I still remember a time when the only décor a chef would accouter his plates with was parsley — at least at the restaurants I went to. I hardly gave the little green sprig a second glance as I ate the rest of my meal. But there’s been a revolution over the years, with the little green “decorations” playing a more expansive role in the foods we eat. Recently, I went to a five-course tasting at a local restaurant with a stellar reputation, and my expectations were high.

Having never had this type of meal, I wasn’t sure what to expect. Fortunately, the waiter explained the types of dishes that the chef would be preparing and the order of the plates, so we knew more or less how it would go. As the dishes came out, I noticed that, regardless of whether it was pasta, fish, meat or dessert, there were colorful microgreens, herbs or both accompanying nearly every dish. And we ate every last one; each added a spark of flavor and complemented the other elements on the plate. For their size, they certainly packed quite the punch.

Rob Laing, CEO and founder of Farm.One, is well aware of the culinary power and popularity of these tiny plants. The vertical farm has grown 580 different varieties of rare herbs, edible flowers and microgreens in approximately 1,500 square feet of growing space across two locations in the heart of New York City. Chefs appreciate that they can get fresh herbs, edible flowers and microgreens delivered to their restaurants sometimes mere minutes after being harvested, and have a much wider selection than they would if they relied solely on seasonal items. This month, we’re sharing Farm.One's story.

Also in this issue, we have the second installment of the Urban Agriculture series, which focuses on lighting in vertical farms and greenhouses. Leslie Halleck shares her take on how to clearly communicate your production methods to your customers. If you’re interested in growing lettuce, don’t miss Christopher Currey’s Hydroponic Production Primer this month. We hear from growing media guru Brian Jackson of North Carolina State University in our article on wood fiber, and food safety expert Lisa Lupo on the Produce Safety Rule. We round out our issue with growers’ perspectives on maintaining the cold chain and the latest from The Produce Moms.

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Genetics: Lettuce (Lactuca sativa) is one of the most popular food crops grown hydroponically in greenhouses and controlled environments (Fig. 1). Though iceberg is the most widely grown type of lettuce produced in fields, this is not what hydroponic growers are producing. Rather, butterhead and loose leaf are the most widely grown types of lettuce in greenhouses and controlled environments, though romaine or cos is also grown to a lesser extent. Though in limited quantities when compared to lettuce, other greens including spinach (Spinacia oleracea), kale (Brassica oleracea), bok choy or pac choi (Brassica napa), arugula or rocket (Eruca sativa), swiss chard (Beta vulgaris subsp. vulgaris; Fig. 2) are increasingly being grown hydroponically.

Production systems: Lettuce and leafy greens are most commonly grown in nutrient-film technique (NFT) or deep-flow technique (DFT) systems. These systems are best suited for producing leafy crops such as lettuce and other greens.

Propagation and young plant production: All lettuce and leafy greens are propagated by seed; no vegetative propagation is used for these species. Seedlings can be grown in a range of acceptable substrates. Stone wool and phenolic foam are the most ubiquitous substrates used for seedling production. However, coconut coir or peat moss that has been stabilized chemically with a binder or physically using a mesh wrap may also be used. Some lettuce and greens seed may be covered during germination, though this is not required; it is used to help manage moisture. Maintaining a temperature between 65 and 75° F results in good germination; air temperatures warmer than 75° F can diminish germination. Mineral nutrients should be provided by using solutions that are a ¼ to ½ the electrical conductivity (EC) of nutrient solutions used for finishing. Seedlings can be ready to transplant in 10 to 14 days from sowing for smaller plugs; larger plugs will take longer until they should be transplanted.

Nutrient solution: Different species of lettuce and greens will have different nutrient requirements. For example, the recommended EC for lettuce is around 1.4 mS/cm, whereas the recommended ECs for watercress and spinach are 1.2 and 1.8 mS/cm, respectively. The pH should be maintained between 5.4 and 6.0. For lettuce varieties that are susceptible to tip burn, be sure there is ample calcium in the nutrient solution. Since lettuce and greens are most often produced in NFT and DFT systems, the concentration of dissolved oxygen (DO) in the nutrient solution should be monitored and maintained at around 8 ppm. While some species may be tolerant of lower DO concentrations in the nutrient solution, keeping the solution well-aerated or injecting oxygen to maintain a constant and ample concentration will help promote healthy growth.

Temperature: Day temperatures range from 68 to 75° F and should never exceed 77° F, while night temperatures range from 60 to 65° F. While there are few specific guidelines for air temperatures for other leafy greens, many can be grown in the same conditions as lettuce. While many lettuce and greens will tolerate cool temperatures, the time it takes to reach harvestable sizes will be longer, which in turn reduces crop turns and costs of production. However, spinach is one green that requires cooler air temperatures for production, as warm temperatures promote bolting.

