I imagine that would weigh a ton when wet. And row cover is usually white and allows light through. It would be pretty gloomy under brown fabric.

Tulle fabric works. As does insect netting. Neither one will help protect against freezes, however. For that you will need actual row covers. And as long as it isn't hot out you don't need to remove them in the daytime.

Rodney

But plastic keeps water out and can cook in the sun. How does it work?

Plastic would qualify as cloches rather than row covers, I believe. I don’t find it cooks plants in winter but then my climate is probably different from yours.

What are you looking to protect your plants from? Bugs, frosts? A light netting or row cover fabric can be great for bugs, but you want to use a row cover that is meant to be thermally protective or a heavier fabric for frosts. In a pinch if you are surprised by a frost, you can use old bedsheets for an evening.

I don't put the plastic over the hoops until the weather has turned really cold. Before that, I use a standard row cover.