Genetics: Microgreens are not classified by the species of plant produced; the term is simply a classification based on the stage the plant is harvested. Unlike leafy greens and herbs that are grown to finished-sized plants, when these species are grown as a microgreen they are harvested when their first true leaf is unfolded. Any species can be grown as a microgreen; production is based simply on the species having a desirable flavor and/or color. Many of the most popular microgreen species are in the mustard family (Brassicaceae), including arugula (Eruca sativa), radish (Raphanus raphanistrum subsp. sativus), mustard (Brassica juncea), kale (Brassica oleracea), and watercress (Nasturtium officinale). Other leafy greens such as Swiss chard (Beta vulgaris) are also used. Many culinary herb species are also popular as microgreens as well, such as basil (Ocimum basilicum), dill (Anethum graveolens) and cilantro (Coriandrum sativum).

Production systems: Microgreens can be successfully grown in a variety of production systems. One of the most common systems is very similar to the nutrient-film technique (NFT) system used to grow lettuce and herbs; the primary difference between those systems used for lettuce and herbs to those used for microgreens is the size of the growing channel. Channels used for microgreens are two to three times wider than the traditional 4-inch-wide NFT channel (Fig. 1). Another system that works well for growing microgreens is marketed for producing hydroponic fodder for livestock. The primary difference between traditional NFT systems and fodder systems is that there are multiple layers on the fodder system, while there is a single layer of channels on the regular NFT systems. These hydroponic systems utilize a fiber mat that comes in rolls or even burlap as the growing substrate that seed is sown into and grown on. Microgreens can also be grown in containers such as large open packs or flats filled with soilless peat or coconut coir-based substrates.

Propagation and young plant production: For microgreens production, propagation is essentially the entire production cycle, as they are harvested when the first true leaf is developed. All microgreens come from seed; there are no vegetatively propagated microgreens. An important consideration when sowing seed for microgreen production is the seeding density or the number of seeds per unit or area. Seeding density impacts microgreens yield; as the seeding density increases, the weight per individual plant decreases due to competition among seedlings, while the total yield increases from the increased number of seedlings in each area.

If seeds need darkness for germination, flip a black tray over the top of the flat or place a polystyrene board over the channel and remove once most of the stand has germinated.

Nutrient solution: Microgreens can germinate without any fertilizer application. However, providing mineral nutrients to microgreens will increase yields. Providing 75 to 150 ppm N from a complete, balanced, water-soluble fertilizer will enhance growth, though the concentration to use depends on your production system. For sub-irrigated systems like the NFT-like channels, use the lower range, while for microgreens that are irrigated overhead you can apply concentrations on the higher end of the range.

Temperature: Microgreens can be grown across a wide range of temperatures. Much of this is due to the wide range of plant species that can be grown as microgreens. Temperatures that are commonly used in greenhouse and controlled environment crop production work well, such as 68 to 72° F. While some species, such as many of the species from the Brassica family, can be grown at cool temperatures, the time it takes for seedlings to reach the marketable stage increases if grown too cool; warmer temperatures help your crops finish quickly and increase the number of crop turns in your facility.

Light: As evident when grown in multi-tiered hydroponic fodder systems, microgreens can be grown under low-light conditions since the seeds have stored energy they can use to germinate. However, microgreens will grow more and increase yields as light intensity increases. For greenhouse producers, supplemental light will only be required in the late fall, winter and early spring, when light is limiting. If you are growing in a controlled environment with no sunlight and rely on artificial lighting, it is not a matter of when you are going to provide light, but rather how much (the instantaneous light intensity).

CO₂: Like other food crops grown in greenhouses and controlled environments, microgreens are responsive to carbon dioxide (CO₂) concentrations. Supplemental CO₂ should be used in greenhouses when outside temperatures are cooler and venting is reduced, and can also be used indoors when necessary.

Pollination: Since culinary herbs are produced for foliage, not fruit, no flower pollination is required. Some herbs are produced for their seed, but they are produced in the field, not in controlled environments.

Pruning and training: Microgreens do not require pruning or training.

Pests & Physiological disorders: Due to the short crop time for microgreens, there are few severe pests or physiological disorders.

Diseases: The most significant disease in microgreens production is damping off of recently germinated seedlings. Seed can be sterilized prior to planting to minimize disease incidences.

