This is a neat and quick way to create a cold frame for your low temperature tolerant greens and to get a head-start on in-ground planting. If you have some old glass doors or windows hanging around, simply lay them across some straw bales or other support.
The biggest problem with a cold frame is that the veggies can get cooked on a hot day. Be sure to vent the cold frame on hot days or remove the glass completely. You can grow most greens like kale and lettuce well into winter and sometimes year-round.
Image courtesy of ozarksalive.org