Testing the Compost-Heated Greenhouse in an Urban Setting

Final Report for FNE12-739

Project Type: Farmer
Funds awarded in 2012: $5,042.00
Projected End Date: 12/31/2014
Region: Northeast
State: New York
Project Leader:
Jesica Clark
South Pine Street City Farm
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Project Information


The purpose of this project was to translate the method of a compost-heated greenhouse to an urban setting by taking advantage of free materials produced in the waste stream of the city and using them as the basis for compost to directly heat the planting substrate and the surrounding air in a 12’x25’ greenhouse.  Over the fall, winter and spring seasons between 2012 and 2013 we used three different compost mixtures, piled them in cinderblock bays inside of the greenhouse and measured the temperatures of the active compost itself, the soil above it, and the air temperature both inside and outside of the greenhouse.  We measured the level of volatile ammonia and carbon dioxide produced by the composting mixture to determine whether it was safe to have around growing plants and its progress during composting.  The finished compost was tested for its usefulness and safety as a soil amendment.  We grew both winter crops and spring seedlings in the greenhouse to test the compost’s ability to keep the various plants warm and register the value of the produce and seedlings.  I found that while the greenhouse air temperature was positively affected by the compost, raising it an average of 10 degrees above the outside air temperature, this was sometimes not enough to keep it above freezing.  Fortunately, the soil temperatures were often at 50 degrees or above, keeping the winter produce from enduring any frost damage.  Spring seedlings were started as early as they would have been in a fuel-heated greenhouse and again while the air temperatures early on dipped below freezing, the hardy seedlings were not frost damaged and all seedlings were healthy and on-time for field plantings.  Gross sales for the winter produce were $150 and the gross retail value of the seedlings was $1300.  The compost produced during the project tested well and would make a good soil amendment.  Overall, I believe a compost-heated greenhouse would be a valuable addition to an urban gardening community for its contribution in both efficient growing space and area for community involvement, as long as it were placed in a safe location and improvements made for winter produce profitability.


Farm Profile
South Pine Street City Farm (SPSCF or South Pine) is a quarter-acre urban garden located in Kingston, NY.  Consisting mostly of permanent raised beds, the farm grows mixed vegetables for sale at farmers markets and an on-site farm stand. The farm is also connected to the urban community through educational events, collaboration with other organizations and urban farms in the city, and an “open gate” policy where any visitors to the farm are welcome to walk the grounds and enjoy the green space.  In early 2014, the farm was incorporated into a new urban farming venture, Revival City Farms, increasing its connectivity with other urban gardens.  The greenhouse for this project is located at the Kingston YMCA, for both space and community development reasons.

I, Jesica Clark was until recently the owner of South Pine Street City Farm and I currently live in Kingston, NY.  I have been working in organic and/or sustainable agriculture for the last 11 years, most recently as Assistant Farm Manager at Poughkeepsie Farm Project, another urban farm.  For this project I had an assistant role filled by two people: first Elena Batt and then KayCee Wimbish who ended up taking on the growing responsibilities for SPSCF during the 2013 season and also starting an urban farm at the YMCA.  Both Elena and KayCee assisted with coffee grounds pickup, filling and emptying compost bays, and some data recording.  My technical advisor was Teresa Rusinek, Senior Extension Educator at Cornell Cooperative Extension in Ulster County.  She helped me site my greenhouse, identified fungal and disease issues and referred me to a compost expert at Cornell for help with interpreting my compost data.  This project would not have been completed without the help of my assistants and advisors.

Project Objectives:

This project had goals that fell into two categories, agricultural and community oriented.

Our agricultural goals were to translate and refine the idea of a compost heated greenhouse to an urban setting, allowing urban farmers to grow winter produce for sale as well as increase their ability to grow their own seedlings while producing a usable soil amendment in the form of finished compost.  By closing loops and using materials efficiently, especially waste materials, small urban farms have a better chance at turning a profit.

One of our community goals was to reduce food waste by diverting it to produce heat and ultimately nourish new crops.  In addition, a shared compost-heated, urban greenhouse space allows both the maintenance and the benefits to be divided among different communities such as urban farms, community gardens and school or youth gardens.  Another benefit was the relationships developed between the different coffee businesses, the municipality that provided the brown materials, and the gardens.


