Beginner gardeners often start seeds at the wrong depth. Never sow a seed deeper than twice its diameter. Thus, seeds should be placed twice as deep as broad.
But there's no need to get out the measuring tape and microscope. Just keep in mind these basic guidelines:
Bean and squash seeds need deeper containers and more soil to germinate. Exposed roots dry, eat, or germinate. Plant 2-3 times seed width deeper. These seeds thrive in deep sowing.
Large Seeds
Medium seeds like brassicas and tomatoes need a double-sized opening. Backfill gently so the seed can reach the surface.
Medium-sized Seeds
Press tiny basil and lettuce seeds into the soil and lightly sprinkle seed starting mix or “topper” mix.
Small Seeds
These crops are light- and moisture-sensitive. Sowing too deep kills seed.