I'm in Alabama, and live on a very heavily treed half acre, and generally cover my pool at the end of August. While the water temp is running 86/87 currently, next week our overnight lows will start dipping into the 60's and together with shorter days/less sunlight, that water temp quickly will drop into the high 70's, and anything below 80 is too cold for me.

I don't use polyquat, just shock it with the proper number of jugs of household chlorine, but I do run the pump for half days until October, and I've never opened to anything other than a pristine pool 8 months later in May! I think that since no organics are going into the pool and it's getting no sun exposure, algae is highly discouraged.