Vegetables under plastic

Weighing the costs and benefits of plastic vegetable greenhouses over conventional vegetable production.

By Liza Lester, ESA communications officer

USDA NRCS "seasonal high tunnel"

The USDA Natural Resources Conservation Service offers “seasonal high tunnel” kits as part of a three year trial to assess the potential of the plastic houses for conserving water and soil, reducing pesticide use, and improving yields for small farmers. Credit, NRCS.

THE economic benefits, for small-holders in particular, of intensive vegetable cultivation inside plastic greenhouses have driven a rapid mushrooming of long plastic tents in farmlands worldwide – but principally in China, where they cover 3.3 million hectares and produce approximately US$60 million in produce (2008 figures). In fact, 90% of all greenhouses are in China, and less than 1% of Chinese greenhouses are glass.

Covering vegetables with hoops of plastic sheeting conserves water, binds up carbon, shrinks land use, protects against soil erosion and exhaustion, and mitigates problematic dust storms. But this change from conventional vegetable farming has harmful environmental effects as well. Jie Chang and colleagues review the current research and identify gaps in our knowledge in the February issue of ESA’s journal Frontiers in Ecology and the Environment.

Plastic greenhouses are cheap, and easy to construct from locally available materials like wood, bamboo, brick, and plastic sheeting. The Chinese government encourages construction with greenhouse-friendly policies, including credit programs to cover start-up costs. The protection from the cold extends the growing season. In the temperate north, this means farmers can have two seasons, and more than double their production. Most farms (90%) in China using plastic greenhouses are small, only 0.1-0.2 hectares, and the extra money is important to farmers with low incomes. It also makes vegetables available to more people in China, particularly during the off-season.

Because the plastic catches evaporation and channels the water back to the crop, greenhouse-raised vegetables consume less water. In Shouguang Province on the northwest coast, traditional flood irrigation of conventional vegetable fields uses 8690 cubic meters of water per hectare. Farmers can reduce demand to 7049 m3ha-1 if drip irrigation is installed. Greenhouses need only 4500 m3ha-1 of water, and can get by with only 1800 m3ha-1 using drip irrigation. The difference grants a big advantage to greenhouse farmers in areas where industrial, agricultural and urban users compete fiercely for limited water resources.

The plastic cover keeps soil and water in, and some harmful airborne pollutants out. Greenhouse farmers find it more worthwhile to cycle inedible vegetable biomass back into the fields and apply manure. Greater reliance on organic fertilizers means that the soil binds up more carbon than conventional open fields. Greenhouse vegetables need less pesticide and inorganic fertilizer per unit of production than conventionally grown vegetables.

But more crops and longer growing seasons mean more water, pesticides and fertilizers are used in total, with attendant problems of soil acidification and salinization, and water quality. Plastic waste is left behind in soils, degrading soil quality and crop yields. Disease may also be concentrated under plastic. Chinese farmers usually remove plastic covers during the high summer months to reduce the disease risk.

ResearchBlogging.orgChang, J., Wu, X., Wang, Y., Meyerson, L., Gu, B., Min, Y., Xue, H., Peng, C., & Ge, Y. (2013). Does growing vegetables in plastic greenhouses enhance regional ecosystem services beyond the food supply? Frontiers in Ecology and the Environment, 11 (1), 43-49 DOI: 10.1890/100223

Author: Liza Lester

ESA's Communications Officer came on board in the fall of 2011 after a Mass Media Science and Engineering fellowship with AAAS and a doctorate in Molecular and Cellular Biology at the University of Washington.

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  1. As an ESA member who’s also an organic farmer and user of plastic-covered greenhouses, I’d like to offer a few comments. Let me say at the outset that I think these structures are, on balance, a good thing. Our 0.1 hectare of plastic-covered greenhouses contribute $40,000/year to the gross income of our farm, producing tomatoes, cucumbers, peppers, eggplant, ginger, and basil during the warmer half of the year (with some supplemental heat during March, April, and May), and hardy salad and cooking greens during the cold half (with zero supplemental heat). This is in the generally cool, wet, maritime climate of Nova Scotia, Canada. All produce is sold directly to consumers at local farmers markets. During the winter, the only alternative for fresh greens available to local consumers are greens trucked in from California and Mexico and sold in supermarkets.