Light: Butterhead lettuce has an optimal daily light integral (DLI) requirement of 17.1 mol·m^–2·d^–1. This can be a challenge to achieve in greenhouses in northern latitudes in the late fall, winter and early spring. Supplemental lights can be very useful for boosting light intensity in the greenhouse during these times. With respect to other types of lettuce and leafy greens, the light requirements are not well-defined. However, a DLI of 12 mol·m^–2·d_–1 is a reasonable target for promoting good growth. In addition to light promoting growth, light also affects the coloration or red-leaved lettuce cultivars. The reddish pigment in lettuce leaves, anthocyanin, is not produced as much when light levels are low. Providing ample supplemental light or using blue light from light-emitting diodes can increase the coloration of the red foliage.

CO₂: Providing supplemental carbon dioxide (CO₂) can enhance the growth of lettuce, particularly in the winter months when the amount of air from outside is minimal and CO₂ is not being replenished from venting. Supplemental CO₂, whether produced from burners or injected from liquid, can be used to maintain a concentration of 1,000 to 1,500 ppm CO₂.

Pollination: Since lettuce and other greens are grown for their foliage, and not for any fruits, there is no need to manage flower pollination.

Pruning and training: Though some lettuces and greens may be harvested more than one time, no pruning is required to maintain plants. Additionally, lettuce and leafy greens are relatively short crops with no training or support needed.

Diseases: Powdery mildew and Botrytis are two of the most common diseases that affect lettuce and leafy greens. Botrytis can cause tissue to develop necrotic lesions and, as it develops, more obvious fungal growth. Powdery mildew, as the name implies, develops into white blotches on foliage. Both diseases can reduce or eliminate the marketability of crops. Good greenhouse sanitation between crops can help minimize successive incidents of powdery mildew, but Botrytis is so popular it can be a challenge to eliminate. Both diseases are promoted by high humidity as well, so greenhouses should be vented at the end of the day while the heat is one to push humidity out of the greenhouse. Pythium is another disease that can take a toll on greens, so keep a close eye on roots to look for signs of infection as this can spread quickly through the nutrient solution.

Physiological disorders: The two primary physiological disorders of lettuce and greens are tip burn and bolting. Tip burn, the necrotic tissue on the margin of leaf tips, is the result of a calcium deficiency in the foliage (Fig. 3). While this can be caused by a lack of calcium in the nutrient solution, it can also be due to a lack of calcium uptake from diminished transpiration. Avoiding high humidity, increasing light intensity with supplemental lighting or using vertical airflow fans, which blow down into lettuce heads and reduce humidity right around the foliage, can all promote transpiration and calcium uptake. The other most prevalent physiological disorder on lettuce is bolting. Bolting is when the stems in head-forming or rosette plants start to elongate in preparation for flowering. Bolting can make crops less attractive, but more importantly, it can make crops taste bitter and less palatable. Warm temperatures are the primary factor that promotes bolting. Try to keep maximum air temperatures below 80° F and, if this is not possible, harvest your crops before flowering is induced and bolting starts.

Harvesting: When you should harvest will depend on the type of crop, as well as the type of product you are harvesting. Head-forming lettuces should be harvested when a head starts to form and plants have reached sufficient size, usually 150 g (~5 oz.) to 250 g (~9 oz.). When harvesting lettuce heads, the root system can be left on the plant for a “living lettuce” that can result in longer postharvest performance. Other non-head lettuces are harvested for sale as loose leaves and sometimes multiple types are blended to make a mix. Other greens such as kale, spinach and some chard cultivars are sold as bunches, where multiple plants are combined to make a salable-sized bunch. Head lettuce and loose leaves can be sold in clamshell containers or plastic bags. Bunches of greens may be sold simply as bunches or placed inside a plastic bag.

Postharvest care: The postharvest shelf life of lettuce and greens varies with species and product. Some of the main causes of product loss in postharvest include desiccation, microbial spoilage, discoloration, off flavors and/or odors, and loss of texture/nutritional value. Most greens are very cold tolerant and should be stored just above freezing to prolong freshness. Some varieties can bruise easily, so handling lettuce should be minimized and careful.

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

Photo courtesy of The Produce Moms

Originally, The Produce Mom was Lori Taylor’s pet project at the Indianapolis Fruit Company. In her marketing position, Taylor developed the digital brand to help her then-employer improve its produce branding and play up produce’s importance in a healthy diet.

“There was not a comprehensive resource for consumers or produce industry professionals to learn about all of the different [options] in the produce department,” she says. “And I’m a mom in my real life. I wanted to learn more about the produce department, and I was hungry for a solution.”

Three years ago, however, the vision she saw for The Produce Mom became more than what the Indianapolis Fruit Company envisioned. So, Taylor bought the brand from the company and struck out on her own, eventually renaming it The Produce Moms. Below, she answers questions about the company’s evolution, its mission and the #ProduceChallenge.

Produce Grower: Why did you change the name from The Produce Mom to The Produce Moms?

Lori Taylor: It was inspired by Mark Zuckerberg’s recent mission statement update at Facebook. Facebook changed its mission statement to put an emphasis on communities to build a better world. And when I read that, I thought, ‘We are in a position where we built a community and it’s united under the passion of fresh fruits and vegetables. We are also excited about introducing our two [new] produce moms. One is Candice, who has been recognized as one of America’s top African-American food bloggers. We are also bringing in Tara, who is a fitness instructor and dietician. They represent what I am not.

