I've solarized a couple of areas using a heavy tarp held down with rocks piled on the corners. It works pretty much any time of year. I solarized a plot of soil that was healthy turf grass for about 8 weeks through February and March - after having turned the soil over with shovel - and it is now virtually weed free and growing roses like crazy. I failed to seal the edges, and there was a bit of space near the edges where the outcome was less than perfect, but the overall result was stunning. There are a lot of overgrown spots on my property, so the tarp is employed full time. BTW, an old rubber rug underlayment works pretty well, too.