    While it’s true that some greenhouse operations are pesticide- and fertilizer-intensive, other practicable approaches do exist. In our greenhouses, pest control is achieved entirely through biological (e.g. release or conservation of predators and parasitoids) or physical (e.g. use of screens to exclude pests from the structure) methods. No pesticides, including organically-allowable ones, are used. Fertilization is achieved through applications of locally-harvested seaweed, processed fish-plant waste, and farm-produced compost.

    Looking at the question of water use, I think the most relevant metric is efficiency of use – that is, amount of food produced per unit of water consumed. While it’s true that the absolute amount of water consumed per hectare of production may be higher in greenhouses than in fields, the amount food produced in greenhouses is typically also much higher, leading to greater water use efficiency.

    Plastic residues left in soil occur when the plastic is held in place by burying the edges in the ground. In North America, the vast majority of greenhouses use plastic fastening devices which are a permanent part of the structure and are reused when the plastic is replaced (which is typically every 5 to 6 years). This eliminates the need to bury the plastic, and allows the use of slightly smaller pieces (since the buried part is eliminated).

    We definitely experience some moral qualms over the amount of plastic being used on our farm. We’ve looked at alternatives, such as greenhouses covered with tempered glass, which would be an essentially permanent covering. Unfortunately, the cost of a glass structure is about 8 times the cost of a plastic one, and the current economics of food production make such an investment impossible to cash flow. Hopefully future research will give us plastics with longer lifetimes and affordable costs.

  2. Keep your eye out on graphene.

    Looks like it will absorb UV, and extend the life of the plastic.

    They are just now figuring out how to bond it to polymers, but it appears that flowing methanol over the part, and inducing some current to flow, will work.

    Also works like a solar cell. Low voltage, but hey, it’s free byproduct.

  3. Hi David, thanks for sharing your experience in such excellent detail. Chang et al gave me the impression that water savings per production unit were quite substantial with plastic greenhouses, especially when combined with drip irrigation. Has this been your experience? Based on the numbers they present, even when doubling the growing season, some farms may be using less water in total after installing greenhouses and improved irrigation.

    Pesticide and fertilizer use was a little less clear. I suspect usage patterns may vary quite a bit and therefore be difficult to summarize. Organic fertilizers are apparently more common with greenhouses than conventional fields in China. Unfortunately, I didn’t get a chance to talk directly with the review authors for this particular post and our email exchange didn’t elaborate much on this point. The increase in pesticide and inorganic nitrogen application seemed to be proportional with increased production through extended growing seasons (and, notably, without increasing the land footprint). Since presumably the vegetables are needed, I’m not sure that is a problem with greenhouses per se.

    Your method of pest control is really interesting. Is this easier to do in an enclosed greenhouse space?

  4. Hi Liza,

    Responding to your questions, yes, the water savings from the combination of a greenhouse and drip irrigation are substantial. I’m basing this statement on experience and personal observation comparing amounts of irrigation water applied to greenhouse vs. field crops, not on actual quantitative data derived from metering irrigation water. It would be a useful project for someone to collect those data! At least part of the reason for the difference in water usage is the protection from wind that the greenhouse provides to the plants inside it; exposure to wind can greatly increase plant evapotranspiration.

    On the subject of pest control, greenhouses provide both advantages and disadvantages to the use of natural enemies to control pests. If the greenhouse provides a significant barrier to the movement of insects either in or out (if, for instance, the doors and ventilation openings are screened), this can be helpful by excluding pests and by keeping natural enemies that have been released by the greenhouse manager inside, where they are forced to concentrate their predation or parasitism on the pests in the greenhouse. The downside of a screened greenhouse is that, in addition to excluding pests, it also excludes beneficial insects such as pollinators and natural enemies that may occur naturally as part of the farm ecosystem.

    As for using natural enemies to control field pests, the big problem with releases is that the beneficial insects may simply disperse to other areas, failing to provide control in the desired area. I think the most promising approach to field pest control is to tackle it on a landscape scale, where everyone in an area collaborates on promoting natural enemies by minimizing or eliminating pesticide use and providing appropriate sources of food, water, and habitat. Of course, it’s very challenging to get this to actually happen in the real world where pesticide application is still by far the dominant approach to pest control…

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