As a general rule, seeds should be planted to a depth equal to 2-3 times the width of the seed. However, some seeds are large and tend to migrate to the soil surface and need to be planted a little deeper. The New Seed Starters Handbook provides guidelines for planting many seed varieties. Most seed packets also contain the appropriate planting depth for each variety of seed. The first step in choosing a cover crop is to determine what results you want to obtain from your planting.
Cover crops can be annual or perennial.
If you want a permanent cover you would choose a perennial cover crop. If you were planting a cover crop only to receive benefits for the next crop season, you would choose an annual. If you need Nitrogen, then a legume or legume mix would be your choice. You also need to consider how soon you may be planting your crop and choose a cover crop based upon the days to maturity. The time of the year is also a consideration as there are warm and cool season cover crops.