We have only had about a week of temps above freezing this month with many days in the 20 and nights near or below 0. It has also been rather cloudy. The result of all this is that the planting bed in my greenhouse hasn't warmed up as much as I needed it to so I could get some things started. My greenhouse is a bit drafty, still missing a few bits to get it sealed up nicely. It is also completely unheated. I'm working on fixing those problems, but it's not getting done fast enough. So, I installed a bed warmer today. It should help capture any heat and hold it through the night. We have a couple of warm days predicted so I'm hoping to get the frost out of the soil today (sunny!!!) and get some things planted tomorrow.
The mini-hoop bed warmer should give me an extra 10 degrees at night which will speed germination. I'll open it up during the day so things don't cook. I make my mini-hoops out of 9 gauge wire and cover it with a piece of greenhouse plastic I trimmed off when we put the cover on the big house.
For my 4 foot beds I use 8 feet of wire. That allows me to poke enough of each end into the dirt that they stand up quite well. I like them much better than pvc hoops. They last longer and cost less. In a few weeks I will be using it out in the production garden to warm a section of bed so I can get my salad greens planted in time for the open of the Farmers Market.