Starting seeds indoors gives you a head start on the growing season — especially for warm-season crops like tomatoes and peppers that need a long time to mature. The key is timing: start too early and seedlings get leggy; too late and you lose weeks of harvest.
How to Calculate Your Start Date
Everything is based on your last frost date. Look up your date by ZIP code (our garden planner does this automatically), then count backward by the number of weeks listed for each crop.
For example, if your last frost date is April 15 and tomatoes need 6-8 weeks indoors, start tomato seeds between February 18 and March 4.
Seed Starting Timing Chart
| Crop | Start Before Frost | Germination | Transplant Ready |
|---|---|---|---|
| Tomatoes | 6-8 weeks | 5-10 days | 6-8 weeks after sprouting |
| Peppers | 8-10 weeks | 7-14 days | 8-10 weeks after sprouting |
| Eggplant | 8-10 weeks | 7-14 days | 8-10 weeks after sprouting |
| Broccoli | 6-8 weeks | 4-7 days | 4-6 weeks after sprouting |
| Cabbage | 6-8 weeks | 4-7 days | 4-6 weeks after sprouting |
| Basil | 4-6 weeks | 5-10 days | 3-4 weeks after sprouting |
| Watermelon | 3-4 weeks | 4-10 days | 3-4 weeks after sprouting |
| Cucumber | 3-4 weeks | 3-7 days | 3-4 weeks after sprouting |
Indoor Seed Starting Essentials
- Seed starting mix — Use a sterile, soilless mix (not garden soil). Peat- or coir-based mixes drain well.
- Containers — Cell trays, peat pots, or recycled yogurt cups with drainage holes.
- Light — Seedlings need 14-16 hours of light. A south-facing window rarely provides enough — use a grow light 2-3 inches above the seedlings.
- Heat mat — Peppers and eggplant germinate much faster with bottom heat (75-85 F).
- Hardening off — 7-10 days before transplanting, gradually expose seedlings to outdoor conditions.
Common Mistakes
- Starting too early — Leggy seedlings that outgrow their pots before it is warm enough outside.
- Not enough light — Seedlings stretch toward dim windows. Use supplemental lighting.
- Skipping hardening off — Transplant shock can set plants back 2-3 weeks or kill them.
- Over-watering — Soggy soil causes damping off, a fungal disease that kills seedlings at the base.
Automate Your Calendar
Rather than calculating dates manually, use our free garden planner — enter your ZIP code and it generates a Gantt-style planting calendar with indoor start dates, transplant windows, and expected harvest ranges for every crop you grow.
For layout ideas, see our raised bed planning guide, and pair your crops wisely with the companion planting chart.