40°F outside, 80°F under the plastic

Last week I swapped out my worn-out frost covers for fresh ones, and it reminded me of the question I get more than any other this time of year: fabric or plastic? I know, plastic sounds like the obvious winner. It's clear, it traps more heat, and it looks like a little greenhouse. But warmer isn't always better for cool-season crops. On a sunny winter day when it's only 40°F outside, a plastic-covered tunnel can easily climb to 80°F inside. Your kale and spinach take that as a cue to drop...