Harvesting: Microgreens can be marketed in several different ways, either cut from the growing substrate and placed into a bag or container, or marketed while still growing in substrate. For microgreens that are sold “live” and attached to the substrate, the trays or growing units are simply collected from the greenhouse and, depending on whether they are sold wholesale or retail, they may be placed in decorative containers, then placed in plastic or cardboard carriers for transport and display (Fig. 2). For microgreens grown in loose soilless substrate and being harvested for sale, care must be taken so the loose substrate does not get mixed in with the microgreens shoots during harvest.

Postharvest care: While cut microgreens are protected when placed in a plastic clamshell, they are tender, and bagged microgreens are susceptible to bruising. For microgreens marketed in bags you should leave ample air that increases the size of the bag, like a brand-new bag of chips, to protect the fragile shoots. To maximize the shelf life of cut microgreens, place them in a refrigerator to keep them cool. It is difficult to recommend a single acceptable storage temperature because of the variety of species grown.

Most species grown will do best in temperatures in the mid-30s down to just above 32° F; basil is the most notable species sensitive to cold temperatures and should not be stored below 50° F.

Christopher (ccurrey@iastate.edu) is an assistant professor of horticulture in the Department of Horticulture at Iowa State University.

May 2017. A Virginia greenhouse recalled 10 lines of packaged produce due to the potential presence of metal as a result of construction at the greenhouse farm.

October 2017. A New York greenhouse recalled assorted greens due to potential presence of E. coli. 

April 2018. A Canadian greenhouse recalled all its microgreen products due to potential contamination of Listeria monocytogenes.

In each of these incidents, the recall was made out of an abundance of caution when it was discovered that the produce may have been compromised, as no illnesses or injuries were known to have been caused by the contaminations. And for that, the greenhouses are to be commended for discovering the issues and recalling product before consumers were impacted — because any of these could have turned out differently.

For instance, take the two leafy green E. coli O157:H7 outbreaks in the U.S. earlier this year. The first, which began in late 2017 and was determined to be closely related genetically to an outbreak in Canada, caused 25 infections in 15 states including nine hospitalizations and one death. There was no product recall in the U.S., however, because the source was never determined. Although the Public Health Agency of Canada eventually identified romaine lettuce as the source of that outbreak, the U.S. was able to state only that the likely source of the outbreak appeared to be leafy greens. This was compounded by the fact that the outbreak was being reported by news channels well before FDA or CDC made any announcements or recommendations for consumption or avoidance of produce.

The second was the E. coli-contaminated romaine lettuce, which resulted in 210 reported infections, including 96 hospitalizations and five deaths. While the source was eventually traced to the Yuma, Arizona, region and contaminated canal water implicated in the outbreak, no single farm was identified. The traceback took nearly four months from the onset of the first illness. In neither of the incidents was a specific grower or supply chain facility identified as the source. But the longer it takes to recall product, trace an outbreak and communicate with consumers, the more the entire industry segment is impacted.

This is where the food producer comes into play. While it is the federal agencies that are tasked with tracing contaminated product back to its source, it is the food producer’s responsibility to conduct a timely recall and provide the relevant agency with enough information to enable traceback to the source and trace forward to the point of sale to ensure all customers and consumers are notified. That is not — yet — a legal responsibility. However, rarely does it benefit anyone for any facility to implement only the required minimum in food safety. Rather the more proactive each grower, producer and retailer is, the better will both the consumer and the industry be protected.

While consumer safety should always be the primary objective, it is important to understand the impact a recall can have on the entire segment in which it operates. As reported by Nielsen, the week the news broke (April 14) on the second E. coli contamination:

• Romaine lettuce sales fell by 20 percent.
• Iceberg sales were down 19 percent.
• Red leaf sales were down 16 percent.
• Endive sales fell 17 percent.

Lessons learned and shared

As evidenced by the recalls discussed, food contamination can be caused by internal, external or unknown factors. Things can happen that one can’t control and doesn’t expect. But, said BrightFarms CEO Paul Lightfoot in a recent Hort Report podcast, “You have to have the SOPs [standard operating procedures] and the cultural factors in place and be ready for these unexpected sources of problems so that you can reduce the likelihood that they are going to happen in the future.”

The podcast, also featuring AeroFarms CEO David Rosenberg, focused primarily on a new controlled-environment agriculture coalition, but Lightfoot also discussed BrightFarms’ Virginia and New York recalls.

In both cases, the issue was caused by third-party suppliers. Both were detected and a voluntary recall initiated by BrightFarms, with no detection by retailers, consumers or government. “In both cases, our voluntary testing protocols were successful and identified the problems before there was any risk of problems to consumers that were buying our products at our retailers,” Lightfoot said.

