How to Heat a Greenhouse With Compost

Organic decomposition generates heat. Publications document that American gardeners have been harnessing that heat to jump-start their gardens since the turn of the 19th century, and probably long before that. Recent experiments have shown both the promise and pitfalls of harnessing compost heat on a large scale. There are many methods and theories, ranging from incredibly simple to enormously complex.

The directions for building a hot bed using manure compost for heat printed in "The Domestic Encyclopedia"--published in 1803--are virtually identical to the instructions presented in the University of Missouri Extension Service's publication on the subject updated in 2008. Summarizing both, excavate for the hot bed 24 to 36 inches deep. Fill the bottom with 18 inches of horse manure mixed with straw bedding at a 2-to-1 ratio. Top with 6 to 8 inches of soil. Place the hot bed frame and sash over the prepared bed. Pack soil and/or manure all around the frame to insulate it. Let it cook off for a few days before planting, and you're ready to start growing.

It's hard to criticize a method that's been in practice for no less than 200 years, but controlling the heat generation is tricky, and you're limited to about five or six weeks of heat without digging everything out and replacing the manure mix. In more temperate parts of the country, that's more than enough time to get a big head start on the first tomato contest, but in the northern tier, you probably need more time.

In the broad span of years when this method was most popular, horse manure wasn't just plentiful, it was everywhere, all the time, and the use of oil and gas to generate heat was unheard of or in its infancy. As horsepower switched from horses to internal combustion and petroleum products became cheap and abundant, commercial growers in particular switched from labor intensive methods to the sure thing of forced air heat for their greenhouses.

Escalating energy costs have commercial growers desperately seeking alternatives to forced air propane heat. Home gardeners also find it hard to justify heating a greenhouse based on the amount they can save growing their own produce. At the same time, there's a growing interest in creating and using compost as a soil amendment.

For a commercial greenhouse, the biggest concern when contemplating compost heat is a steady, plentiful supply of compost feed stock with minimal transportation costs. The greenhouse almost needs to be next door to a dairy or a large stable, or part of the same farm. Large-scale projects usually involve indirect heating rather than heating the growing beds directly with the manure compost. The composting process generates ammonia and carbon dioxide, both of which benefit plant growth in small doses, but not in the volume generated by the amount of compost necessary to make enough heat for a large commercial greenhouse. Running water coils through the compost, then through a radiator in the greenhouse, is one approach. Another is to run sealed air ducts through the compost and into the greenhouse.

Directly heating a hot bed or the growing beds in a small hoop house with compost is certainly not a new idea. It may be labor-intensive, but if you have access to a sufficient quantity of manure feed stock, the labor will likely be your only cost. An article from the gardening monthly "The Cultivator," published in 1845, presented an interesting design to overcome the short duration of the heat cycle. It suggests that you build the foundation of the starting bed with brick or stone and install doors at the two ends. Place a metal grid shelf above the composting manure and create the planting bed on top of the shelf. Now you can can rake out the finished compost and restock the "heater" with fresh manure without disrupting the planting bed. That's an idea that seems so logical, but may have come to light too late in the first life cycle of the practice.