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This plant won't grow anymore, but I have four more that will be ready in about 5 weeks or so.

The white stuff is the trichomes that have the THC and other goodness of this plant.

Is there a tell for when to know it won't grow anymore?

When the trichomes are at their peak potency, which is milky color and some amber color to them, that is when they are ready for harvesting. If you continue to let them grow, the trichomes will begin to degrade and become less potent.

Exactly what i wanted to ask.

!PIZZA