Seeds vary hugely in size, from the dust-like seeds of orchids to huge and almost indestructible coconuts. Their size is related to how long the seeds need to survive before they find suitable growing conditions in the wild. Orchids, for example, are shed onto permanently moist leaf mould and can grow straightaway. Coconuts, on the other hand, come equipped for a long sea voyage before being buried in the sand on a tropical beach.
The size of each seed acts as its own instant guide to sowing depth - as a general rule, cover seeds with roughly their own depth of compost.
Here are our recommended sowing depths for many popular seeds.