More often than not, E. coli outbreaks in lettuce, spinach, and other leafy greens are sourced back to a field-grown operation in Arizona or California; two regions that, combined, grow 98% (!) of the lettuces consumed in the U.S. Inherently, these fields are susceptible to the risks of contamination from animals pooping in a field (probably while munching on some salad), polluted irrigation water (also probably caused by animals – dead or defecating), and poorly enforced sanitation practices.
Even if just one lettuce plant is contaminated, the chance that other lettuces get contaminated is very high. Not only are the lettuces comingled into a single box or container in the field, but they then follow a long and diluted chain of custody route from the farm to the packing shed, off to regional distribution centers, and then trucked to grocery stores and markets, all before it ever lands on your dinner plate (and that was a condensed version). And in the case of packaged lettuce, like your favorite Caesars mix, those lettuces may have originated from 5 or more farms. That’s a hard bread crumb (or crouton) trail for the FDA to follow.
Enter Controlled Environment Agriculture, or CEA. This method of farming shelters crops from the outdoors by growing plants in greenhouses or buildings. The simplest term used – and almost lost to history – is “protected agriculture,” a term that is most appropriate in the discussion of food safety. By farming inside buildings – regardless of whether it’s made of glass, plastic, or concrete – the risk of animal contamination is all but eliminated. Risk of water pollution also practically evaporates to nothing; as typical hydroponic irrigation systems are built with filters and purification systems to remove any potential contaminants before water is delivered to the plants. Water is also delivered directly to the roots, rather than sprayed or flooded over a field; so even if the water was contaminated, it wouldn’t get on the parts we eat.
As for the chain of custody concerns, they practically disappear with CEA-grown crops. Most, if not all, of produce grown indoors is marketed under a single, specific brand, with little to no comingling of produce sourced from different locations. In fact, that would be a pretty challenging feat, considering the hyper-local market many of these farms serve. Not only do indoor and greenhouse growers brand their product, they also label where it’s grown to appeal to their local market. Indeed, during this most recent E. coli outbreak, indoor farms and greenhouses were the first lettuce producers crossed off the FDA’s list of possible offenders.
The current conversation around indoor food production is often overshadowed by the debate over energy use and whether or not it justifies the reduction in carbon footprint by eliminating long food miles. But the recent E. coli outbreak in Romaine lettuce has highlighted an often overlooked potential benefit of growing indoors: improved food safety. As consumers continue to eat more fresh produce, food safety is going to become a greater factor in their purchasing choices. And until deer or raccoons can figure out how to open doors or cut through plastic, greenhouses and vertical farms will stand as a reliable source of safe and healthy fresh produce.
Humidity control is one of the most important factors in designing and operating an indoor farm. The amount of moisture in the air (aka “humidity”) affects the transpiration rate of plants, which is responsible for moving water and nutrients from the root zone to other parts of the plant. When the humidity levels are too high or too low, transpiration will slow, inhibiting plant health, growth, and development.
There are many terms used to describe the moisture content of the air, including relative humidity (RH), absolute humidity (AH), and dewpoint temperature. In horticulture, vapor pressure deficit (VPD) is most commonly used, as it describes the relationship between water in the air and water at the surface of the leaf. VPD is the difference in water vapor pressure at a given air temperature and RH and the vapor pressure at the same air temperature when it is saturated with water (100% RH). This difference in vapor pressure is literally what “forces” water through the plant.
The majority of horticultural crops are C3 plants, which have evolved to thrive in temperate climates with a good source of water (as opposed to CAM plants, such as cactus, which evolved to thrive in hot and dry climates). Much research has been performed demonstrating that, in general, horticultural crops enjoy a VPD within the range of 0.65 to 1.25 kPa when well-watered. For vegetative plants, such as leafy greens, they tend to do better in the lower half of the range. Whereas fruiting and flowering plants, such as tomatoes and cannabis, tend to like the VPD in the higher range, especially when they are mature crops. When the VPD is lower than 0.65 kPa, stomata begin to close, restricting transpiration and CO2 assimilation. When the VPD is higher than 1.25 kPa, the plant will begin to stress and, in an effort to conserve water, will also close their stomata.
Many growers are afraid of “high” humidity levels, for fear of harboring molds and bacteria, and they will try to drive the RH as low as possible to avoid these pests. Unfortunately, this action can be counterproductive. Just like over-worked humans who have suppressed immune systems, plants that are water-stressed will be more susceptible to microscopic invaders and diseases. And just like your mom telling you to “drink lots of water” and “eat chicken soup,” the same prescription is good for plants. By properly managing the moist air environment and providing an adequate supply of water at the root zone, plants can freely absorb water and nutrients, build up their defense systems, and put more resources into carbon fixation, photosynthesis, and reproduction.
When it comes to managing humidity, plants are like Goldilocks: they like it “just right”; not too high and not too low.