April 8 until June 10, Asheville
US (NC): Passive solar greenhouse opens doors for tours
M R Gardens Owner Megan Riley aspires to grow plants that are as sustainable as possible. Growing seedlings with practically no supplemental heat or high-powered exhaust ventilation is a big step toward that goal.
“Sustainability first inspired me to go into the agricultural field more than 15 years ago,” Riley said. “But greenhouses aren’t always energy efficient. If not designed carefully, they can use a lot of energy.”
Riley grows plants for clients of her native and edible landscape design and garden coaching services. Customers can also order vegetable, herb and flower starts through the M R Gardens website prior to the start of the season or attend plant sales on Saturdays at the greenhouse in spring and fall.
It is rare to find a passive solar greenhouse as large as M R Gardens’ 800-square-foot house, especially one used solely for a plant nursery. While growers have experimented with similar structures since at least the 1960s, most of them are backyard hobby greenhouses for producing food in the ground year-round. A few small vegetable farmers incorporate passive solar techniques in their greenhouse designs, but not commercial growers in the nursery trade.
When laypeople think of solar design, they often picture solar panels with photovoltaic cells which convert sunlight to electricity. But a passive solar greenhouse does not necessarily need these panels. The building remains warm because water in barrels store heat collected on sunny days and then emit it when outside temperatures fall.
For optimal solar gain, the south-facing roof slants at a 55-degree angle so that the bottom edge nearly reaches the ground, ending at a three-foot high kneewall. On the north, west and east sides of the building, thickly insulated walls prevent heat from escaping.
The specific angles of the building determine how much sun rays enter the building at various times of the year. When deciding the specifications of the design, Riley considered the time of year she’d be using the greenhouse the most and what type of vegetation she’d be growing.
In the spring, the greenhouse remains within the range of 55 to 85, which is ideal for the summer transplants Riley is growing at that time. In the winter, it rarely drops lower than 50 degrees, suitable for the native perennials, herbs and spring vegetable starts in the greenhouse then.
The structure is also designed so that it doesn’t overheat on late spring and summer days. As days lengthen and the sun is high in the sky, it moves above the slant of the roof, and less direct rays enter the house. Vents along the sides, front kneewall and roof peak ensure adequate air circulation.
The water barrels not only emit heat during cool nights but also regulate the temperature on hot days. “It’s like living next to a body of water, where the temperature fluctuations are less extreme,” Riley said.
The 2017 winter offered ample opportunity to put the design to the test. On unseasonably warm 60 to 70 degree days, the greenhouse remained below 90 degrees as long as all the vents were opened at sun-up. The greenhouse also stayed sufficiently warm during short, cloudy, cold days of early January. The five inches of snow on January 7 was proceeded by several overcast days with little sun to warm the barrels. Yet even when it dropped to 20 outside, the minimum temperature inside at night was 45 degrees. These results were repeated the following night when the outside thermometer dropped below 10 degrees.
Heating a conventional greenhouse costs $4 to $5 per square foot in the course of the winter. That equals approximately $4,000 per year for a building of similar square footage. That number does not include the large expense of powering exhaust fans that are typically on the ends of greenhouses. Over time savings in these energy costs make a passive solar design economically feasible.
While sustainability is the primary goal of the project, Riley has found that the greenhouse has other benefits. “It’s an ideal environment for seedlings,” said Riley, who has been growing seedlings for sale since 2011. “The plants grow faster and are an even higher quality than past years.” She explains several theories why it’s such a good growing space during greenhouse tours.
Please RSVP by contacting megan@mrgardens.net or 828.333.4151. Tours are offered at a sliding scale with a minimum of $5 per person. For more information, see www.mrgardens.net.