Starting seeds indoors gives slow-growing plants a head start of several weeks, so they're ready to go into the garden as soon as it's warm enough instead of beginning from scratch then. It's also much cheaper than buying a flat of seedlings, and it lets you grow varieties you'll never find at a garden center.
The part that trips most beginners up isn't the growing. It's the timing. Start too late and you've lost the head start. Start too early and you end up with tall, floppy seedlings crammed into tiny pots, waiting weeks for the weather to catch up.
Step 1: Find your last frost date
Your average last spring frost date is the anchor for everything here. It's the point after which a frost becomes unlikely, and it varies a lot more than most people expect. Two towns in the same hardiness zone can be weeks apart, because zones only measure how cold winter gets, not when spring arrives. (More on that in Hardiness Zones vs. Frost Dates.)
You can look it up from a local extension office or a weather-history site. Write it down. Every date below is counted from it.
Step 2: Count back for each crop
Each crop needs a different amount of time indoors, and goes outside at a different point relative to that frost date. Cool-weather crops can go out a little before it; heat lovers wait until after. Here's the timing for common crops:
| Crop | Start indoors | Move outside |
|---|---|---|
| Pansy | 12 weeks before | 3 weeks before |
| Onions | 11 weeks before | 3 weeks before |
| Celery | 10 weeks before | 1 week after |
| Leeks, parsley | 10 weeks before | 2 weeks before |
| Chives | 8 weeks before | 2 weeks before |
| Peppers, eggplant | 8 weeks before | 2 weeks after |
| Kale, collards, kohlrabi | 7 weeks before | 2 weeks before |
| Lettuce, bok choy | 6 weeks before | 2 weeks before |
| Broccoli, cabbage, cauliflower | 6 weeks before | 1 week before |
| Brussels sprouts | 6 weeks before | Around your frost date |
| Tomatoes, basil, marigold | 6 weeks before | 1 week after |
| Chard | 5 weeks before | 1 week before |
| Cosmos | 5 weeks before | 1 week after |
| Zinnia | 4 weeks before | 1 week after |
| Cucumbers, squash, zucchini | 3 weeks before | 1 week after |
| Watermelon, cantaloupe | 3 weeks before | 2 weeks after |
These are typical starting points, not hard rules. Check the seed packet too, since some varieties are faster or slower than average.
A worked example
Say your last frost is around April 15. Counting back:
- Onions: start indoors around January 28, plant out around March 25.
- Peppers: start around February 18, plant out around April 29.
- Tomatoes: start around March 4, plant out around April 22.
- Lettuce: start around March 4, plant out around April 1.
- Cucumbers: start around March 25, plant out around April 22.
Notice how spread out that is. Seed starting isn't one weekend in March; it's a rolling list of small jobs from late winter into spring.
What not to start indoors
Some crops are better sown straight into the garden. Root crops like carrots, beets, radishes, and turnips don't like their roots disturbed, so transplanting them gives you stunted or forked roots. Beans, peas, and corn grow so quickly from seed that starting them indoors doesn't buy you much. Sunflowers and nasturtiums are happiest sown where they'll grow, too.
What you need
You don't need a greenhouse. A shelf or table near an outlet is enough:
- Seed-starting mix: light and fine, so tiny roots can push through. Not garden soil or regular potting soil.
- A seed tray with a clear lid: cells keep each seedling separate, and the lid holds in moisture until they sprout.
- A heat mat: gentle warmth from below. Most seeds sprout days sooner, peppers especially.
- A grow light: a sunny window usually isn't bright enough, and seedlings stretch toward it and flop.
- A plug-in timer: runs the light 14 to 16 hours a day so you never have to remember.
How to start them
- Sow. Dampen the mix, fill the cells, and sow 2 to 3 seeds per cell about twice as deep as the seed is wide. Tiny seeds like lettuce just get pressed onto the surface. Label everything; they all look the same once they're up.
- Heat, then light. Lid on, tray on the heat mat. As soon as most seeds have sprouted, take the lid off, turn the heat mat off, and turn the grow light on, keeping it 2 to 3 inches above the leaves.
- Water from below. Pour water into the tray and let the mix soak it up for 20 minutes, then pour off the rest. Keep it moist, never soggy.
- Thin. Once seedlings have their second set of leaves, snip all but the strongest one in each cell at soil level.
Hardening off: the step everyone skips
Seedlings raised indoors have never felt real sun, wind, or a cold night. Move them straight into the garden and many will scorch, wilt, or stall for weeks. Hardening off fixes that by toughening them up gradually over about a week before transplanting:
- Days 1–2: an hour or two outside in shade, out of the wind.
- Days 3–4: half the day, with some gentle morning sun.
- Days 5–6: most of the day, including some afternoon sun.
- Day 7: all day, and overnight if the night is mild.
Bring them in if frost or strong wind is forecast. Then transplant on a cloudy day or in the evening, at the same depth they sat in their cells, and water well.
Every seed-starting date, worked out for you
GardenWise finds the real frost history for your exact location and gives you the start, harden-off, and transplant dates for every crop in your garden, with a reminder when each one's due.
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