They act like miniature greenhouses. During sunny days the temperature inside can get hot enough to damage or kill the plant, so they should be removed or vented. Venting can be done by placing the bottom edge on a piece of wood or brick, allowing for proper cooling. This also gives you one more thing to forget and one more way to kill the plant. In my opinion, only a handful of plants are worth the time, money and trouble of covering with a cloche, he says.
Tomatoes are at the top of the list.
The earlier a garden-fresh tomato is ripe, the better. Cucumbers, some melons, and a few exotics like artichokes and sweet potatoes can also warrant the use of a cloche. By extending the season, these plants may produce larger and more abundant fruit. I once saw peas growing in a Wall O' Water. That seemed pretty disgusting until I saw someone do the same thing with zucchini, Goodspeed says.