I've had great luck with the heat mats. I use them in my unheated GH and they generate a surprising amount of warmth, especially if I have a dome cover over the flats. You might want to put something underneath the cold frame so the brick doesn't suck up too much heat. Perhaps a thick sheet of styrofoam?

Isn't a cold frame supposed to sit directly on the soil? I'm wanting to build a cold frame soon.

Rain2Fall

like karin_mt, I use several heating mats for seedlings in my greenhouse. They measure 48" X 18" and provide about 100 watts each (about the same as a light bulb). If you use a mat in a cold frame on the brick patio it should work well...as long as you get yourself a 1" thick piece of foam insulation. Make sure the foam piece is larger than the base of the cold frame, as that will help minimize drafts out of the bottom. I also bought myself a thermostat that the mats plug into. For about $35 you can get a thermostat with a probe on a wire that goes into the soil and helps maintain a uniform temperature. Otherwise, you may have temperatures that will not lead to good germination, or possibly cook your seedlings.

How about using a soil heating cable in the cold frame instead? Again -- do I have to put the cold frame on the patio? Can't I put it on the ground, over a piece of weed barrier fabric?

Rain2Fall

Rain2Fall,

You can put your cold frame wherever it works for you. On the ground is the usual way of doing it. If you are going to use soil heating cables or heat mats then a consideration is that you'll need to run an extension cord.

Roger. If your hot bed is going to be raised off the floor on a frame try enclosing the legs (framework) with some scrap plywood then put a couple of 100 watt light bulbs inside the enclosure suspended below the bed. I use sand in the bottom of my raised hotbeds and it helps to spread the heat around. You can move the bulbs closer if needed. I use propagation mats now but started with light bulbs and they worked fine. Happy Planting Steve

For 2 or 3 flats you might want to consider growing them indoor with simple fluorescence fixture.

I always think of it this way: outdoor you have all the free sunlight but you need to provide heat. Indoor you have all the "free" heat (not really free but you are heating your house anyway) but you need to provide some additional light.

For 2 or 3 flats, maybe on light fixture is all you need, and they are cheap.

Dave is right. A single flourescent light fixture is about $10, plus a few bucks for some lightbulbs!

For less than $50, you could easily have two 4 foot long 2 bulb light fixtures, which should easily fit 2-3 flats under them. The issue then becomes where to put them inside your house. (providing a small amount of air circulation, via a small fan, or placing them in a high traffic area of the home is a major benefit as well...more breeze reduces the chance of damping off, and produces stockier plants.)