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  • Teresa Rusinek


Materials and methods:

Greenhouse and Compost Bay Creation
The greenhouse used in this project was purchased outside of the project funding and erected in the early-September of 2012.  It was actually a 12’x25’ temporary shed frame that we covered with a single layer of plastic, installed a door and window for ventilation, and anchored to the ground with ground screws.  The compost bays were created with 8”x8”x16” cinderblocks and the amount of space within each bay was 42”wide x 32” deep x 32” high, which holds a little less than a cubic yard, 24.9 cubic feet.  In all there were six bays in the greenhouse, three on each long side.  The blocks were not held together with anything and were free to be removed to ease loading and unloading of materials.

Compost Maintenance and Greenhouse Planting

Over the course of the project each bay was filled and emptied twice, using three different mixtures.  Unfortunately the availability of materials was different than what I initially expected, so we were not able to test all three mixtures at the same time.  The make-up of our mixtures was also different than expected because we were not actually able to get dried leaves until late winter between 2012 and 2013.  We ended up first using a mixture of 1 to 1 by volume ratio of used coffee grounds to chipped yard waste to fill all six bays, then switching to a 2 to 1 ratio with more coffee grounds to refill three bays in mid-December 2012 because it seemed like my initial mix had a too high carbon to nitrogen ratio.  Also, as winter started to take hold, the compost was not heating up as much as I wanted it to.  When we were able to gather dried leaves in March 2013, we used them in a 2 to 1 ratio as well to refill the last three bays. 

“Brown” compost ingredients where gathered from the municipality at the transfer station and stockpiled at the farm.  The coffee grounds were gathered weekly or twice a week depending on the business and were used close to the date they were collected.  During the winter, the coffee grounds would freeze in the buckets and would have to thaw in the greenhouse for a day before being used to fill the bays.  The bays were filled by layering, depending on the ratio, three or six 5-gallon buckets of coffee grounds with three buckets of either chipped yard wasted or dried leaves.  These created relatively thin layers in the bays and in effect mixed the two ingredients.  Often the grounds were wet enough to support biological activity in the compost, but if the materials seemed dry we would also water the compost as we were filling the bay.  Empty buckets were rinsed and returned to the coffee houses at the next pick up. When a compost bay had cooled down, the materials were emptied and piled outdoors to mature into finished compost, and the compost bay was refilled.

From late-September 2012 to March 2013, the greenhouse was used to extend the season and grow winter produce.  During this time the bays were not filled all the way up, leaving space for a layer of soil on top, separated from the compost by a layer of ground cloth or weed barrier.  The soil was then either directly seeded with salad greens, or was transplanted into with chard, lettuce or spinach.  Starting in February 2013, the emptied bays were filled to the brim with new compost ingredients and then a bread tray was laid over it to hold seedling trays above the compost.  The seedlings were germinated at the grower’s home in a germination chamber before being transferred to the greenhouse.  Seedlings where held in the greenhouse until they were big enough to be planted outdoors and we used the greenhouse for seedling production until July.  All the seedlings for the farm were grown in the greenhouse, so everything from artichokes to zucchini benefited from the compost heat.

Data Collection
Data collection was an important part of the process throughout the project.  We recorded the volumes of compost materials and the dates as they went into the compost bays.  High and low air temperatures were recorded daily from the first filling of the compost bays in the fall of 2012 until the night time temperatures were regularly well above freezing in spring of 2013.  At the beginning we used a data logger to track the air temperature, but in early November it was stolen, and from then on we used a regular hi-lo thermometer.  The temperatures of the compost and the growing medium for both winter grown produce and seedlings were recorded weekly using a regular compost thermometer.  For each ratio of compost, I tested the materials in one of the bays throughout the composting process for the levels of volatized carbon dioxide and ammonia that they were producing as they composted.  For these tests I used the Solvita test for compost maturity from Woods End Laboratory.  Lastly, for each ratio, the finished compost piles were tested at a soil and compost lab to analyze their utility as a soil amendment.

During the project I also noted the health of the crops, the work hours that went into building and maintaining the greenhouse compost piles, the income from sold winter crops and the expense saved from growing our own seedlings.

Research results and discussion:

Horticultural Outcomes
Tables 1 and 2 show my raw data and notes for compost temp, soil temp, air temps, and Solvita test results.

