Seed Starting Indoors: Getting the Timing and Conditions Right

Pull out your seed packets and note the "weeks before last frost" recommendation printed on each one. Then look up your area's average last frost date—your local cooperative extension office is the most reliable source. Count backward from that date by the number of weeks specified.

For example, tomatoes typically need 6–8 weeks indoors. If your last frost date is May 1, start tomato seeds between early and mid-March. Crops like lettuce and kale tolerate frost and can be started 4–6 weeks out. Keeping a simple calendar or spreadsheet prevents over-crowding your setup with plants that mature unevenly.

Fill containers to within half an inch of the rim with a pre-moistened seed-starting mix—not garden soil, which compacts and can introduce disease. Squeeze a handful: it should hold its shape but release only a few drops of water. Label each cell or container before sowing so varieties don't get confused.

A general rule is to sow seeds at a depth equal to twice their diameter. Tiny seeds like basil or lettuce should be barely pressed onto the surface and covered with a fine dusting of mix. Larger seeds like squash or cucumbers go about half an inch deep. Place 2–3 seeds per cell to account for germination variability; you'll thin to one seedling later.

Most vegetable seeds germinate best when the growing medium stays between 65–80°F. The top of a refrigerator, or a dedicated seedling heat mat, can provide bottom warmth. Cover trays with a humidity dome to retain heat and moisture. Check daily—once you see the first sprouts, remove the dome immediately to prevent fungal problems.

After sprouts emerge, light becomes the most critical factor. A bright south-facing window may work in late spring but is often insufficient in late winter, when days are short and light angles are low. Position grow lights 2–4 inches above seedling tops and run them for 14–16 hours per day. Raise the fixture as plants grow to maintain that distance.

Leggy, pale seedlings with long gaps between leaf nodes are a clear sign of insufficient light—not a watering problem, despite how it might look.

Mist the surface with a spray bottle at germination, then transition to gentle bottom-watering once seedlings are established: pour water into the tray and let containers absorb from below for 20–30 minutes, then drain. Allow the top layer of mix to feel barely dry before the next watering. This encourages roots to grow downward and prevents the standing moisture that causes damping off.

Once seedlings develop their first true leaves (the second set of leaves, which look like miniature versions of the mature plant), thin to the strongest one per cell. Use small scissors to snip extras at the soil line rather than pulling, which disturbs neighboring roots. Thinning feels wasteful but is essential—crowded seedlings compete for light and nutrients and rarely catch up to properly spaced ones.

Beginning about 1–2 weeks before your intended transplant date, start moving seedlings outdoors for short periods each day. Start with 1–2 hours in a sheltered, shaded spot. Gradually increase sun exposure and time outdoors over 7–10 days until plants are spending full days outside. Bring them in if temperatures drop significantly or heavy wind or rain is expected.