Contributed from Garden and Greenhouse Magazine
by Michelle Moore
Keeping Accurate Records is the Key
Just a few miles upriver from the self-proclaimed “windsurfing capitol of the world” lies the small town of Dallesport, Washington. When you head East along the Columbia River, the landscape changes quickly from thick forests to open land and rock. The small town, population 1200, boasts incredible views of Mt. Hood and the scenic Columbia River Gorge. Just a few miles from its greener and more temperate neighbors, Dallesport is a climate of extremes. Winter temperatures dip into the single digits, and summers hit 100° plus. Frequent winds of 55 to 60 miles per hour race through the gorge, although those winds are silent during the heat waves. This area of the world possesses a stark and striking beauty and enjoys sunny skies for 8 months of the year. It’s also home to one of the only greenhouse growers in the Gorge.
Jeff Dearborn and Karen Schwartz of J & K Growers began constructing their greenhouse in the fall nearly four years ago. Initially, the couple planned to cover their two-inch galvanized frame with six mil poly sheeting. After using double poly in their previous greenhouse, they planned to get by for a while with this method. The plan worked…for about 30 days. The first heavy winds of about 55-mile per hour gusts ripped the plastic from the frame. Dearborn and Schwartz quickly decided they required a more durable glazing. Having investigated and used various products including: glass, corrugated single-wall polycarbonate, double-wall polycarbonate and Solexx, they choose to use a combination of borrowed, donated and purchased materials.
The Setup
This spacious structure is light, bright and warm. Plywood and insulation cover the northern wall of the 4000 square foot greenhouse for heat retention. The East and West facing end walls are covered with “borrowed” double-walled polycarbonate. The Southern walls are covered with single-wall corrugated polycarbonate. Some of the Southern wall has a layer of 6 mil poly over the polycarbonate to reduce the heat loss. The roof of the structure is covered with 5mm Solexx. The sidewalls are roughly 11 feet high with the roof peaking at 20 feet. The greenhouse is 33 feet from side to side and approximately 140 feet long. Exhaust fans are located on the end walls, and circulation fans are placed throughout the structure. The structure does not have roof vents and does not use cooling devices.
The winter greenhouse is heated with a combination of propane and hot water running through the propagation benches when warm weather crops are grown. The propane is used only when growing warm crops tomatoes. Cool crops require far less energy and heat providing a more sustainable winter harvest. Metal Halide and High Pressure Sodium grow lights provide supplemental light for crops requiring longer day lengths, but are used only if soil temperatures are high enough to support growth. All benches are placed at a comfortable working height and designed to be accessed and moved by forklift. Metal posts imbedded in concrete provide support for the benches and rows are configured to allow easy access. Planters consist of heavy plastic sheeting framed by wood. The plastic is filled with a mixture of coco fiber and pumice. Synthetic stings threaded through drainage holes direct excess water out of the planters onto the gravel flooring. Heating costs are reduced by running warm water through tubing placed in the benches rather than along the bottom. Water running through tubing in the growing media doesn’t need to be as hot as water along the bottom of the bench but is much more efficient. Drip irrigation delivers nutrients directly to the plants as needed.
Indoor Climate
Karen Schwartz keeps extensive records. She knows the high and low temperatures and what the outside temperature will do to the inside greenhouse environment. She knows that on a winter day, the inside temperature will be roughly 80°. The night time low in the greenhouse this year was 20°, which is too cool for fast growth but warm enough to keep everything alive. Growing a warm crop, such as tomatoes, requires more heat than some things like spinach, lettuce or broccoli. These cool crops are economical and sustainable to grow until late spring.
When the sun is out from March through October, temperatures increase substantially. Since the greenhouse does not have any cooling mechanisms in a hot climate, the greenhouse can get very hot in the summer. The highest temperature recorded was 122°. Although that is extremely hot, Dearborn and Schwartz determined the heat is not a problem in their greenhouse because the light coming through the Solexx roof is diffuse rather than direct. According to Dearborn, as long as plants have water, they do not burn. Dearborn believes minimizing temperature swings and creating a more stable environment is more important than controlling the temperature.
Another factor contributing to a consistent environment is the greenhouse insulation. The foam insulation on the northern wall provides a heat sink which both minimizes heat loss as well as reduces summer heating. The insulated Solexx roof also helps to maintain consistent temperatures. Dearborn and Schwartz report a significant savings from the insulation. They are able to maintain an indoor temperature 20° greater than the outdoor temperature without added heat.
During a research trip to Holland, Dearborn discovered the average heat loss from a glass greenhouse was 1 BTU per square foot per degree of temperature differential. A BTU is a British Thermal Unit, which is simply a measure of heat output. Most heaters have a BTU rating which is used to calculate the size of heater needed for a space with a given temperature target. Dearborn explained a 20° outside temperature and a 40° inside temperature would have a total heat loss of 20 BTU’s per hour per square foot. J & K Growers’ greenhouse has a heat loss of ½ to 1/3 BTU per hour resulting in significant savings over glass and other coverings. They have also discovered their corrugated polycarbonate has significantly more heat loss than glass. The corrugation designed to scatter light increases the surface area of the material by 40% and heat loss or retention is directly related to surface area.
Dearborn and Schwartz also report significant growth differences with diffused light. Plants growing within six feet of the clear sidewalls have decreased growth. This is attributed partially to the greater degree of temperature changes but also from direct versus indirect light. Plants located six feet or more from the sidewall do not have shadows or intense direct light. Schwartz routinely measures the plant growth and reports the results are consistent and significant regardless of the crop planted.
J & K Growers began growing strawberries, then added tomatoes to their offerings. Local restaurants and farmer’s markets gobbled up their heirloom crops and asked for more. This year Dearborn and Schwarz are growing a wide variety of vegetables. Their winter crops include arugula, peas, beans, spinach, lettuce, kohlrabi, broccoli and cauliflower.
After nearly four years of tracking and record keeping, J& K growers is ready to expand. They are adding three more greenhouses and will have 18,000 square feet of greenhouse space. They partly attribute their success to their detailed record keeping. They learn from trial and error and record their mistakes to make sure they don’t repeat them. By tracking detailed information they are able to see patterns develop which helps them do more with less. J & K Growers are not strictly organic, but they are pesticide free. They grow heirloom seeds for flavor rather than maximum productivity, and they pride themselves on high quality produce.
While most people won’t start a commercial growing operation, there are many opportunities to learn from the commercial growers. Try new things. Track your progress, keep records and don’t be afraid to fail. Mistakes lead to some happy discoveries.
Michelle Moore is the General Manager of the Greenhouse Catalog where she has worked with greenhouses for 20 years. Michelle writes and lectures about greenhouse gardening as an Oregon State University Master Gardener and member of Garden Writers Association. Michelle is a Fulbright Scholar and earned an International MBA from Thunderbird, School of Global Management. Michelle and her husband live in Oregon, but they enjoy hints of summer all year in their greenhouse! You can contact Michelle at: Michelle.Moore@GreenhouseCatalog.comThis e-mail address is being protected from spam bots, you need JavaScript enabled to view it or www.GreenhouseCatalog.com.
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