Enter your last frost date and check which plants you want to start — get the exact indoor start date for every vegetable.
Select all plants you want to start indoors:
Starting seeds indoors at the right time is one of the most important decisions a vegetable gardener makes each year. Start too late and you miss the harvest window; start too early and you end up with overgrown, root-bound plants that transplant poorly and set back weeks behind seedlings started on schedule. The seed starting calculator above works backward from your last frost date — the average date when temperatures are expected to stay above 32°F for the rest of spring — to give you the ideal sowing window for each crop.
Your last frost date is determined by your USDA Hardiness Zone and your specific location within that zone. Gardeners in Zone 5 (central Midwest) might have a last frost around May 1–15, while Zone 8 gardeners in the Pacific Northwest or Southeast might see their last frost in late February or early March. Urban gardeners often run 1–2 weeks warmer than surrounding rural areas due to the heat island effect. The Old Farmer's Almanac and your local Cooperative Extension office are the most reliable sources for your specific last frost date.
Vegetables fall into two broad categories that determine your starting timeline. Warm-season crops — tomatoes, peppers, eggplant, melons, and basil — cannot tolerate frost at all and must be started well in advance indoors. They require warm soil (65–80°F) to germinate quickly, which is why a seedling heat mat is essential for these crops. Cool-season crops — broccoli, cabbage, cauliflower, kale, and lettuce — can tolerate light frost and can often be transplanted while nighttime temperatures still dip to 28–32°F. Some cool-season crops like spinach and peas are direct-seeded outdoors well before the last frost and should not be started indoors at all.
Most vegetables are started indoors 4–12 weeks before your last frost date, depending on the crop. Tomatoes and peppers need 6–8 weeks; eggplant, celery, leeks, and onions need 10–12 weeks. Broccoli, cabbage, and kale do well with 5–6 weeks. Basil and marigolds only need 4–5 weeks. Starting on the late end of the recommended window typically produces stockier, healthier transplants than starting extra early. If you start tomatoes 12+ weeks before transplant date, you'll almost certainly end up with leggy, root-bound plants that require significant recovery time in the garden.
The last frost date is the statistically average date after which your area has a 50% chance of receiving another frost. It is not a guarantee — frosts can occur 2–3 weeks after the average date in bad years. Many experienced gardeners wait until 2 weeks past their listed last frost date before transplanting frost-sensitive crops. To find your date, search your zip code at the Old Farmer's Almanac frost dates tool or contact your local USDA Cooperative Extension office. If you are in a frost pocket (low-lying area that traps cold air), add 5–7 days to the listed average.
Leggy seedlings — stems that are tall, thin, and floppy — are almost always caused by insufficient light, not insufficient fertilizer or water. Most indoor environments simply cannot provide the 14–16 hours of bright light that seedlings need during early spring. A south-facing window is rarely enough in January or February north of latitude 35°. The solution is supplemental grow lights positioned 2–4 inches above the seedling canopy and run for 14–16 hours per day. Starting too early also contributes to legginess because seedlings outgrow their cells waiting for outdoor temperatures to warm up, then get increasingly stretched as they compete for available light.
These are two distinct numbers that are often confused. "Days to maturity" on a seed packet refers to the number of days from transplanting (or outdoor direct seeding) until the crop is ready to harvest. A tomato labeled "75 days" will produce ripe fruit 75 days after you put the transplant in the ground. "Weeks to transplant" — what this calculator uses — refers to how many weeks before transplant date you should start the seed indoors. A tomato started 7 weeks before your last frost date (transplant date) that is labeled "75 days" will be ready to harvest roughly 75 days after transplanting, well into summer.
Celery, celeriac, leeks, and onions have the longest indoor start windows — typically 10–12 weeks before transplant date. These crops have slow germination (celery can take 2–3 weeks to sprout) and slow early growth. Peppers are close behind at 8–10 weeks, especially hot pepper varieties which germinate slowly and grow more slowly than sweet peppers. Eggplant also needs 8–10 weeks. Tomatoes are the most forgiving of the slow-start crops — 6–8 weeks is the sweet spot, and even 5 weeks produces acceptable transplants in a pinch. Most other vegetables do not benefit from indoor starting at all and should be direct-seeded outdoors.
Individual cells or plug trays are strongly preferred for most crops because they minimize root disturbance at transplant time. Crops that resent root disturbance — carrots, beets, cucumbers, squash, and melons — should never be started in a flat and pricked out; use individual biodegradable pots or direct sow outdoors. Tomatoes, peppers, and brassicas tolerate transplanting well and can be started in 72- or 128-cell plug trays, then potted up to larger containers if the season is long. Start with a sterile seed-starting mix (not garden soil or potting mix) to prevent damping off, the fungal disease that collapses seedling stems at the soil line.