Although AeroFarms has not had a recall, Rosenberg noted his agreement with Lightfoot and the crucial importance, especially when working with big retailers, of “making sure one builds the SOPs in which the response time can be fast,” he said. “Of our about 250 SOPs, half are related to food safety.” These include areas such as holding mock recalls so everyone knows what to do if one should occur, reducing risk from a sanitation standpoint and ensuring that SOPs and design go through the right stage gates.

As also evidenced by the BrightFarms recalls in which the cause was linked to third-party suppliers, such analyses and preparation needs to include that of external conditions as well. One example Rosenberg gave was that of air pollution for urban operations. Greenhouses need to have protocols in place for understanding what’s going on outside and putting in the right filtration (such as HEPA filters) to minimize the effect to the plants, he said.

“It was our recognizing that it was sort of ‘black swan’ things and uncontrollable things happening that made us realize that we should start sharing what we’ve learned with the rest of the industry to reduce the chances that, God forbid, someone would be made ill in the future by someone in the industry,” Lightfoot said.

Hear more about the growers’ sharing of experiences through the CEA coalition here.

Very unique greens are growing right now in a greenhouse in Hixton, Wisconsin.

These greens are distinctive in several ways — first and foremost, they float on a rather large water tank of 850,000 gallons. Indeed, they are located at what the company says is the largest aquaponics facility in the world and utilize salmon waste in their growth. They are also certified organic.

Superior Fresh has been growing various greens and Atlantic salmon since 2017 with an eventual projected greenhouse harvest of 1.8 million pounds and fish harvest of 40,000 pounds per year. The greenhouse is quite large (123,000 square feet with more than 1,100 LED lights) and employs 40 people, including the adjacent packaging facility.

Baby romaine, spring mix, head lettuce, red leaf lettuce, swiss chard, bibb and green romaine are currently being marketed by Superior Fresh to various retailers, schools, hospitals and restaurants between Minneapolis and Chicago. “Our goal is to keep everything within 400 miles of our farm so that our customers get the freshest product possible,” says chief operating officer Brandon Gottsacker. “We currently package both washed and raw product in plastic clamshells, bags and wholesale cases.”

Semi-decoupled system

Some aquaponics systems have a setup wherein the plants grow in floating beds right in the fish tanks, but Superior Fresh uses what it calls a semi-decoupled system. The system has interconnected but separate greenhouse and aquaculture operations, with the same water usually flowing through both, then cleaned and recirculated. Gottsacker says there are several benefits to this system. “It allows us to have a large-scale, high-production environment for completely different organisms, fish and plants,” he says. “In addition, water parameters ranging from temperature to nitrates and micronutrients are measured and adjusted as needed, but in our setup, we have the option to completely decouple the systems. That is, we can circulate the water continuously or control the greenhouse environment independently from the fish system but still utilize all the available nutrients from fish waste.” The plants are grown in standard floating foam boards, which Gottsacker and his team are optimistic will last for years.

Superior Fresh head grower Adam Shinners says running an aquaponics greenhouse in Wisconsin had initial challenges with fine-tuning the nutrients, handling pest and disease pressure and also managing light, but at this point, he and his team have had many months to optimize processes. The goal, as it is in any greenhouse, is to achieve a consistent environment to ensure consistent product, but it’s obviously more complex due to the fish factor. Shinners had had to ensure that the nutrient mass balance between fish waste load and plant uptake is accurate, consult with pest and diseases experts to ensure good management practices minimize risk, and work closely with Superior Fresh’s LED supplier to help understand the best lighting strategy, “with plant yield, energy consumption and plant morphology in mind.”

The pests that Superior Fresh greens have had to contend with include mites, thrips and aphids, all of which must be controlled using beneficial insects in order to maintain organic product certification. “We are currently using Swirski mites and Chrysopa as our beneficial insect of choice to control for thrips and aphids,” Shinners says. “This practice is seasonal and pressure-dependent. When the pressure is low to undetectable during the winter months, the usage is discontinued. When pressure increases during the summer months, using these beneficials seems to control the population of these pests very well.” With all that water in the greenhouse, mildew is another challenge, but it’s being addressed well with precise humidity control and the use of preventative biological fungicides. These are sprayed through an atomizer, Shinners explains, ensuring good leaf coverage while using a minimal amount of product.

