Why starting seeds indoors matters
In most climates, the outdoor growing season is too short for warm-season crops like tomatoes, peppers, and eggplant to go from seed to harvest if started directly in the ground. Starting them indoors gives you a head start — typically 6 to 12 weeks — so by the time the risk of frost is over, you're transplanting a plant that's already well established.
Indoor seed starting also gives you access to a much wider range of varieties than what's available as transplants at nurseries. And it costs a fraction of the price of buying plants.
The single most important number you need: your last frost date. Every seed starting schedule is calculated backward from this date. Without it, any timing guide is just a guess. Find yours from your local agricultural extension service, or automatically via Niwa when you enter your location.
How to calculate your seed starting dates
The process is straightforward. Every crop has a recommended number of weeks to start indoors before transplanting outside. You count backward from your last frost date by that many weeks to get your indoor start date.
For example: if your last frost date is April 30th, and tomatoes need to be started 6–8 weeks before transplanting (which happens after last frost), you'd start tomato seeds in early to mid-March.
Different crops also have different relationships with frost. Some (like lettuce and spinach) can go outside before the last frost date — they tolerate light frost. Others (like tomatoes and basil) must wait until all frost risk is gone and soil temperatures have warmed. The chart below accounts for all of this.
The master seed starting chart
All times are given in weeks before your last spring frost date (LFD). Negative numbers mean the plant can go outside before last frost — positive numbers mean after.
| Crop | Start indoors | Transplant outside | Direct sow? |
|---|---|---|---|
| Onion / Leek | 10–14 weeks before LFD | 4–6 weeks before LFD | No (slow growing) |
| Celery / Celeriac | 10–12 weeks before LFD | 2–4 weeks before LFD | No |
| Artichoke | 10–12 weeks before LFD | At LFD | No |
| Pepper | 8–10 weeks before LFD | 2–4 weeks after LFD | No |
| Eggplant | 8–10 weeks before LFD | 2–4 weeks after LFD | No |
| Tomato | 6–8 weeks before LFD | 1–2 weeks after LFD | No |
| Broccoli / Cauliflower | 6–8 weeks before LFD | 2–4 weeks before LFD | Possible (fall crop) |
| Cabbage | 6–8 weeks before LFD | 3–4 weeks before LFD | Possible (fall crop) |
| Kale | 5–7 weeks before LFD | 3–4 weeks before LFD | Yes |
| Lettuce | 4–6 weeks before LFD | 2–4 weeks before LFD | Yes (preferred) |
| Swiss chard | 4–6 weeks before LFD | 2–3 weeks before LFD | Yes |
| Basil | 4–6 weeks before LFD | 2 weeks after LFD | Yes (after LFD) |
| Cucumber | 3–4 weeks before LFD | At or 1 week after LFD | Yes (after LFD) |
| Zucchini / Squash | 3–4 weeks before LFD | At or 1 week after LFD | Yes (after LFD) |
| Melon | 3–4 weeks before LFD | 2 weeks after LFD | Yes (warm climates) |
| Pumpkin | 3–4 weeks before LFD | 1–2 weeks after LFD | Yes (after LFD) |
| Spinach | 4–6 weeks before LFD | 4–6 weeks before LFD | Yes (preferred) |
| Peas | Not recommended | 4–6 weeks before LFD | Yes (preferred) |
| Beans | Not recommended | At LFD | Yes (preferred) |
| Carrots | Not recommended | 3–4 weeks before LFD | Yes (only option) |
| Beets | Not recommended | 4 weeks before LFD | Yes (preferred) |
| Radishes | Not recommended | 4–6 weeks before LFD | Yes (only option) |
| Corn | Not recommended | At LFD | Yes (preferred) |
LFD = Last Frost Date. "Not recommended" for indoor starting means the crop prefers direct sowing — it either grows fast enough or dislikes root disturbance from transplanting.
Month-by-month schedule
This schedule assumes a last frost date of April 30th — the average for USDA Zones 5–6, which covers much of the US Midwest and Mid-Atlantic. Adjust earlier or later based on your own last frost date.
January is for onions, leeks, and artichokes — crops that need a very long lead time to reach transplant size. Most gardeners should resist starting tomatoes or peppers now; they'll be massively overgrown before it's warm enough outside. If your last frost is in June (northern climates, Zone 4 and below), January is when peppers and onions should start.
For most gardeners with a late April last frost date, February is when peppers and eggplant go under lights. These are slow-growing crops that need 10–12 weeks to reach a good size. Also the right time for celery and celeriac, which are notoriously slow. Onions started in January should be thinned and fertilized lightly.