Document 3 contains two graphs pulled from the data in Table 1, showing the temperatures of the different composts and the soil situations in Bay 1 as an example.  From these graphs we can see that the soil temperatures are held quite high when in contact with the compost, up until the point when the compost is cool.  When the soil is not in direct contact, as is the case for the seedlings, the soil temperature is cooler, but still above 40 degrees Fahrenheit and within a comfortable growing temperature for the plants.  The compost piles themselves reached respectable peak temperatures of 115 to 130 degrees within a couple of weeks of being built and held a warmer temperature for several weeks without being turned or actively aerated. 

Document 4 is a graph of the daily low air temperatures, both inside the greenhouse and outside the greenhouse.  The low temperatures give us an idea of how well the compost is heating the air inside the greenhouse without the heat gain from the sun.  The air temperature inside the greenhouse seems to be slightly warmed by the rising heat of the compost, a little over 10 degrees Fahrenheit on average, which was sometimes not enough to keep the inside air temp above freezing.  Considering that frost tolerant, but more tender plants such as chard and lettuce were able to survive the winter with no visible frost damage, it seems like the higher soil temperature combine with some extra protection from the wind was enough to keep plants healthy through the winter.  Some of the early seedlings experienced light frost air temperatures in the greenhouse, but they too survived the early spring and thrived throughout the season.  We were able to start seedlings in the compost-heated greenhouse as early as we would have put them in a fuel-heated greenhouse.  The only plants that did not do well were arugula crops that I seeded in the late fall. This might have been due to older seed or nutrient or salinity problems in the soil as they looked yellowed but not frost damaged.

The Solvita tests charts in Document 5 show that our plant-based compost had very little volatized ammonia while remaining active.  High levels of ammonia can “burn” plants, damaging their roots and leaves, and this is sometimes an issue with manure-based compost.  From these results, it seems the compost ingredients could have an even higher nitrogen ratio without causing significant harm to the greenhouse plants while composting.  Refer to the Solvita Test Manual in Document 6 for more information about the test and how to interpret results.  The results from the Agricultural Analytical Services Laboratory at Penn State University of the finished composts are in Documents 7, 8 and 9.  Note thatthe sample in Document 9 was actually the leaf and coffee grounds mixture, contrary to what the sample title says.  A few important things to note from the tests are that for all three samples the pH is almost neutral, the soluble salt concentration is very low and the C:N ratio is less than 20.  All of these qualities make for useful finished compost that will not harm plants and contribute usable nitrogen as it mineralizes.

For pictures of the greenhouse and what was growing, please see the PDF of my PowerPoint presentation in the Outreach section.

Financial Outcomes

Our financial goals for the project were to increase income through winter produce sales and decrease expenses through raising our own seedlings instead of buying them in or renting a greenhouse outside of the city.  Although the winter crops in our greenhouse seemed to grow well and I had no trouble selling them at our farmers market, the sheer scale of the greenhouse was just too small to turn a meaningful profit.  Gross sales from the greenhouse produce totaled $150 for the winter.

On the other hand, growing seedlings in our own greenhouse proved to be a great cost-savings.  Even in such a small greenhouse we were able to produce all our own seedlings with room to spare.  In total, the gross retail value of the seedlings we grew in the greenhouse was $1300, assuming a $0.50 average retail price per seedling. 

The only materials expenses besides the greenhouse itself and growing materials were for cinderblocks ($300 for a pallet delivered) and buckets to collect coffee grounds ($45 for 12 food safe 5-gallon buckets).  The biggest expense was labor; 60 working hours were spent over the course of the project to collect materials and maintain compost piles, but in a community or urban setting, these tasks could be delegated to volunteers or others sharing the greenhouse and if the greenhouse were only used for seedling growing, the work hours would be cut in half.

Community Impacts
In every urban garden the community impacts and influences matter almost as much as the agricultural and financial.  Our greenhouse space was able to be shared with the YMCA community gardeners as well as the YMCA youth gardeners so that everyone could take advantage of a warm, sunny space to grow seedlings.  These greenhouse participants contributed by helping to repair damage from vandalism, taking on some of the watering duties and ultimately helped move the greenhouse to its new location inside a fenced area.  This coming season, two urban farms will be sharing the space as well as the compost maintenance.

Waste reduction was another positive community impact driven by the greenhouse.  In total, almost 2500 gallons of waste were diverted from the waste stream and instead were used to both provide heat and nourishment for plants in future seasons.  The owners of both coffee houses were happy to have their staff separate used coffee grounds into the buckets and the staff appreciated not having to put the heavy, wet grounds into garbage bags for disposal.  There was a bit of a training period to ensure that staff did not accidentally put plastic or other non-compostable items in the buckets or also accidentally put the filled buckets out for trash collection.  After a few weeks, everyone got into the rhythm of the bucket pickups and there were few issues.  Collection of yard waste and leaf materials was easy, free, and able to be done any time the transfer station was open.