Production challenges

At the same time Superior Fresh has handled the basics of greenhouse production, the greenhouse team is also working to ensure the right varieties are grown at the right times of year to maximize quality and yield. “We have been testing varieties for over a year now, so we are slowly collecting data as to which varieties we should grow at certain times,” Shinners says. “We offset the natural decrease in biomass during the winter by injecting carbon dioxide into the air and extending the photoperiod to 20 hours using our LEDs.” The LED spectrum provided is suited for greens production: blue and red wavelengths, at a rate of 12 to 14 mols, depending on how much sunlight is available on a given day.

Water for the greens is drawn from two wells on the property, flowing first to the fish and then to the greens, with 99 percent recirculation. The remaining 1 percent is removed along with fish waste and used to irrigate alfalfa fields onsite. All fish processing waste and greenhouse waste is also composted onsite and applied to these same fields. An ozone system is used to treat the water, but only unwanted bacteria are targeted. “Beneficial bacteria are a very important piece of the puzzle when integrating fish and plants and we do not want to kill them,” Gottsacker explains. “The ozone helps flocculate the solids, which helps keep the water free of unwanted or unnecessary bacteria.”

The nutrient level from the fish waste is very consistent and now displaces about half the fertilizer that would otherwise be required on an ongoing basis. Initially, however, Shinners says the fish waste was not high enough in macronutrients to meet the demands of the greens. This has changed as the fish matured and feeding rates have increased, and some micronutrients are added into the greenhouse fertigation system.

In terms of other hurdles, Gottsacker says one of the biggest has been, “trying to get buyers and/or distributers to understand what we are offering. … Many buyers have a difficult time changing their conventional ways. All of our customers have been incredibly responsive to this futuristic style of growing and truly enjoy coming to see the farm and all the foods that we are producing. The hardest part is getting the attention of buyers and getting them to make a change as well.” He adds that because salmon are a coldwater fish species that require very good water quality, keeping that in check “requires constant monitoring and is incredibly challenging.”

Gottsacker gives huge credit to the staff. “Everyone here puts in farmer’s hours when needed no matter what day of the year it is,” he says. People with farming mentality were hard to find, but today we have a great group of people onsite every day that do a great job keeping our fish and plants thriving.”

He adds that if Superior Fresh only focused on a few varieties of lettuces, it could hit its goal of 1.8 million pounds a year very quickly. However, Gottsacker explains that “we as a team have the vision to grow what people want. … This vision comes with some production volume sacrifices but shows our customers that we can grow them what they want.” He notes that standalone aquaculture systems, especially for salmonids, take two or more years to see any cashflow, but by farming both fish and plants, Superior Fresh was “able to alleviate the long lead time to positive cashflow with produce sales early on in the development of the business.”

Looking forward, Gottsacker says that “as people begin to recognize the positive benefits that comes along with growing food the way we do, they become life-long customers.”

Treena is an Ontario-based national and international award-winning writer who focuses on science, technology and industry issues.

This article series is from the Resource Management in Commercial Greenhouse Production Multistate Research Project.

Home gardening of edibles in indoor environments is an important urban trend and a growing market for companies supplying plants and growing systems. This article discusses research at the University of Florida (UF) focused on irrigation, nutrition, lighting, plant material and consumer preference that support “micro-scale” urban agriculture.

Irrigation

Two main growing system options for homegrown edibles are hydroponics or container growing in a root substrate. In hydroponics, the reservoir size is a key consideration. Larger reservoirs require more nutrient solution for replacements, an increased physical footprint in homes, are more difficult to move due to increased weight and have higher costs. However, if the reservoir is too small, more frequent nutrient solution replacements may be needed. For example, in greenhouse trials with hydroponics conducted at UF, four basil plants grown in a 2-gallon reservoir for eight weeks required water replenishment every other day, which is clearly a burden for most home growers. In contrast, a single tomato plant in a 6-gallon reservoir only required weekly top-ups (Fig. 1).

With container production, water and nutrient supply are determined by the growing substrate, container size and water reservoir. Even though a substrate that dries out quickly during the production phase may be easier to grow into a saleable plant, you are not doing the customer any benefit if their plants dry and wilt quickly. Water-holding additives such as gels are options, but the most water you can possibly fit into a pot is limited by the container volume and the substrate water-holding capacity (typically about 65 to 80 percent of the container volume). This water reserve can be greatly increased by providing a reservoir.

