(Editor's Note: This article was originally published on March 14, 2008. Your comments are welcome, but please be aware that authors of previously published articles may not be able to respond to your questions.)
I'm that cheap gardener, or at least I’d like to think I am. So I devised ways to start my own seeds like tomatoes, peppers, eggplant and warm weather annuals such as zinnias, melipodium and gomphrena extra early without starting inside with a light set up, in a cold frame. I’ve used a mini-greenhouse in the past too. All of my perennials are winter sown, however, in zone 7/8 the variable spring temperatures can fool the gardener into thinking it’s warmed up enough to seed very tender plants…and then zap! We get a week of very cold temperatures in late spring. So I learned to use a cold frame and more of a “hot frame” and the mini-greenhouse to grow the plants along after they get too tall for the cold frame.
However, that brings us back to the colder weeks or nights we occasionally get. Since I don’t have enough space to need a real heat source like a portable heater, or even a light bulb like a friend uses (they would fry plants in a small space) I use much more basic and primitive methods.
My favorite is a milk jug full of water. I leave the jug of water with the cap on it, beside the cold frame. By spring, most of our days are warm enough to heat the water up to a nice temperature. By placing the jug in a closed cold frame before it gets dark, the water can slowly release it’s heat – just enough to keep the plants from freezing or dropping dramatically in temperature. When we are in for a real dramatic temperature drop, I will augment the milk jug with an old sheet and throw an old shower curtain over that. However, I am always careful to vent the structure every day. My last attempt, if it is wildly cold, even during the day is to throw some bricks or rocks between the sheet and the shower curtain. The extra later of air is insulated, and will heat up in the sun, plus the bricks and rocks heat in the sun.
It may sound like a lot of extra work, but I keep the milk jugs and three bricks right by the cold frame. The sheet and shower curtain go in a plastic box, tucked out of sight somewhere near the cold frame. So when the need arises, I just take a minute or two and run out to save my plants, and in the long run, save a lot of cash by growing my own warm season annuals!