If you want to replace it a minimum of once per year, then go light. Otherwise, go as heavy as you can find that's still relatively see-through or at least opaque. I'm not sure what UV protection is going to do for plants... they get enough UV in nature. If there's something to it, I'm sure I'll be corrected.

Here is a link that might be useful: Sinfonian's gardening adventure...

The UV protection is for the plastic.

I would be more concerned with plastic capable of withstanding storms or high winds. Sure, a thin plastic is fine except for those couple of hours when perhaps you'll need them most.

UV protection is recommended for plastic covers you plan to keep up for more than a few months.

Thanks very much for the tips. I think I'm going to go the cheap route first and see what happens...

That cheap sheet plastic from X mart you use to fix leaky windows is fine, and one package will be enough to last years. Keep the cover vented unless it's below 26 degrees or so. The risk of frying on a hot day is high, so consider keeping the ends open at all times. For added wind/cold resistance, throw on an old blanket on cold nights. You can use plain old sticks to hold this little set-up aloft over plants. I've had lettuce seedlings that germinated during a mild spell of spring weather come through 12 degree nights under sheet plastic topped with an old quilt.

Here is a link that might be useful: my website

When I think of row covers spun polyester is the material that comes to mind. Its sold under names such as Remay or Agribon and for these the weight of the fabric, usually indicated in oz. per sq. yard, indicates the degree of protection.

As bcomplx mentioned, plastic is risky when it is just above the plant canopy. Opening ends of rows probably isn't sufficient. Most people using clear plastic for crops use a slitted or drilled plastic to allow the heat to pass through. I strongly encourage you to use a utility knife to cut many small slits along the highest points of the plastic cover. And inspect your plants hourly for the first few sunny days under the cover for signs of heat stress.

If you were to switch from plastic to the spun poly materials you will have far less stress to yourself and your plants. Those materials will tear but typically can be reused for many years.