Answer: Here's a great bulletin from Ohio State University that gives the basics of building hot beds:
They focus on electrically heated hot beds, but you can construct one that gets its heat from decomposing organic matter. Essentially, you will build a compost heap in your cold frame. The hottest of all manures is poultry manure, but any manure will work just fine as long as it is fresh. Here's a link to an article on a traditional hot bed heated with horse manure -- kept the bed hot for a month!
Be sure you pay attention to venting the hot bed during the day. A lot of heat can build up quickly and you don't want to cook the plants. There are heat-activated automatic window openers available from Charley's Greenhouse Supplies (http://www.charleysgreenhouse.com).
Otherwise, just use the same principles as you would for a cold frame. Watch your moisture levels, cover the frame on the coldest nights with a blanket, bags of leaves or straw to keep in the heat. Keep a thermometer inside to keep an eye on things. And, be sure to harden your plants before putting them out in the garden. Good luck - it's a great way to garden!
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