The best seed starting date depends on when a young plant can move outside. Starting earlier is useful only if you can keep the seedling healthy until that day. A crowded tray of oversized plants can be harder to establish than a younger, compact transplant.
Start with the outdoor planting date
Find your local average last spring frost date, then check how your crop handles cold. Warm-season plants often need extra time after frost; hardy crops can sometimes go out earlier. An average is a planning anchor, so keep checking the forecast as planting day approaches.
Subtract the growing period
Read the seed packet for the indoor growing period. If a packet calls for six to eight weeks before transplanting, count back from the transplant date rather than blindly from the frost date. For a May 15 transplant, that produces a March 20 to April 3 window.
Make room for the whole journey
Seed trays occupy little space at sowing and much more after potting up. Schedule a hardening period before planting out. If you have limited lighted space, start fewer varieties or sow toward the later end of a recommended range. Your setup is part of the timing decision.
- Record the variety and sowing date on every tray.
- Reserve space for larger pots before you need them.
- Keep a few days of flexibility for poor outdoor weather.
Enter your frost date and select a few crops in the calculator. Compare the indoor sowing window with the separate plant-out date before buying more trays.
Plan my gardenFurther reading
Our guides offer general planning advice. Check the instructions for your seed variety and your local extension service before planting. We do not claim hands-on tests of products mentioned in buying guides.