Unfortunately, there were also a couple of negative outcomes from siting this project in a densely populated area.  Vandalism in the form of ripped greenhouse coverings was a major issue throughout the winter, reducing the effectiveness of the greenhouse and also possibly affecting air temperature data for the project.  When our data logger was stolen, that increased the amount of work needed to gather data for the project and reduced the accuracy and range of that data.  Ultimately the greenhouse cover had to be replaced twice and we moved the greenhouse into a more secure space at the YMCA.

Research conclusions:

Following my presentation of the project at the Northeast Organic Farmers Association-New York (NOFA-NY) winter conference, I received several emails from urban, and rural, farmers saying that they were creating their own compost-heated greenhouses and asking for further information on my process.  I have also passed down my presentation to my current farm apprentices who are interested in creating compost-heated greenhouses on their future urban farms.

Participation Summary

Education & Outreach Activities and Participation Summary

Participation Summary:

Education/outreach description:

This project was able to appeal both to garden scale producers as well as small scale farmers, so my outreach was quite varied.  Early on in the project I was able to give tours to both the YMCA  gardeners and participants in a NOFA-NY field day event.  In June 2013 I gave a tour of the greenhouse in action to members of the Hudson Valley Garden Association as a follow up to a web article I wrote about the greenhouse, shown in Document 1 and at http://www.hvgardenjournal.com/2013/01/31/heat-a-greenhouse-with-compost-even-in-the-city-2/ .  In January of 2014 I gave a workshop on how to create an urban, compost-heated greenhouse and the SARE project at the NOFA-NY winter conference in Saratoga Springs.  The workshop was well received and as mentioned earlier I had many inquiries from farmers wanting to create their own greenhouse after the conference.  My project was featured in an article in the Winter 2014 issue of Small Farms Quarterly, put out by the Cornell Small Farms Program.  The article is on page 6 in Document 2 and on the web here: http://smallfarms.cornell.edu/2014/01/14/local-compost-materials-heat-community-greenhouse/ . A few inquiries also followed the publication of that article.  This summer I plan to present my workshop through a gardening workshop series in Poughkeepsie and possibly through a Cornell Cooperative Extension event.  Initially I planned to create a section of my farm’s website dedicated to the project and my process of creating the greenhouse, but transitions in growers for the farm and ultimately the transfer of South Pine to a new farmer and a new business means that the website was in a constant state of flux.  To make up for it I have created in depth notes within the PowerPoint presentation I created for the NOFA workshop (Document 3) and will be making it all available at both the website of the new business, Revival City Farms, as well as the South Pine Street City Farm website.

Project Outcomes

Assessment of Project Approach and Areas of Further Study:

Potential Contributions

My attempt at creating an urban compost-heated greenhouse was successful enough that I can recommend it to other urban farmers who would like to heat their greenhouse without fossil fuels.  It is a useful way to enable otherwise limited urban growers to grow their own seedlings and have a wider variety choice without the need to connect to utilities or propane tanks while also reducing waste in the community.  Seeing where my attempts were not as fruitful can also spark new ideas on how to improve the profitability of the greenhouse in winter and also how make the maintenance less labor intensive.

Future Recommendations

Considering the amount of waste we were able to divert, the fact that we were able to heat a greenhouse in a location that had no access to energy utilities and the health of our own farm-grown seedlings, I would say that this project has been a success.  The farmers at South Pine Street City Farm and the newly created Kingston YMCA Farm Project have just begun putting seedlings into the greenhouse for this season and they plan on sharing the space and maintaining it as a compost-heated greenhouse.  Now that it is moved to a more secure location, we are crossing our fingers that it will no longer be damaged by vandals.  I believe the new farmers at South Pine may consider growing produce with a higher rate of return for the winter, to increase the likeliness that it can turn a profit.  Another area they are trying to improve is aeration of the compost piles, to increase the heat production and possibly heat distribution in the greenhouse.  During the workshop at the NOFA conference, participants were inspired to come up with their own ideas for improvement on my process in terms of labor saving, winter produce profitability and increasing growing area with the same amount of compost area.  I look forward to hearing about their ideas panned out.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.