Polyquat can be used year round. You can add an ounce or two every week to help prevent algae during the swim season. It's not necessary if you follow the B-B-B method, but it won't hurt and can help if you have to neglect the pool for a few days. It's the ONLY algaecide we recommend using.

But be aware: after adding Polyquat you may see your FC level drop--that's OK just add enough chlorine to bring it back up.

I use a full quart when I close my pool. I raise FC to shock level, add the Polyquat, wait 48 hours, then raise FC to shock level again and close. I rarely have algae in the spring and then only if I open late (like mid May).