Improved Productivity in Winter Greenhouse

Final Report for FNC06-642

Project Type: Farmer/Rancher
Funds awarded in 2006: $1,565.00
Projected End Date: 12/31/2008
Region: North Central
State: Minnesota
Project Coordinator:
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Project Information


Garden Goddess Produce is a small CSA (Community Supported Agriculture) specializing in winter delivery. We grow storage crops in the summer to combine with fresh produce grown all winter in a passive solar greenhouse.

I utilize organic production methods including IPM (Integrated Pest Management) for controlling insect infestations in the greenhouse.


GOALS: First, I wanted to find out if a raised bed with a soil heating system would improve production in a passive solar greenhouse. I wanted to know if the added expense and maintenance of a heated raised bed would be offset by added productivity.

Second, I wanted to test various greens varieties to find out how well they would produce during my most challenging production period; the three weeks before and after winter solstice (when days are shortest).

PROCESS: It's important to note for other producers that this research was conducted in a cool winter greenhouse. Produce varieties raised in that structure are selected for their cold hardiness and ability to grow even during short day length periods. The passive solar structure has as its thermal mass a 1.5 foot bed of large gravel below the structure. That gravel bed has perforated drainage tile laid down in a grid through the gravel bed. The tile is connected to PVC pipe that goes up the east and west walls of the greenhouse to the peak, where the PVC pipes connect with black stove pipes which run the length of the peak.

Thermometers in the PVC pipe trigger small fans to kick in at a certain temperature. The fans pull the hot air at the peak of the greenhouse down through the pipe and into the drainage tile in the gravel bed. This method of storing thermal mass below the structure uses heat that would otherwise be wasted at the peak of the structure to heat it from below. The added benefit of the system is that it also warms the soil. The greenhouse has an insulated cement block foundation that goes down 4 feet, protecting the soil inside from frost. This structural feature is outlined here to explain why the structure by itself keeps soil temperature around 55-65 degrees even in mid-winter. The question our research posed is whether warming the soil temperature to a consistent 70-72 degrees would result in increased production that would more than offset the cost of the heating system.

To test the heated raised bed, we selected a low cost, easy to install (and remove) raised bed material from FarmTek, a company well-known by most sustainable agriculture farmers. They offer a raised bed that is made of their heavy duty pond liner. It is approximately 1 foot high, designed for a 4 foot wide bed and comes in several lengths. We chose 40 feet; a length that would allow us to create a raised bed in two of our ground beds. This material was easy to install (it comes with metal stakes and post driver), and so was the wire cloth and heating coil that were set into it for heating the soil).

I wanted to have a soil material that matched the composition of our existing raised beds (which are approx. 5 inches tall). In those beds, I have equal amounts of black dirt, compost and peat, along with a mineral fertilizer (equal parts) of blood meal, rock phosphate and green sand. We were surprised at the amount of added soil material required to fill it. More needed to be purchased than was originally estimated.

The wire cloth and heat coils were placed about 4 inches below soil surface in the lined bed so that the heat would be close to the root zone of the crops. Transplants were placed in this bed and in our normal beds that were seeded in the same flats and transplanted at the same time. Three crops were tested: broccoli, pac choi and chinese cabbage. Soil thermometers were used to keep track of the soil temperature in both types of raised beds. These were checked and recorded 3 times a week. All plants were watered at the same time, in the same amount. The goal was to limit the variables between planted beds to just soil temperature.

For the second component of our winter production research, we tested new varieties of greens for their growth capabilities during the three weeks before and after solstice (December 21), when day length is shortest. Again, to explain growing methods, we utilize planters for greens that are hung in harnesses in the greenhouse to make full use of the valuable growing space in that structure. Planters are made from 3.3 foot lengths of plastic rain gutter, with end caps and holes drilled in the bottom. These planters are filled with a soil mix, set on germination mats and seeded with many varieties of greens to be harvested at a size of 3 to 4 inches for salad mixes. Our propagation area has room for 8 planters to be seeded at a given time and this is done weekly, as older plantings wear out and need to be replaced. When seeds germinate, the planters are placed in slings that hang from the rafters. Each sling holds three planters. The top one is at about 5 feet from the floor. The second is at about 3.5 feet from the floor and the bottom one is at about 2 feet. There are two rows of these slings that run the length of the greenhouse, which doubles the growing space available in the greenhouse.

The trick to maximizing production of greens (which are very popular with customers), is to select varieties that are not phototropically sensitive to short day length. Some plants will simply not grow during the short days of solstice while others continue to do so. It is important for anyone growing in winter to be aware of the most successful greens varieties so their planting schedule can reflect the need to get these varieties started at a time when they will become the primary source of salad greens during the solstice weeks. We selected 6 new varieties to test during this time. We seeded two new varieties along with our usual selections each week and noted the growth rates of the new varieties along with the familiar ones. The same soil mix and water were used for all the planters.

PEOPLE: The only person who assisted with this research was my business partner, and husband, Chuck Waibel. He constructed the raised bed and installed the heating coil. He also helped with the monitoring of soil and air temperatures.

