Actually, that's a good question. It will keep for a while but you have to keep it moving. We have enough growing that we use it pretty steadily even outside the greenhouse. The first dry spell we ended up with a couple of gallons in the bottom of the white tank that wouldn't pump out. It got green and ugly. I cleaned it out and now when that happens and rain is not in sight, I just keep it filled with tap water and use that. At least it keeps the tanks clean that way.