You can add a month on either end of your growing season with a tunnel cloche.
The cloche uses hoops made of 1/2″ EMT steel electrical conduit pipe. You can buy this in 10′ pieces at Home Depot for about $2.00 each.
Using a conduit pipe bender make 60 degree bends in the middle of the pipe, and at 30″ on either side of the middle, creating a hoop that looks like this. Push the ends of the hoops into the earth, at 4′ intervals.
Cover the hoops with 10′ wide 4 mil polyethylene film. Attach the film to the end hoops with clips. You can purchase these, but they are expensive. I make my own, by cutting 5″ sections from old worn out 3/4″ graden hose, and cutting along the length of the hose to open it up.
To hold the film in place, bury the film in the dirt along one side, and weigh down the other side with rocks. The film can be turned up on the ‘rock’ side in order to weed or water. The ends of the cloche can be left open for ventilation when the weather is sunny, or covered up with salvaged window glass or plywood at night or when the weather is cloudy. Your total investment to cover 4’x30′ will be about $16 for the conduit and $10 for the plastic film. The conduit hoops last nearly forever (mine are 3 years old and show no sign of rust) and the PE film will last for 2 or 3 seasons.