Hot Beds and Cold Frames

  • Downhome Magazine
  • Posted: Feb 22, 2012 3:41 PM
Hot bed

I started gardening as a member of the 4-H club when I was growing up in St. George's on the west coast of Newfoundland. One of the first things we learned in the gardening project was how to build a hotbed or a cold frame to start new plants. These structures can be used for starting seeds, rooting cuttings and grafting plants. They are among the most useful items you can add in your garden, and they are cheap to make and to operate.

A cold frame is a structure shaped like a box with a sloping top. A hotbed is the same structure with a source of heat. The simplest frame is one made with two-inch planks, and it should be designed to be at least six inches higher in the back than in the front so that water will run off readily when it rains. The top panel of the frame needs to be removable or hinged so that it can open for ventilation on sunny days. It can be made with glass, plastic or greenhouse fiberglass. (Glass is the best covering; you can even design your hotbed or cold frame to fit discarded windows.)

Cold frame

The hotbed or cold frame should be located on the south side of your home and not shaded by trees. If this is to be a permanent location, part of the frame can be sunk into the ground to provide extra insulation. Of course, you need to provide drainage so water will not accumulate. The sides of the frame can also be materials like brick or concrete block. For a portable frame you can use lightweight plywood. If you really are not all that handy you can buy a hotbed or cold frame already built from a garden supply company.

The cold frame, if it is portable, can be used in the garden to start seeds directly in the soil, then moved around to start another crop, such as lettuce or spinach. The cold frame can also be used to protect tender plants such as tomatoes, squash and cucumbers during the early part of the season and extend the season in the fall. In the fall you can use the portable cold frame to extend the season. You can sow seed in the cold frame in September and then harvest crops like spinach, leaf lettuce, turnip greens or radish.

And it provides good storage for pots of dormant plant material through the winter. When you have a heat source in the hotbed, you can use it to root cuttings in the fall for spring planting. Another good use for the cold frame or hotbed is to protect bedding plants that you may buy early in the season.

I remember when I made the hotbed in the 4-H club, we used horse manure as a source of heat. The two feet of fresh, compacted horse manure would start to ferment and produce enough heat for a month. It was amazing and I was really impressed with the results. Nowadays horse manure is not a practical source of heat for hotbeds. Instead you can buy thermostatically controlled heating cables or heating mats. When you're starting seeds you simply place the containers on the heating mat or the heating cable that is buried in sand. You dial up the proper temperature and ventilate on sunny days. Extra protection can be provided during cold nights with rigid insulation or an insulating blanket.

When you're starting seeds it is important to have good, clean soil. I always recommend buying soil from a commercial greenhouse operation that sells the same soil as they use themselves; a lot of the soil mixes on the domestic market are very poor quality. Containers for growing plants should be used rather than putting the soil directly in the hotbed or cold frame. Watering is as much an art as it is a science. The watering technique comes from experience. Avoid watering on dull days and when you do water make sure the soil is completely wet. Make sure the containers have good drainage holes. In a week or so after the seeds germinate they should be watered with a diluted solution of fertilizer. Soluble seaweed fertilizer can give small seedlings an extra boost.

So you can see one of my first lessons in the 4-H club paid off. The hotbed and cold frame is one of the most useful structures I have had in my garden. They have stood the test of time!