This is a very low-cost, low-tech way to protect tender vegetable plants early in the season. Just cut off the bottoms and push the jugs into the soil around the plant. Leave the caps off for air circulation, unless a frost is threatening, then screw the caps on for more protection. As the plants grow, cut the jugs back until they’re superfluous, then just slice them down the side and pull away from the plant. Not only do the jugs protect plants from frost, but also help retain heat, making the plants warmer at night, which helps them grow. This is especially beneficial at higher elevations where evenings are cool; we routinely drop into the 40s at night.
I was short a couple jugs short for my tomatoes. Turns out puppies like chewing on milk jugs, and since they’re cheaper than a couch, for example, I decided to sacrifice a couple. A few more bowls of cereal and those plants will have jugs too.