I just went through my first Winter with a greenhouse.  All I can say is that if you don't have one- get one! (assuming you live outside the tropics)  Do whatever you have to do and get one!  Imagine plumerias, brugmansias, and hibiscus blooming year-round.

Picture shows plumeria plants shortly after greenhouse completion.

Its a jungle with the brugs. 

Consists of a 30 in. auger stake, turnbuckle hook and eye and rafter clamp.  This greenhouse uses ten of them.  The wind hasn't blown it away yet.

I have a dozen black five gallon buckets filled with water lining one side of the greenhouse.  They absorb heat during the day (if the sun is shining) and the heat is released at sundown.

Our first and only snow of the season- February 14, 2004.

For rooting cuttings.  Its a sand box with a water bed heater.