Harvest time for seeded plants depends on why you are harvesting them, for seeds or medicine. If you are harvesting for seed, then it depends on when they were pollinated. When I harvest for seed, I put a collection trough below the plant and just let the seeds fall into it as the plant dies and am not concerned with the plant for any reason other than seed. If you are harvesting for medicine then it is just as Dutch says, harvest when the trichomes are to your desired ripeness.

JSG.