Nutrients

Water quality, and the rate at which water and nutrients are taken up by plants, is likely to vary between home growing locations, and most home growers are unlikely to measure pH or electrical conductivity. Therefore, it is important to use a simple and resilient approach to fertilization. Solution pH is likely to drift up or down in hydroponic nutrient solutions, and an iron chelate such as iron-DTPA or iron-EDDHA is needed that remains soluble at high solution pH. Micronutrients should be provided at a higher concentration than in container production because of less efficient root systems and the lack of substrate components such as peat that also provide micronutrients at low pH. Any essential plant nutrient can become limiting, but nitrogen and phosphorus often drive growth rate, whereas magnesium, iron and manganese greatly affect leaf greenness, and calcium deficiency leads to tip burn in leafy greens and blossom end rot in fruit.

An approach to simplify hydroponic fertilization is to provide nutrients in the form of controlled-release fertilizer (CRF) in the transplant you provide to the customer, with the reservoir primarily providing water. A growing substrate provides both nutrient and pH buffering, and is less prone to drift than a nutrient solution.

With container production, performance for the consumer is likely to be improved if the grower provides residual fertilizer that releases over time in the home environment (Fig. 2). Growers can select from several strategies to provide residual fertilizer depending on cost and feasibility for your production system:

• Apply a high concentration of liquid fertilizer immediately before sale. This is cheap, easy to apply and improves performance for a few weeks.
• Top-dress with CRF immediately before sale. This requires extra labor, but CRF dosage equipment can reduce application time. Use a product that will not fall out of the container during shipping.
• Incorporate enough residual CRF to last beyond the production phase. Run trials to ensure that nutrient concentrations are not too high early in production.
• Use a double-coated CRF that begins to release during the consumer phase.

Lighting

In an indoor environment, photosynthetically active radiation (PAR) is a common limiting factor to plant growth. In commercial production, the goal is to maximize economic productivity, with recommended light levels for lettuce and tomatoes exceeding 15 moles·m^-2·day^-1 of accumulated PAR. However, it may be unrealistic to achieve this light level in a home environment. In addition, the goal of many indoor home growers is to have a continuous production with an acceptable plant performance and enough yield to enjoy at mealtime, rather than a costly beacon in the living room blasting out purple light through the night.

We ran trials with red-leaf lettuce to determine the minimum light required to grow leafy greens indoors, and recommend at least 6 moles·m^-2·day-¹ to support active growth and acceptable appearance of leafy greens (Fig. 3). Fruiting crops require more light than leafy greens, but more research is needed to evaluate this minimum light threshold.

Light quality is another important factor to consider when growing plants indoors — blue light is particularly important for anthocyanin pigmentation in leaves. “White” (i.e., phosphor-coated blue) LEDs or fluorescent lamps provide adequate blue light for leafy greens, but lack the far-red light required by long-day flowering plants. As more research is undertaken, light recipes for different plant types will be improved.

Plant material

Breeding companies are releasing an exciting array of compact and attractive edible plant varieties that are suitable for indoor home production. Two attributes that improve consumer satisfaction are: (a) attractive foliage under low light, which provide interest before flowers and fruit development; and (b) resistance to calcium deficiencies (e.g., tip burn for leafy greens, blossom end rot for tomatoes and peppers) and other disorders.

Plant preparation before sale is also important. For example, rather than expecting a home grower to successfully grow a tomato or pepper crop indoors from start to finish, providing a nearly harvestable plant that is already bearing green fruit may ensure a successful harvest.

Consumer preference

Home growers have different goals when they produce an edible plant product. The two types of gardens typically grown, depending on the home grower’s desired outcome, are:

Hobby gardens — Growers are motivated by the pleasure of growing their own food from start to finish, possibly even building their own hydroponic systems.

Cooking-type gardens — Consumers want a constant supply of high-quality fresh produce from either a plant that has already been grown and is ready for harvest or a plant that has been pre-finished and will last one to two weeks and will then be disposed of.

The worst outcome is for consumers to come away with guilt and shame from having a black thumb. If a plant dies, many will consider it their own fault and turn to other alternatives to acquire produce or spend their disposable income. Considering irrigation, nutrients, light, plant material and consumer goals will help ensure a successful gardening experience.

Acknowledgements: Financial support was received from the USDA National Institute of Food and Agriculture, Multistate Research Project NE-1335: Resource Management in Commercial Greenhouse Production. We thank USDA-ARS Floriculture and Nursery Research Initiative #58-3607-8-725, the Scotts Co., and industry partners of the Floriculture Research Alliance at the University of Florida (floriculturealliance.org) for supporting this research.

Paul and Celina are professors, Elisa is an MS student, and Maria is a biological scientist at the Environmental Horticulture Dept of the University of Florida.