings. A south-facing window gives a fraction of the light a seedling needs — hang this 2–4 inches above the leaves for 14–16 hours a day and the problem disappears.

Buy on coverage, not wattage claims. A 4 ft fixture spans two standard trays.

Check it on Amazon →

Seedling heat mat

Peppers and tomatoes germinate on soil temperature of roughly 75–85°F, which is above most room temperatures. Without a mat they sit dormant for weeks and gardeners assume the seed was dead.

Get one with a thermostat probe. Unregulated mats can cook a tray.

Check it on Amazon →

Soil thermometer

This site tells you to sow on soil temperature rather than air temperature or calendar date. A cheap probe is what makes that advice actionable — a 68°F room can hold a tray at 62°F.

Cheapest genuinely useful tool in the seed-starting list.

Check it on Amazon →

Seed starting mix

Garden soil compacts in a tray, drains badly and carries damping-off fungus that can flatten a whole tray overnight. A fine sterile mix is not the same product as potting compost.

Wet it before filling cells — dry peat-based mixes repel water.

Check it on Amazon →

Cell trays with humidity dome

Consistent cell size means consistent watering, which is most of seed-starting success. The dome comes off the moment seedlings emerge — leaving it on is a fast route to damping-off.

Heavier-gauge trays last years; the thin ones crack in a season.

Check it on Amazon →

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Common questions

When should I start seeds indoors?
It depends entirely on the crop and on your last frost date. Slow starters like celery, onions and leeks want 10–12 weeks; peppers and eggplant 8; tomatoes 6–8; brassicas 5–6; and cucumbers and squash only 3–4. Count back that many weeks from your local last frost date to get your sowing date. The chart on this page lists all 35 crops we hold data for.
What seeds should not be started indoors?
Root crops and most large-seeded vegetables resent transplanting: arugula, beet, bush bean, carrot, okra, parsnip, pea are all better sown straight into the ground. Carrots and parsnips in particular fork permanently if their taproot is disturbed.
What happens if I start seeds too early?
You get leggy, root-bound transplants that sulk for weeks after planting out — often finishing behind seed sown directly at the right time. Starting too early is the single most common seed-starting mistake, and it is caused by counting from the wrong frost date or forgetting that your seedlings cannot go out until the soil is warm enough for them.
Do I need grow lights?
In almost every case yes. A south-facing window supplies a fraction of the light a seedling needs, which is why windowsill seedlings stretch toward the glass and flop. A basic LED shop light hung 2–4 inches above the leaves and run 14–16 hours a day solves it. The light needs to be that close: intensity falls off sharply with distance.
What temperature do seeds need to germinate?
Warm-season crops germinate on soil temperature, not air temperature — peppers and tomatoes want roughly 75–85°F, which is above most room temperatures, so a heat mat noticeably speeds them up. Cool-season crops like lettuce and brassicas germinate happily at 55–70°F and can actually fail in heat. Once seedlings emerge, drop the temperature: cooler growing conditions produce stockier plants.