Guide
Starting seeds indoors gives warm-season crops a head start so they mature and produce before fall frost arrives. But timing is everything: start too early and your seedlings become leggy and pot-bound before it's safe to plant out; start too late and you lose the head start entirely. The trick is to count backward from your last spring frost date.
Every crop has a recommended number of weeks it should spend indoors before transplanting. To find your indoor sowing date, take your last frost date and subtract that number of weeks. If your last frost is May 15 and tomatoes need six weeks indoors, you'd sow seeds around April 3. The frost date planner does this math automatically for 50+ vegetables, herbs, and flowers once you enter your zip code.
| Crop | Weeks indoors before last frost |
|---|---|
| Peppers, eggplant | 8–10 weeks |
| Tomatoes | 6–8 weeks |
| Broccoli, cabbage, kale | 4–6 weeks |
| Basil | 4–6 weeks |
| Cucumbers, squash, melons | 2–4 weeks |
Fast-growing vine crops like cucumbers and squash resent root disturbance, so they only need a couple of weeks indoors — or can be direct-sown after frost. Slow starters like peppers and eggplant benefit from the longest indoor lead time.
Say the planner gives you a last frost date of May 15. Counting back the recommended weeks turns the table above into actual sowing dates:
| Crop | Lead time | Sow indoors around |
|---|---|---|
| Peppers, eggplant | 8–10 weeks | Mar 6 – Mar 20 |
| Tomatoes | 6–8 weeks | Mar 20 – Apr 3 |
| Broccoli, cabbage, kale | 4–6 weeks | Apr 3 – Apr 17 |
| Basil | 4–6 weeks | Apr 3 – Apr 17 |
| Cucumbers, squash, melons | 2–4 weeks | Apr 17 – May 1 |
Notice how staggered the dates are: peppers go in more than two months before tomatoes and squash. Sowing everything on the same day is the single most common seed-starting mistake — the fast crops outgrow their pots while the slow ones are barely up.
Root crops such as carrots, beets, radishes, and turnips are direct-sown because transplanting disturbs the developing root. Beans, peas, and corn also germinate quickly outdoors and rarely benefit from an indoor start. Knowing which crops to sow directly saves you tray space for the plants that truly need the head start.
Warmth drives germination; light drives strong growth afterward. Iowa State University Extension notes that most seeds sprout when the soil temperature sits between 68° and 86°F, which is why a seedling heat mat speeds up warm-season crops on a cool windowsill. Once seedlings emerge, they need 12–16 hours of light a day, and a bright window rarely supplies enough — hang a fluorescent or LED grow light just 4–6 inches above the leaves and raise it as the plants stretch up. Interestingly, seedlings grow best at roughly 10°F cooler than their germination temperature, so once they're up you can ease off the heat. A gentle breeze from a fan for a few hours a day thickens stems and helps prevent the tall, floppy growth that plagues indoor starts.
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The most heartbreaking seedling loss is damping off — a fungal disease that topples healthy-looking sprouts at the soil line overnight. University of Minnesota Extension ties it to overwatering, poor air movement, and reused containers. To keep it away, start in fresh sterile seed-starting mix in clean seed-starting trays with a humidity dome, wash and sanitize any reused pots and trays, water from below and let the surface dry slightly between waterings, and run a small fan for air circulation. Remove any humidity dome as soon as the seeds germinate — trapped moisture is exactly what the fungus needs to take hold.
Seedlings raised indoors have never felt direct sun, wind, or temperature swings. Moving them straight outside can shock or kill them. Instead, harden off over 7–14 days, increasing exposure a little each day. A typical progression looks like this:
Water a little more often during this stretch — wind and sun dry small pots quickly — and skip putting seedlings out on the harshest days. Only after this transition, and after your frost risk has passed, should tender crops go into the ground. Transplant on a cloudy afternoon or in the evening rather than under blazing midday sun, and water them in right away to settle the roots.
Most seed-starting mixes contain little or no nutrition, so once seedlings develop their first set of true leaves, begin a weak dose of liquid fertilizer — roughly half or quarter strength — every week or two. If a plant outgrows its cell and roots begin circling the bottom before it's time to plant out, pot it up into a larger container so growth never stalls. Peppers and tomatoes in particular reward a mid-season bump into a bigger pot with stronger, better-rooted transplants.
Enter your zip code in the frost date planner to get exact indoor-sowing and transplant dates for every crop in your garden.