Study: Plastic greenhouses in China pose pros and cons

Dr. Laura Meyerson

Plastic-covered greenhouses are efficient low-cost structures that can boast vegetable yields plus provide other benefits but they also pose some environmental issues, according to a published study that involves a URI researcher.

Dr. Laura A. Meyerson, of the URI Department of Natural Resources Science, is one of nine researchers (most of them Chinese) who decided to study the extreme proliferation of plastic greenhouses in China where it is estimated that some 3.3 million hectares of land (about 8.3 million acres) are now covered with the structures.

Plastic greenhouses were introduced in China around 1970 and since that time they now cover a land area 100-fold. The plastic greenhouses are relatively cheap to construct and maintain than glass greenhouses which require sturdy permanent frames and sometimes foundations.

A plastic greenhouse consists of one or two layers of clear plastic sheathing stretched over a light-weight frame that be made of plastic or metal pipe and even wood. When the plastic becomes dirty or torn it is easily replaced. In China, the vast majority of plastic greenhouses are used to raise vegetables under controlled conditions. They also serve as growing season extenders.

The paper appeared in “Frontiers in Ecology and the Environment” by the Ecological Society of America.

The researchers says the Chinese account  for more than 90 percent of plastic greenhouse use worldwide and in 2008 they produced 160 million tons of vegetables in them. That yield was made possible by the fact that plastic greenhouses foster intensive cultivation and extended growing seasons especially in the colder parts of the country. The yield statistics for plastic greenhouses are far better than conventional vegetable growing in open fields, the researchers said. The paper compared the various aspects of using plastic greenhouses to raise vegetables vs. conventional field growing

The plastic greenhouses provide a number of environmental benefits, said the authors. Their use sequesters more atmospheric carbon dioxide, mainly attributed to a greater reliance on organic fertilizers (a mix of animal manure and vegetable waste).  Farmers tend to apply more organic fertilizer in plastic greenhouses because they reap greater economic returns, said the researchers.

Plastic greenhouses also help conserve water with the use of drip irrigation which puts water directly into root zones and results in less evaporation. The plastic covers help condense transpired water for reuse. Plastic houses also protect soils from erosion and help reduce dust storms, said the researchers.

On the societal side, plastic greenhouses help conserve land, increase income for farmers and can help alleviate poverty in developing countries because the houses are relatively cheap and easy to construct.

But all is not rosy with plastic greenhouses, say the authors. Such houses can lead to more carbon dioxide and nitrous oxide emissions than conventional field ground operations because more construction materials such as steel, plastics and pesticides are used. Plastic greenhouses also have a direct cooling effect as the plastic increases surface albedo and then there is the waste plastic left behind when the houses are taken down—the thin plastic does not lend itself to recycling. Plastic houses also have high humidity which can promote plant diseases and an environment that encourages insect populations, leading to the use of more pesticides.

In conclusion, the researchers said “…in order to better understand the trade-offs between ecosystem services and environmental impacts, future research priorities should focus on quantifying the net greenhouse effects of” plastic greenhouses “on climate, the economic feasibility of reducing soil salinization and acidification and the socioeconomic benefits of this agricultural practice. In doing so, we may find a ‘win-win’ strategy that promotes food production, provide other ecosystem services and improves human well-being worldwide.”