Exploring Low-Tech Possibilities for Heating Hoophouses with Compost

Proposal funded December, 2008
Report submitted September, 2010

My original objective with this project was to explore possibilities for using compost heat to boost soil temperatures in an unheated hoophouse, and to study whether I could increase yields, improve crop quality and hasten spring harvest dates for a number of floral crops.

In a nutshell, the answer for some crops is yes, yes and yes. However, the amount of labor output required to get results by far cancelled out any economic gains I may have achieved with this experiment. I would not recommend any farm to repeat my steps, but some interesting results were achieved.

Materials and Methods

After much trial and error, I discovered that an eight-and-a-half yard compost pile was necessary for the project. Each new pile, built from fish scrap, farm debris and sawdust, regularly heated to a temperature of 160F, and would hold above 130 degrees for 2-3 weeks before requiring a turning. At the time of constructing each pile, I would snake 250 feet of hot water hose through the pile, being careful to maintain an uninterrupted area of approximately 2½’ X 5’ X3’ where the pile could keep its core heat.

Powered by a 2.2 amp/270 watt in-line pond pump, water circulated through the hot water hose where it would pick up heat from the compost pile, exiting the pile at an average temperature of 85 degrees. The heated water would then disperse through a network of ½” pvc piping buried 1½” below soil surface and running 1’ apart, parallel in the study area. Finally the water would return at a trickle to a 50 gallon insulated rain barrel where it would again recirculate. Rate of flow was .665 gpm.

Again through trial and error, I learned to run the system just ten hours each day so that the pile could recover its lost heat. I chose to start running the system during the early morning hours of each day, figuring that plants might best benefit from soil heat during daylight hours. The only exception to this pattern was during two periods of arctic weather when I ran the system at night instead to protect the crops and keep my system from freezing up.

It was difficult to measure soil temperature with the simple thermometers that I had, but on average I observed a temperature difference while the system was running of 2-3 degrees between the heated area and the control throughout the duration of my trials.

A 300 square foot area was planted with eight trial crops. Adjacent to that, a control area of equal size was planted with the same crop schedule, but the soil was not heated.

Crops trialed were campanula Champion series (seeded early August and transplanted into study area 10/15/09), ranunculus ‘Gigi White’ (presoaked and planted 11/27/09), stock ‘Cheerful Yellow’ and ‘Cheerful Midseason Yellow’ (seeded late November and transplanted 1/13/10) , godetia ‘Grace Salmon’ (seeded late November and transplanted 1/13/10), sweet pea Winter Elegance Mix (seeded 11/13/10 and transplanted 12/25/10), lupine assorted varieties (seeded early August and transplanted into hoop 9/2/09), and anenome ‘Galilee White’ (presoaked and planted 12/12/09).

Trials received soil heat from 11/8/09 to 3/6/10, a period of four months. In addition, both the trial and the control areas received daylight interruption via fluorescent shop lights from 4:30 p.m. to 10:30 p.m. each day.


My results varied hugely with different crops. When the first arctic weather of the year hit in early December, air temperatures in the hoop plummeted to 15 degrees. I lost all ranunculus during that event. I harvested the first anemones from the trial bed on 2/20/10 and that crop came into production two weeks ahead of the control. There was no discernable difference in yield or crop quality on the anemones.

The stock and godetia crops in both areas performed the same. There were no differences in harvest time, yield or crop quality. Stock bloomed from mid April to mid May. The godetia came on a little later than the stock and bloomed into June. Sweet peas in the trial area came into production two weeks sooner (first bloom on 3/30/10) than the control (first bloom 4/12/10), with no difference in crop quality or overall yield. The lupine crop got every kind of mildew and languished in both the trial and control areas, eventually offering up a few blooms at exactly the same time as our field crop lupines started producing.

The Champion campanula provided my most startling results. First stems of white and blue were harvested from the heated area on 4/12/10 with an average main stem height of 25-1/2″. Side stems had an average height of 23″. First stems of white and blue were harvested from the control area one week later with an average main stem height of 20″. Side branches from the plants in the control area were minimal and too short to bunch and sell for premium price. Because we could harvest and sell almost every side branch, we achieved at least four times the salable yield from the plants in the area with heated soil and it was a far superior crop. Of note: the campanula was the earliest crop planted besides lupine.


All told, I made over forty yards of compost for this experiment. Because flexible hose was snaked through the pile, I had to build and unbuild each pile by hand, a total of ten hours for each cycle. Benefits: a totally buff body in the winter, lots of great, high quality compost to spread around the farm and a few square feet of top quality campanula.

Costs: about 100 hours of labor to set up the system and make enough compost to keep it running.

I learned that just a few degrees difference in soil temperature can cause dramatic changes in plant behavior. In order to achieve economic success heating soil with compost, a more efficient method of extraction would need to be engineered.

This grant was supported by the ASCFG Research Fund.
To see how you can apply for an ASCFG Grower Grant, go to www.ascfg.org and click on Research Activities.

When I wrote for this grant, my goal was to create a farm-friendly system for harnessing compost heat. My work has been read with interest by at least one engineering firm seeking ways to collaborate with the agricultural sector. Below are excerpts from correspondence with Donald R Flett, P.E., Flett Associates, Environmental Engineers and Scientists, Toms River, New Jersey.

I heard about your research efforts on line at the Specialty Flowers web site. Your report is very interesting and helpful in my design of small, community based, decentralized wastewater management systems.

The three goals of my small decentralized wastewater management systems are: environmental, economic and energy neutrality. I call it the “E3 Principal”. In other words, the system should have no negative environmental effects, use no fossil fuel derived energy and must economically cost the tax payers nothing (seriously). Melding my wastewater treatment and disposal system with community based agricultural activity allows me to effectively start approaching these ambitious, but logical, goals.

The biosolids from our wastewater facility can be composted to produce heat for young plants, and as a soil improving amendment. We also plan to use anaerobic digestion of organic materials to produce biogas. This biogas can fire a combined heat and power (CHP) unit to produce electric and heat energy. We can provide CO2 from this unit for growing operations. We will use the captured exhaust heat for winter heating and summer cold storage (by means of absorption chiller).

There are many other collaborative activities and more are being developed all the time. I believe you are making a useful contribution to this effort.