Having a piece of greenhouse plastic or a gauzy sheet of what’s called Reemay ( aka floating row cover) around may make the difference between vibrant, green plants that survive a frost and blackened, sickly things that don’t. If you’re working the edges of the season, pushing the limits of cool-season crops, trying to hasten along warm-weather crops and/or prolonging summer’s harvest late into the fall, some kind of moveable, removeable row cover is essential. It can also serve other functions, such as excluding pests as the season progresses. If you go for a more greenhouse-y effect and get the plastic, make sure it’s greenhouse-duty grade and hence UV-stable. The thin, clear-plastic sheets from your local hardware store will ultimately degrade into a billion smithereens after a couple seasons.
As with most things, sometimes a piece of gear requires its own gear. So although the lighter Reemay may rest or “float” directly on top of the plants it protects, the heavier plastic option will require a set of hoops (metal or bamboo) to support the cover. They will both require some kind of garden staple or weight to affix them to the soil. Do a good job of this or your neighbor some distance away may be the beneficiary of your newly-purchased and now wind-tossed, wind-lost row cover.
Also consider that as the sun gets higher in the sky, a tunnel or cover of greenhouse plastic will require ventilation; otherwise those green, healthy plants, which you prevented from turning black and sickly from the cold, will become bright green and mushy from solar cooking!!! Reemay can be left in place and doesn’t need venting because it’s permeable to air and water and actually reflects away a bit of the sun’s light.
Rowcovers, whether plastic or polyesters, are a cheap investment into a much longer growing season. Get some now and put it to use right away!
And if you want to learn more about my rule-breaking, experimental approach to gardening, check out the class I recently released with @anneparmeter entitled “Things They Didn’t Teach You in Garden School”
https://www.udemy.com/course/gardenschool/?referralCode=6A7389A3FC7B22E8FA91