Grab a small pocket microscope, 15x is adequate but you really need a 30x one (Ive heard Radio Shack do them in the US, I suppose you could get one from eBay) and look at the trichomes. When theyre all cloudy you can harvest then for a more sativa oriented head high, or you can wait a little longer until you have 50% amber, this will give an indica couchlock feeling.
Of course it also depends on how sativa/indica the strain is too

Edit: you can also harvest by looking at the pistils (hairs), but this isnt exactly accurate. If you do this, then harvest at 50% amber/red.
Tom Swierzbinski Reviewed by Tom Swierzbinski on . ready for harvest? Alright this is my first grow so im hoping you guys can answer these harvest ?'s cool so ive got two plants outside in a pot and the buds are huge, but i heard you can tell when its time to harvest by how many red hairs there are, and there aren't many. its probably about 80-90% white hairs to 10-20% red. but its been like that all month so im wondering if this strain just doesnt have a lot of red hairs. all that seems to be changing about the plant this month is the thickness and number of Rating: 5