Most seed packets tell you how deep to plant. A rule of thumb is to sow seeds two or three times as deep as they are wide. Take care not to plant seeds to deep. A seed contains a limited supply of stored food to nourish it during germination. If you plant it too deep, it will run out of food before it reaches the light and grow big enough to start producing its own. Some seeds require light to germinate. Sow these seeds on the surface of the planting mix.