Twice the seed's thickness, which is shallower than most people plant
The rule
Cover a seed to about twice its own thickness. A lettuce seed is barely covered; a broad bean goes in about five centimetres.
Too deep
The commonest sowing failure. The seed germinates, runs out of stored energy before reaching light, and dies underground unseen.
The exceptions
A few seeds need light to germinate and are sown on the surface. There are about a dozen common ones and they are listed.
