This is why I think black plastic is better than transparent. Because the light can get through the transparent plastic, what you've got is a kind of "mini greenhouse" and it actually helps the weeds to grow. Black plastic, on the other hand, prevents light from reaching the plants and that's what kills them dead. No light - no can grow. I use some recycle small black animal trough over some really stubborn weeds like thistle that insist on growing in the main walkway to my house. Kills 'em dead every time!

Also, I'd like to note that this year's ferociously hot summer we've had probably helped the plastic to degrade since most plastics are designed to disintergrate in the sun.

The UV/heat-induced degradation of the plastic was a reason that the Save The Bay organization couldn't successfully use plastic to solarize ground in their efforts to eradicate invasive species of grasses from certain areas, for habitat restoration. They experimented with some kind of plexiglass instead, with good results. That may not be worth the investment for you two, but it's an idea.