The results from the soil temperature experiment were surprising. I anticipated that we would see some improvement in production in the heated bed but wondered how that would balance out with the cost of the materials and added electricity for the heat coils. To my surprise, plants in our regular raised beds actually out-produced those in the heated raised bed. In fact, during the third week of harvest, the Chinese cabbage from the regular beds were twice as tall and weighed twice the amount of those in the heated soil bed. Pac choi was the crop with the least difference in size and weight. While the choi in the regular beds was somewhat larger, the weights were not as different. The broccoli in the heated bed produced two weeks later than that in regular beds and the plants themselves were 6 inches shorter.

Clearly, the crops we grow in raised beds in a cool winter greenhouse prefer to have cooler soil. These are, after all, crops selected to prefer cool conditions and their response to increased soil temperature indicates that this attempt to improve growing conditions is unnecessary and counter-productive. We have since removed the wire cloth and heating coil from the raised bed with liner, as crops do better in that system without added soil heat.

The system that we constructed certainly did the job it was intended to do. The soil in that raised bed stayed around 70 degrees consistently. Soil temperatures fluctuated more in the natural raised beds because the thermal mass system is affected by air temperature in the greenhouse, which reflects cloud cover and temperatures outdoors. This heated raised bed system could be more effectively used in other situations. I have read that plants can produce well with cooler air temperatures when soil temperature is raised. This probably works better with fruiting crops like tomatoes and peppers. A producer using a hoophouse or low tunnel might see impressive results with a raised bed system like we constructed.

In our case, I wanted to know if the design we created for a very specific purpose (minimal carbon footprint for cool weather crops grown in winter greenhouse structure) was getting the most out of the crops being grown. It appears to do just that, so the addition of a raised bed with heated soil is not only an unnecessary expense, it is also counterproductive.

As to the tested greens varieties for the weeks around solstice, one stood out above the rest. We discovered that Tokyo Bekana (a Chinese Brassica greens variety) was far and away the best solstice variety to grow in mid-winter. It has a light green, frilly leaf that looks much like lettuce but heftier and with more flavor. It grows quickly even during short, cloudy days in mid-winter. It now features prominently in the plantings at that time and is a consistent grower with no problems and gives at least 4 to 6 harvests before it's spent. Another good variety discovered at this time was Ruby Streaks another Brassica but with deeply lobed leaves and an attractive maroon color. It looks like mizuna but does not have the strong flavor of a mustard green. However, it did not grow as quickly as some others that perform well during solstice so it is now included in the fall and post-solstice plantings when it grows a little more quickly and adds attractive color and shape to the salad mix. Yukina Savoy was another successful variety. It too is a Brassica and grew larger than the variety it has now replaced, tatsoi. Yukina had a larger leaf size than tatsoi, a more oval, less spoon shaped leaf, and was harvest size one week ahead of the tatsoi. This kind of growing capability in mid-winter is critical for maximizing production. Green Wave was another variety tried, but it was unimpressive in mid-winter. It is slow to bolt, however so it works well in the early months of spring when the greenhouse gets heated up by warmer temperatrues outdoors. Ho Mi Z was another Brassica that did well enough to keep growing in mid-winter. It's not as productive as Tokyo Bekana but is a full oval leaf shape and after the first cutting, it grows back fairly quickly and often gives more than four harvests before needing to be replaced.

This experiment taught me that it is necessary, interesting and rewarding to try new greens varieties every year. I continue to do this, starting them out in mid-winter trials and adjusting their planting schedules according to the outcome of that first experiment. This helps me constantly improve my planting schedule but so far, none of the varieties have been able to top Tokyo Bekana. It's the champ.

I was glad to find out that the design features selected for our greenhouse are effective and efficient. This research proved that additional expenses to heat soil in raised beds are not necessary, but the materials we used to find that out have good possible uses in other circumstances. I was also gratified to find out some new and successful varieties of greens for mid-winter and that discovery sparked a desire to continue testing new greens. Salad greens are the most popular produce we grow. I want to keep customers happy with this product and our CSA surveys indicate that they love the amount of variety in our salad mix.

Our passive solar winter greenhouse is a unique structure that has received much attention since we began production in that facility in 2004. We have given many workshops and presented at many conferences in Minnesota, North and South Dakota, Wisconsin, Kansas and Canada. We also give many tours of our facility to interested visitors who are looking into building a similar structure. Because of this burgeoning interest, we wrote a book about our design features and production methods that was published in 2009, The Northland Winter Greenhouse. At all our speaking events, workshops and tours, we tell people about our SARE research and its outcome. When speaking at conferences, we tell farmers about the SARE research grants for farmers/ranchers and encourage them to use this resource to answer their own questions about production.

The Northland Winter Greenhouse, sold out its first printing of 1,000 and will see a reprint of 2,000 in March, 2011. Pages 93-94 and 102-103 mention our SARE grant research and outcomes. At our website,, we encourage growers to contact us with their questions and I mention when applicable the results of our SARE research. The number of people who have toured our facility, saw the pond-liner raised bed and heard the story about the research numbers total well over 400. The handouts that I offer when we do conference presentations include contact info for SARE and I do talk about the results of our research because it's important to share that information with others who are considering how to build their own similar facility. I've also done at least a dozen radio and newspaper interviews since we did our SARE research and have mentioned our results in those conversations.


Participation Summary
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.