Even during the coldest winter months, tender plants and tiny seedlings can be toasty warm outside if they’re tucked into a hot bed. Like a miniature greenhouse, a hot bed allows gardeners to start seeds, overwinter plants and even grow vegetables out of season.
Hot or Cold
At first glance, a hot bed and a cold frame look identical. In fact, they are identical except for one important difference -- how they’re heated. A cold frame uses only the sun’s heat to warm the plants inside, but a hot bed has an additional source of heat. Typically, electric cables snake through a hot bed’s floor to supply bottom heat that keeps plant roots warm. While cold frames are suitable for cold-season crops and marginally hardy plants, hot beds can sustain even warm-season crops and tender plants during a cold winter.
Location, Location, Location
Before building a hot bed, choose a suitable site for it to maximize its ability to protect the plants inside. Because a hot bed is typically bottomless, the soil on which it is placed should drain well. Optimally, a hot bed should be in a place that is higher than the surrounding ground so that rainwater does not puddle inside the bed. Build the hot bed so it faces south to take advantage of the sun’s heat during winter. The hot bed needs water and electricity. So a location close to your house allows for simple access to both sources, plus shorter trips for you.
The Outside Frame
A hot bed is simply a box with a higher back than front and a transparent lid. Its size can be anything suitable for your needs. The back wall, which faces north, should be 6 inches higher than the front for rainwater to run off the lid and the sun’s rays to penetrate easily. The bed's frame can be made of concrete blocks or wood, and the lid can be made of windows, plastic or polyethylene sheeting attached as a double thickness to a wooden frame. A wooden frame with an attached, hinged lid gives easier access than a non-hinged lid to the hot bed.
The Inside Floor
Before you put the frame of a hot bed in place, the site needs work. Remove the soil to a depth of 12 inches. Fill the bottom 4 inches with gravel, and on top of the gravel place a sheet of burlap. On top of the burlap, layer 2 inches of sand, and on top of the sand place heating cables that are 12 to 16 watts. The cables should be spaced, with 2 feet of linear cable for every 1 square foot of ground. Layer another 2 inches of sand on top of the cables, being careful not to cross the cables over themselves. The last layer, which fills the hole, is potting soil or a soilless mix, depending on the particular seed or plant needs for the hot bed.
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