How do i know when to harvest ?

my week is looking pritty damm good .. 'touchwood' .. anyways the buds ovously still got a bit of growing to go not all that big a few are i can see white crystal/powder forming around the budsites .. some of the hairs cant member the name have turned brown ? .. i hope its not overnutes i been giving them half .. loads leaves went yellow this week, i know it happends around the end of flowering .. but im suppost to have a good 4-5 weeks on my grow ...? growing 90%sativa... suppost to take around 12 weeks .. is there anything i will notice that will deffantly indicate time to harvest .. what happends if i harvest a week or two early .. or a week or two late ??

too early, less bud with less THC, too late, and you'l get less THC. The right time to harvest is when the trichomes on the plant have turned from clear to milky colour. This is when there at their maximum levels of THC. If left they will begin to turn amber and this is a depletion of THC and will also result in a more sleepy effect from smoking those.

When the buds smell like a mother fuka..

A long flowering sativa will require a nitrogen-rich fertilizer for longer than a plant with a shorter flowering period. You don't really want to allow the plants to become very N-deficient until there is only a couple weeks left until harvest.

When to harvest depends on what kind of high you like. The longer you wait the more couch-lock you are prone to experience. As the buds mature some of the pistils will begin to change color (usually to orange) as the buds continue to swell. As they mature and approaching harvest time the majority of those pistils will darken thoroughly. But, the best way to tell is by checking the trichomes with a hand held microscope. These magnify from 60-100x and are available at Radioshack for about ten dollars. The capitate-stalked trichomes you are interested in go from clear to cloudy to amber. I usually cut with just a few amber ones.
Cutting to early gives you buds that are less potent than they could have been, but if you wait too long the cannabinoids begin to degrade.

Your question is answered here

https://www.rollitup.org/newbie-central/1326-101-already-asked-questions.html

JP1 said:

too early, less bud with less THC, too late, and you'l get less THC. The right time to harvest is when the trichomes on the plant have turned from clear to milky colour. This is when there at their maximum levels of THC. If left they will begin to turn amber and this is a depletion of THC and will also result in a more sleepy effect from smoking those.

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are trichomes visible with the naked eye or will i need a microscope to see them , are they the little tiny specs of powderish looking stuff over the buds and leaves or are trichomes the longer white pistils?...

Trichs are the crystals you see. You can get a general sense by that color but if you really want to see you need a magnifier. I tried the radio shack 60x one and it's too powerful for practical use, it's impossible to scope out a bud on the plant because you just can't hold perfectly still..plus the view is inverted, so when your instinct is to move a certain direction to see something, you're ass-backwards. I just got a 30x loupe and it works much better, I can look at/inside the buds

obijohn said:

Trichs are the crystals you see. You can get a general sense by that color but if you really want to see you need a magnifier. I tried the radio shack 60x one and it's too powerful for practical use, it's impossible to scope out a bud on the plant because you just can't hold perfectly still..plus the view is inverted, so when your instinct is to move a certain direction to see something, you're ass-backwards. I just got a 30x loupe and it works much better, I can look at/inside the buds

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nice one thanks