Why Start Indoors
Starting seeds indoors gives you a crucial head start, especially for warm-season crops like tomatoes, peppers, and eggplant that need a long growing season to produce. In most climate zones, these crops need 6 to 8 weeks of indoor growth before they're ready to transplant after the last frost. Indoor starting also gives you access to hundreds of cultivars that nurseries don't stock, at a fraction of the cost per plant.
Timing
Count backward from your last frost date. Tomatoes and peppers go in 6 to 8 weeks before last frost. Lettuce and brassicas (broccoli, cabbage, kale) go in 4 to 6 weeks before. Herbs like basil go in 4 weeks before. Check seed packets for variety-specific timing, and look up your frost dates through your local cooperative extension service.
Containers and Soil
Any small container with drainage holes works: seed trays, cell packs, peat pots, or recycled yogurt cups with holes punched in the bottom. Use a seed-starting mix rather than garden soil — it's lighter, drains better, and is less likely to harbor damping-off fungi. Pre-moisten the mix until it feels like a wrung-out sponge before filling containers.
Plant seeds at a depth of roughly 2 to 3 times the seed's diameter. Tiny seeds like lettuce go almost on the surface with a light dust of mix on top. Larger seeds like squash go 1/2 to 1 inch deep. Cover trays with clear plastic wrap or humidity domes until seeds germinate, then remove immediately to prevent mold.
Light
Seedlings need 14 to 16 hours of light per day. A south-facing windowsill works for some crops, but most seedlings grown in window light alone get leggy (tall and weak) because the light intensity is insufficient. A basic fluorescent or LED shop light hung 2 to 4 inches above the seedlings delivers consistent, even light that produces stocky, strong transplants. Raise the light as seedlings grow to maintain the 2 to 4 inch gap.
Temperature and Water
Most vegetable seeds germinate best at 65°F to 75°F soil temperature. A seedling heat mat under trays speeds germination significantly, especially for heat-loving crops like peppers and eggplant. Once seedlings emerge, ambient room temperature (60°F to 70°F) is fine. Water from the bottom when possible — set trays in a shallow dish of water and let the mix wick moisture upward. This prevents disturbing tiny roots and reduces the risk of damping-off disease, which strikes when the soil surface stays constantly wet.
Feeding
Seed-starting mix contains little to no nutrients, so once seedlings develop their first true leaves (the second set of leaves that appear), begin feeding with a diluted liquid fertilizer at half strength every 7 to 10 days. Fish emulsion or seaweed extract are gentle, effective options.
Hardening Off
Transplanting directly from indoors to the garden shocks seedlings, often fatally. Hardening off is the process of gradually acclimating them to outdoor conditions over 7 to 10 days. Start by setting trays outside in a sheltered, shaded spot for 2 to 3 hours the first day. Gradually increase outdoor time and sun exposure each day. By the end of the week, seedlings should be spending full days outside, including in direct sun. Then transplant into the garden on a cloudy day or in the evening to minimize transplant stress.
For the tools to help with transplanting, see our Best Garden Tool Sets. And for more planting guidance, check our Seasonal Planting Calendar.