How the seed starting calculator works
Every crop has a head start it needs indoors, measured in weeks before your average last spring frost. Peppers need about 8 to 10 weeks, tomatoes 6 to 8, cucumbers just 3 to 4. The calculator takes your last frost date and counts back for each of the 31 crops in our list that are worth starting indoors, then sorts them by start date so the chart reads like a schedule.
Your frost date comes from the NOAA U.S. Climate Normals for 1991–2020, using the nearest of 7,305 weather stations to the center of your ZIP code. If you already know your own date, from years of notes or a local garden club, enter it instead. You can also switch to the safe (10%) date, which has only a 1-in-10 chance of a later frost; see the frost dates page for what the two dates mean.
For warm-season crops the calculator checks one more thing: when your normal daily average temperature reaches the soil warmth each crop needs, 55 to 65°F depending on the crop. Where summers start late, as in much of the Pacific Northwest, that is weeks after the last frost, so the start date moves later too. There is no point in raising a tomato seedling that has to wait a month for warm soil.
Seed starting chart
The standard timing for each crop, before it is matched to your dates. Weeks are counted from the average last spring frost.
| Crop | Start indoors | Plant out |
|---|---|---|
| Celery | 12–10 weeks before | Frost date to 2 weeks after |
| Leek | 12–10 weeks before | 2 weeks before to frost date |
| Bell pepper | 10–8 weeks before | 2–4 weeks after |
| Eggplant | 10–8 weeks before | 2–4 weeks after |
| Hot pepper | 10–8 weeks before | 2–4 weeks after |
| Onion | 10–8 weeks before | 4–2 weeks before |
| Oregano | 10–8 weeks before | Frost date to 2 weeks after |
| Parsley | 10–8 weeks before | 2 weeks before to frost date |
| Thyme | 10–8 weeks before | Frost date to 2 weeks after |
| Broccoli | 8–6 weeks before | 4–2 weeks before |
| Brussels sprouts | 8–6 weeks before | 4–2 weeks before |
| Cabbage | 8–6 weeks before | 4–2 weeks before |
| Cauliflower | 8–6 weeks before | 2 weeks before to frost date |
| Cherry tomato | 8–6 weeks before | 1–3 weeks after |
| Chives | 8–6 weeks before | 2 weeks before to frost date |
| Kale | 8–6 weeks before | 4–2 weeks before |
| Marigold | 8–6 weeks before | 1–2 weeks after |
| Sage | 8–6 weeks before | Frost date to 2 weeks after |
| Tomato | 8–6 weeks before | 1–3 weeks after |
| Basil | 6–4 weeks before | 1–2 weeks after |
| Bok choy | 6–4 weeks before | 2 weeks before to frost date |
| Kohlrabi | 6–4 weeks before | 4–2 weeks before |
| Lettuce | 6–4 weeks before | 2 weeks before to 1 week after |
| Red leaf lettuce | 6–4 weeks before | 2 weeks before to 1 week after |
| Cantaloupe | 4–3 weeks before | 2–4 weeks after |
| Cucumber | 4–3 weeks before | 1–3 weeks after |
| Swiss chard | 4–3 weeks before | 1 week before to 1 week after |
| Watermelon | 4–3 weeks before | 2–4 weeks after |
| Winter squash | 4–3 weeks before | 1–3 weeks after |
| Yellow squash | 4–3 weeks before | 1–3 weeks after |
| Zucchini | 4–3 weeks before | 1–3 weeks after |
What you need to start seeds indoors
- Containers with drainage: cell trays, small pots or recycled containers with holes punched in the bottom.
- Seed-starting mix: a light, fine, sterile mix, not garden soil, which packs down and can carry the fungi that cause damping-off.
- Light: a shop light or LED grow light on a timer, 14 to 16 hours a day, kept a few inches above the seedlings and raised as they grow. A windowsill alone usually gives thin, leaning seedlings.
- Warmth for germination: peppers, eggplant and tomatoes sprout slowly in a cool room. A seedling heat mat speeds them up; take the tray off once the seeds come up.
- Air and water: water from below or with a gentle spray, let the surface dry slightly between waterings, and run a small fan nearby for sturdier stems and fewer fungal problems.
From seed to garden in five steps
- Sow on the start date, two or three seeds per cell, about two to three times as deep as the seed is wide.
- Thin to the strongest seedling per cell once the first true leaves appear, by snipping the others at soil level.
- Feed and pot up. Once seedlings have a few true leaves, water with a diluted fertilizer. Move tomatoes and peppers to bigger pots if they outgrow their cells before planting time.
- Harden off over seven to ten days before the plant-out date: a few hours outdoors in shade at first, then more sun and longer spells, until they stay out overnight.
- Plant out in the plant-out window, ideally on a cloudy day or in the evening, and water well.
Crops to sow straight into the garden
Not everything belongs under lights. potatoes, pumpkins, radishes, turnips, arugula, bush beans, pole beans, peas, green onions, carrots, parsnips, cilantro, dill, beets, spinach, sunflowers, calendula, zinnias, sweet corn and nasturtiums all do better sown directly where they will grow. Root crops dislike having their roots disturbed, and big-seeded crops such as beans, corn and squash catch up quickly from seed in warm soil. Their outdoor sowing dates are in the planting calendar.
Cucumbers, squash and melons sit on the border. Start them indoors only three to four weeks ahead, in individual pots so roots are not disturbed, if your season is short; otherwise sow them outdoors.
Common seed starting mistakes
- Starting too early. Big, leggy seedlings waiting for warm weather are harder to grow on than young, stocky ones. Stick to the dates.
- Too little light. Pale, stretched stems mean the light is too far away or on too few hours.
- Overwatering. Soggy mix invites damping-off, where seedlings collapse at the soil line.
- Skipping hardening off. Seedlings moved straight from a windowsill to full sun can scorch white in a day.
- Planting heat lovers too soon. Tomatoes, peppers and basil set out into cold soil sit still for weeks. The plant-out dates already allow for this.
Once your seedlings have a home date, map out where they will go in the garden planner, and check good neighbors in the companion planting chart. Each crop’s guide, such as peppers or onions, has more on growing it.