Row cover over tomato cages

[janbee]

I live in easternmost California in an arid climate with occasional winter snow, hot summers, and a growing season of about 150 days (USDA Zone 8a, Sunset Zone 10). Our average last frost date is May 25, but July and August are hot enough that tomato production slows or stops. To get a jump on the season, I typically set out my plants in the middle of April in Walls of Water. This year, for reasons that are too complicated to explain, I had to put them out at the end of March. At this point, they have grown above their Walls of Water but are still vulnerable to frost.

To protect them, I put up tomato cages and wrapped each cage in row cover. Each cage has its own lid, also made from row cover. I'm worried that my plants will bake on warm spring days. When days are warm (mid-seventies to low eighties, say), should I remove the lid, or will the plants benefit from the extra warmth? I will of course remove all the row cover altogether once the danger of frost is past.

Jan