Lisa, you could use heating cables for those early spring nights when frost is forecast. They come in different price ranges, but the cheaper ones would be quite sufficient for your purposes. Just measure the square feet and get the proper length and follow the instructions on placement. In mine I cut a piece of screen to cover the entire area of the cold frame to separate the cable, which is very slightly buried in the soil along with its built-in thermostat, from the flats and trays. These heating cables work for me even when the temps dip into the 20s. Hope this helps. Gary

I would like for the temperature in the coldframe to be at least 60 degrees inside, when the temperature outside is below 32 degrees.

That is a BIG gap! Likely only an active heat source like propane, kerosene, or electricity could come close to closing that gap. Can I ask why 60? Normally the goal in a cold frame is just to keep them from freezing.

But I also have to ask is this a cold frame or a greenhouse? They are very different structures and the solution to your problems would be very different for the 2 structures.

A cold frame is a very low to the ground structure, sits on the ground, has a lid, and a limited amount of usable space and a small volume of air to be heated so passive heat sumps like stones, bricks, jugs of water, etc. in one along with the ground heat can often be all you need to keep things from freezing.

A greenhouse in a taller walk-into structure with door or doors and a much larger volume of air to heat but only the air at the level of the plants has to be heated so one can either raise or lower the plants or apply the heat source only to the air around the plants themselves and let the rest get cold. The biggest problem with a GH is cooling rather than heating.

Dave

The old fashioned way of heating a cold frame was to bury a layer of fresh manure in it that would heat it as it composted.

Lisa,

If your coldframe is insulated, most of the heat will be lost through the glazing. You can put a sleepin bag over the top to retain more heat at night.

If it is a small to medium cold frame, put an incandescent lite inside on cold nights. Some use outdoor christmas lights or light ropes. If it is about 3 x 4 by 2 high, one 60-100 watt bulb should do it. This will keep plants from freezing but not at 60 degrees!

Good Luck

Rick

Thanks, to all of you who have taken the time to answer my post, I really appreciate it. To answer a few of your questions, I had my husband build a coldframe, but it is up on a large table instead of on the ground. Therefore, heating cables running through soil isn't going to work unless I move it to the ground. The table is a thick work table and the coldframe sits on it perfectly, so cold air shouldn't be able to get under the coldframe. I put it up on a table since it is easier for me to tend to my plants. Maybe I will try to put it on the ground and try the heating cable thing, but I have animals and when the lid is open they like to eat my plants. I am either losing plants to cold snaps or animals and I am quite tired of it.
Since it is always recommended that you wait until the night time temperatures are in the 60's to transplant outside, I figured that it needed to be at least 60 degrees inside the coldframe or greenhouse at night to keep them from freezing and dying. So I wanted to keep them as toasty as possible. So now my question would be how cold is too cold for the little seedlings or plants? To answer one of the other questions that was asked, I'm not really sure what I would consider the contraption that my husband built for me. Looks like and is built like a coldframe, but could be more like a minature greenhouse, but regardless it isn't working for me the way I visioned that it would.....Lisa

I place my cold frames on fresh manure like someone else mentioned. If its goin to be really cold, I will put milk jugs with water in them in the frame and let them soak in the sun all day. They emit heat for a good portion of the night.

Up on a table is going to defeat the purpose of a cold frame. Sorry. Not only do you lose the benefits of all the ground heat that accumulates during the day and then normally see the plants through the cold spells but you now have an unheated air space below it to drop the inside temps even faster. So it is getting hit with cold air from above and from below.

It is the same reason (cold air above and below) that you see the highway signs - Caution: Bridges freeze before highways.

Dave

Thanks, for the help everyone. Dave, everything that you said makes perfect sense, I'll have to move it to the ground for better insulation. Everyone has been very helpful, maybe now that I know this information I'll be able to keep my plants alive. I only wish that I would have asked sooner..Lisa

Excellent thread I've learned a lot. I'm planning to have something built to keep plants in the spring and also to protect inground plants in the fall-early winter so this thread gave valuable information to consider. Thanks, to everyone who contributed and sorry Lisa-regina for the loss of your hard work.

Take Xmas light start at the bottom left and tack up at 45 degree angle to the top tight corner and then straight down and reverse. You have a cross X from side to side and this will provide additional warmth. You could also place a blanket over the top of the cold fram.