March is peak indoor seed starting month for most of North America and Northern Europe. Start tomatoes (6–8 weeks before last frost), broccoli, cauliflower, cabbage, and kale. In late March, start your first lettuce flats — these can go outside under a cloche or cold frame well before last frost. Basil can start in late March for a late April last frost.
Start cucumbers, zucchini, squash, melons, and pumpkins in early April (3–4 weeks before last frost). These are fast-growing crops that get leggy if started too early. Begin hardening off your brassicas and kale — move them outside for increasingly long periods each day. Cold-hardy seedlings started in March should be going outside to harden off in the final 7–10 days before transplanting.
Once frost risk is past and soil has warmed above 60°F (15°C), transplant tomatoes, peppers, eggplant, basil, and cucurbits outdoors. Direct sow beans, corn, and any remaining root vegetables. Continue succession sowing lettuce, radishes, and spinach every 2–3 weeks for continuous harvest through summer.
Crops that should NOT be started indoors
Some vegetables strongly prefer direct sowing. Starting them indoors and transplanting is either unnecessary or actively harmful because they dislike root disturbance.
| Crop | Why direct sow | When to sow outside |
|---|---|---|
| Carrot | Taproot is disturbed by transplanting | 3–4 weeks before last frost |
| Radish | Matures in 25–30 days, no head start needed | 4–6 weeks before last frost |
| Beet | Direct sown beets outperform transplanted ones | 4 weeks before last frost |
| Pea | Prefers cool soil; hates root disturbance | As soon as soil can be worked |
| Bean | Germinates fast; transplanting not worth it | At last frost date |
| Corn | Grows quickly; needs to be in blocks for pollination | At last frost date |
| Spinach | Germinates well in cool soil; easy direct sow | 4–6 weeks before last frost |
| Dill | Long taproot, dislikes transplanting | At or just after last frost |
Common seed starting mistakes
Starting tomatoes too early
This is the most common mistake. Tomatoes started 10–12 weeks before the last frost date will be root-bound, leggy, and stressed by the time it's warm enough to plant them out. 6–8 weeks is the right window for most varieties. For large-fruited heirloom varieties, 7–8 weeks is optimal.
Not enough light
A windowsill is rarely enough for healthy seedlings. Seedlings need 14–16 hours of direct light per day, which only a south-facing window in a sunny climate can provide reliably. For most gardeners, a simple LED grow light positioned 2–4 inches above the seedlings is essential. Leggy, stretched seedlings are almost always a sign of insufficient light, not poor seeds.
Overwatering
Seedling roots need oxygen as well as moisture. Consistently wet soil — with no chance to dry out slightly between waterings — leads to damping off, a fungal disease that collapses seedlings at the soil level. Water when the top inch of the growing medium feels dry. Use a mister for the first week after germination, then water normally.
Skipping hardening off
Seedlings grown indoors are not adapted to wind, direct sunlight, or temperature fluctuations. Moving them directly from the grow room to the garden on transplant day will stress or kill them. Harden off over 7–14 days: start with 1–2 hours outside in a sheltered, shaded spot, and gradually increase exposure to direct sun and wind each day until they're outside all day.
Using garden soil in seed trays
Garden soil and potting mix compact badly in small cells, preventing root development and good drainage. Use a dedicated seed starting mix — lightweight, fine-textured, and sterile. It doesn't need nutrients at the seed stage; start feeding only after the first true leaves appear.
Never guess your planting dates again: Niwa calculates your exact seed starting and transplant dates for every plant in its library based on your local frost date. Enter your location once, and the planting calendar updates automatically for everything in your garden. Download free →
How to track your seed starting schedule
The easiest system is a physical notebook or a calendar where you write each crop's start date, expected transplant date, and what variety you're growing. Make notes each year about which varieties performed well and whether your timing was right — after two or three seasons, you'll have a personalized guide that's more accurate than any generic chart.
Alternatively, a garden planning app that connects your crop list to a frost-date-based calendar does this automatically. Niwa does exactly this: add a crop to your garden, enter your location, and your personalized planting calendar is generated instantly — including when to start indoors, when to transplant, and when to expect harvest.
Succession planting for continuous harvest
One seed starting date gives you one harvest window. Succession planting — starting new trays of fast-maturing crops every 2–3 weeks — gives you a continuous supply throughout the season.
Good candidates for succession starting: lettuce, spinach, cilantro, bush beans, and radishes. Start a small tray every 2–3 weeks from your first possible start date through mid-summer. As one batch is harvested, the next is catching up.
