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…

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?

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.

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.