Indoor Sowing Windows
Start seeds 6 to 8 weeks before your last expected frost date for optimal transplant readiness.
Heirloom tomato seeds need consistent warmth—ideally 70 to 85 degrees Fahrenheit—for reliable germination. Use a heat mat and bright, artificial light (14 to 16 hours daily) to avoid leggy, weak seedlings. Fill small containers with sterile seed-starting mix, sow seeds a quarter inch deep, and keep the medium lightly moist but never soggy. Germination typically occurs in 5 to 10 days under ideal conditions.
Once cotyledons emerge, reduce daytime temps to 65 to 75 degrees and maintain good air circulation to prevent damping-off. Thin crowded seedlings early, either by snipping extras at soil level or repotting carefully. When the second set of true leaves appears, begin a diluted liquid fertilizer weekly. Avoid overfertilizing, which can stunt roots and delay transplant readiness.
Seedlings are transplant-ready when they are 6 to 10 inches tall with sturdy stems and 4 to 6 true leaves. If growing in cool conditions, slow growth may delay this stage—do not rush. Check your local last frost date and count back six weeks; adjust if using unheated indoor spaces like garages or sun porches where temps dip below 60 at night. Healthy seedlings support longer productive seasons.
