Two-bit Guru | Plant Heaters | Photo of a cold frame.

For a couple of years now I’ve fooled around with adding heat to cold frames to extend the growing season in both spring and fall. In a recent DIY project I made a low-wattage electrical heater that heats the soil rather than the air above the soil. In the spring, I might use this unit to encourage seed germination. This winter I’m experimenting with another use for heated cold frames, actually the reason why I made this unit the way I did. I plan on writing about that later. For now, here’s how to make outdoor plant heaters.

The base of the heater is a galvanized metal tray that I bought at an estate sale, not knowing how I would wind up using it.

Two-bit Guru | Plant Heaters | Photo of a galvanized metal tray.

The 5-foot long 75-watt rope heater (from a surplus store) fit perfectly in the metal tray.

Two-bit Guru | Plant Heaters | Photo of a 75 watt rope heater.

I wired the ends of the heater rope to an ordinary electrical cord (that was just laying around) using wire nuts (laying around) to make the connection. The wire-nut connections are located in a standard 4-inch electrical box (laying around) with a cover (purchased at the hardware store).

Two-bit Guru | Plant Heaters | Photo of various items needed to make an outdoor plant heater for a cold frame.

There happened to be a hole drilled in the tray at just the right place to feed the cord into the electrical box. The box was just the right size to fit in the tray. Coincidences abound when you’re having fun.

Two-bit Guru | Plant Heaters | Photo of the electrical box inside the galvanized metal tray.

A 75-watt heating element isn’t much, think incandescent light bulb. Even so, when in operation the bottom of the metal tray can get don’t-touch-me hot.

Two-bit Guru | Plant Heaters | Photo of the wiring, electrical box, rope heater and galvanized metal tray.

After making the electrical connections I tested the unit.

Two-bit Guru | Plant Heaters | Photo of the wiring connections.

After I screwed the cover on, I blocked the small gaps with duct tape. I wouldn’t advise anyone else to use these techniques because, after all, we’re dealing with electricity here.

Two-bit Guru | Plant Heaters | Photo of duct tape covering the gaps of the electrical box.

I happened to have a whole lot of lava rock, you guessed it, just laying around. I scooped the rock into the metal tray, covering up the rope heater.

Two-bit Guru | Plant Heaters | Photo of a lava rocks being scooped into the tray.

The porous rock acts as a pretty good insulator so not a lot of heat is lost out the top.

Two-bit Guru | Plant Heaters | Photo of the completed plant heater, with lava rocks filling the tray and covering the rope heater.

My do-it-spiffy gene wanted me to make a metal cover for the unit but my that’s-good-enough-for-now-maybe-someday gene overrode the urge. A cover would prevent spillage of the lava rock in handling the unit, and protect the rock from any condensate dripping on it. If, or when, I put a cover on the unit it could be buried in the ground.

Two-bit Guru | Plant Heaters | Photo of a cold frame with the completed plant heater installed.

The sustainable me has raised an eyebrow at using electricity to heat up the cold frame, but the two-bit wizard in me says it’s only 75 watts. The two-bit guru in me says okay.