By Liz Mcdonald 
Traditional clothes and bell jars from glass can function like a magnifying glass, focusing heat from the sun inside the cloche. Because cloches are usually quite small, the intense heat inside can kill plants. This makes ventilation critical.
A milk jug cloche makes that ventilation simple. In order to hack your own inexpensive cloche to help extend your planting season, simply cut the bottom of a
plastic milk jug and set the jug directly over a plant that needs a little extended help. Remove the cap of the milk jug for ventilation; if additional airflow is needed, you can prop the side up with a stick or piece of wood. Since the milk jug is not made out of heavy glass, there is less heat generated. Keep in mind, however, that without as much heat generation, the system does not provide as much heavy frost protection.
Using a milk jug as a cloche is an inexpensive way to hack a longer growing season; it protects plants and so extends production.