I'd try to keep them indoors until the outside temps could at least stay around 65 deg F or better, even at night, lest your plants become stressed and slowed down in their growth, with diminished THC and other cannabinoid levels.... But, if it doesn't get cold enough to kill them altogether, it'll just turn them purple, which under those conditions does nothing for the quality but take it down... If stigmas, or calyxs, or leaves won't turn purple under "norml" temps of 70-80 deg F, then in my book, it just ain't a real purple, AT ALL... I wouldn't risk it if it were me...