In Europe and North America, eating fresh perishable produce out of season usually means hauling it in refrigerated containers from regions where it’s in season, or growing it locally in heated greenhouses.
Our large greenhouses tend to use technology developed in Holland: Huge boilers burn through prodigious amounts of natural gas, coal, wood, or other fuel to fill a network of pipes with hot water, radiating heat into vast leaky structures clad in a single pane of glass. Although these greenhouses boast extremely high yields, the amount of fuel needed to heat them generally far exceeds the amount that would be needed to haul an equivalent amount of produce from a region where it’s in season.
There are alternatives. One is the high tunnel, a simple unheated hoop structure clad in clear plastic. Small farmers in North America are increasingly building high tunnels to extend their growing season without spending a lot of money on energy-intensive technology.
Another alternative is a greenhouse design developed in northern China in the mid-1980s that relies primarily or entirely on solar energy for heat. By 2000 these greenhouses already covered about 260 thousand hectares (650 thousand acres) in northern China, and by 2004 they were supplying residents of northern China with 90% of their fresh produce in winter. That’s when Manitoba vegetable farmerWenkai Liu first brought the technology to North America. Other Manitoba growers have adopted the design since then, but, to my knowledge, it has yet to be tried in other regions of Canada or the United States.
I learned about the Chinese-style greenhouse from horticulturist Sanjun Gu, who worked down the hall from me between 2004 and 2007, before being hired as a state horticulture specialist at Lincoln University in Missouri. In the late 90s, when I was working as a pest management consultant in gigantic Dutch-style houses in southern British Columbia, Dr. Gu was working in the newly-developed Chinese-style houses at Shandong University in northern China. He showed me the picture above, taken in April, 1998, when we worked together in 2005. Those are ripe cucumbers on the vine, ready for harvest in April in Shandong. Shandong is about the same latitude as Frankfort, KY, where we have to wait until late May to get our first cucumbers from the high tunnel, and late June to start our field harvest. That caught my attention.
The Chinese greenhouse design is simple. A thick wall and partial roof on the north side act as heat sinks to absorb solar energy during the day and radiate it back into the house at night. Arched struts extend from the peak of the roof to the soil surface, up to 12 m (39 feet) south of the north wall. These are clad in a single layer of plastic. An insulating blanket rolls down over the plastic at night to retain heat.
Researchers in China and Manitoba have documented remarkable effects of these passive-solar greenhouses on indoor temperatures. A team led by G. Tong, at Shenyang Agricultural University, recently published a complex set of models that successfully predict temperature variation within a Chinese-style greenhouse from incoming solar radiation, windspeed, air exchange rates, and temperatures of sky, air, and soil.
On a chilly mid-February night, the temperature inside their greenhouse remained 10-15 °C (18-27 °F) warmer than outside. The effect was even more pronounced in Manitoba, where indoor temperature on a cold February night remained 30 °C (54 °F) warmer than outdoors. In both northern China and southern Manitoba the greenhouses kept conditions favorable for cool-season vegetable production through the depth of a very cold winter without burning fuel for heat.