Thanks for responding. I follow the closing as closely as I can. I have a mesh cover and a lot of trees around the pool so I close before the leaves really start coming down. Water is usually in the low 60's when I close. I check the water during the winter and spring and it's always clear. It turns green when the temp is in the low 50's and I have no explanation for that.
But you said you open to a green pool every Spring? So, when you check the pool, it's not clear in the Spring....please clarify.
Here's what I think is happening - you're closing (i.e., not chemically treating) your pool while the mesh cover is on and the leaves are dropping. So basically your pool is loading up with fine organic debris (your mesh cover is only catching big stuff) and that is sitting in the water until Spring. You're also doing this while the water temperature is probably bouncing around in the low 60's. The fact is, 60F is not a magic number - algae isn't killed by cold water (just ask all the algae and plankton that grows in the waters off Antartica). All 60F means is that the algae reproduction rates are very slow. So you have water that's loaded up with organics and little or no sanitizer in the water. That is a recipe for a nice slow winter stew of algae and that is probably why you're seeing a little bit of green when the water is cold. Then, when the water starts to warm up even a smidge over 60F and, POW, the algae takes off.
Polyquat 60 is not going to help. It's an algaecide but it doesn't last the whole winter and it will dissipate. You might benefit by adding borates to your water as they are a mild algaecide and you might also benefit from treating for phosphates as the organic load is no doubt increasing your phosphate levels. Phosphates are nutrients for algae growth and, when low or no sanitizer is present, high phosphate levels make the water more conducive to algae growth. Treating for phosphates is not hard but it requires certain criteria to be met or else it's a pointless process.
But here's the real problem - your pool, with the mesh cover, is basically open to the environment all winter long and it has little or no sanitizer in it. So anything that gets in the water is going to find a happy home to grow in. I would say that, if you truly want clear water, you need to actively manage the water all through leaf fall (with the mesh cover in place) and then, when the water is finally cold enough (below 60F), you need to sanitize as prescribed in the Pool School article (SLAM plus Polyquat) and then close it with a solid cover. That way, the pool water gets little to no sunlight, it has no organic load flowing into it and the FC will hold for a long enough period of time to kill anything in it. Then, you need to open early and get the water going.
I'm sure none of that is what you wanted to hear. As I said, if I had a short swim season (less than 3 months) and had to open and close a pool every year, I would simply not own a pool, it would not be worth it in my opinion. But you do have a pool and it obviously is useful to you so, unfortunately, you have to deal more forcefully with it.
Sorry, but that's my opinion on the matter. Maybe other pool owners with a similar climate can give you better advice.