PG: How have The Produce Moms been able to challenge common misconceptions about fresh produce?

LT: We did a $10 produce [buying] challenge to show people the affordability of produce with Kroger. We made it a game. I had to buy one fruit, one vegetable and one organic item [with $10] and, as the series went on, we were able to take it a step further and include one locally grown item. It was a three-month long series on Facebook Live and that series drove the point home.

PG: Back when you were still The Produce Mom, the brand was known for the produce challenge, a monthly calendar that challenges people to eat a different produce item each day. What was the inspiration behind it?

LT: Approximately four years ago, we started the produce challenge, and it was inspired by my Facebook friends sharing things like the squat challenge, the plank challenge or the sit-up challenge. It was very much inspired by the fitness industry and things fitness industry influencers were doing to have people do one specific action. Each challenge came with positive messaging, too — stuff like ‘start a new healthy habit today.’ I participated in them myself, saw them rise in popularity and on social media and thought, ‘Wow, we need that in produce. We should be encouraging people to try all of the varieties in the produce section.’

For more information, visit theproducemom.com

This interview was edited for style and clarity.

This year, the Hydroponic Production Primers columns will focus on key points for producing some of the most popular food crops grown in greenhouses and controlled environments. First up: peppers.

Genetics: The most popular type of pepper (Capsicum annuum) grown is the bell pepper, standard block fruits. In addition to the standard, large bell peppers, snacking or mini peppers are increasing in popularity. Nearly all peppers hydroponically produced in greenhouses and controlled environments are sold ripened and colored, not green. Red is, by far, the most popular color, followed by yellow and orange.

Production systems: Peppers are commonly produced in systems that are most well-suited for vine crops, including stone wool or coconut coir slabs and Dutch buckets (Fig. 1) filled with perlite. Some producers also grow peppers in large nursery cans or containers filled with a loose peat-based growing substrate.

Propagation and young plant production: Peppers are propagated by seed. They should be sown in small propagation cells or cubes for germination. Air temperature and substrate temperature should be maintained in the upper 70s (°F) for seedling germination. During propagation, a nutrient solution with a low EC around 0.5 mS/cm should be used. When transplanting seedlings into 4-inch stone wool or coconut coir blocks, the root cube can be rolled onto its side to bury part of the pepper stem; this should not be performed with grafted plants.

Nutrient solution: The nutrient solution for pepper plants can be divided into two different phases: early, and full production. Once the young plants are transplanted into their final systems, stronger fertilizer solutions (2.0-2.5 mS/cm) than those applied during seedling production are used. During this early phase after transplanting into systems, plants are establishing in production systems and vegetative growth is promoted. To promote vegetative growth during establishment, a higher concentration of nitrogen is used. Once plants are established in production systems, the nitrogen concentration can be decreased, while the concentration of potassium should be increased.

Temperature: Target air temperatures during the day for peppers is 70-73° F; while shade can be used to reduce the greenhouse temperature in the summer, the lower light intensity resulting from the shade when air temperatures are warm may result in fruits that can’t fully develop. Night temperatures from 60-64° F are conducive to the development of flowers and fruits. If night temperatures are too warm, flowers may abort from stress.

Light: During the late fall, winter, and early spring, peppers respond positively to the increased light intensity from supplemental lighting and yields can be improved. Supplemental lighting is not required during the later spring, summer, and early fall.

CO2: Peppers respond positively to supplemental carbon dioxide, both during the seedling and finishing stages. The use of supplemental CO₂ is especially beneficial during times of the year when greenhouses are not venting to replenish CO₂ concentrations in the greenhouse.

Pollination: Peppers do not require pollination for adequate fruit production. However, by pollinating the quality of fruit can be increased while production time and disorders are decreased. While both hand and bee pollination are effective, bumblebees are a more efficient choice.

Pruning and training: Peppers are most commonly grown on plants with two stems trained vertically in the greenhouse (Fig. 2). After several leaves have unfolded and expanded, pepper plants will naturally bifurcate into two stems. These stems are trained on a trellis like tomatoes and other vine crops. However, unlike tomatoes the stems are not leaned or lowered and are trained straight up. Pepper flowers form in leaf axils. The flower formed where the stem splits or bifurcates is removed, as are the flowers formed on the first leaf axil on each of the two stems; the flower on the second leaf axil or node is kept and a fruit can develop. After this, flowers are removed from every other axil/node, allowing a flower to open and fruit to form on every other node. Side shoots, or suckers, are not entirely removed from plants. Rather, they are left on plants but pruned back to leave two leaves.

Pests: The most common pest found on hydroponically grown peppers in greenhouses are aphids; the green peach aphid is the primary pest, though other aphid species can also infest peppers. Honeydew is one of the key symptoms of an aphid infestation and is problematic because it needs to be washed off fruits prior to marketing or can become a food source for sooty mold on foliage and fruits. Two-spotted spider mites are another problematic pest. Signs of mite infestations include webbing and speckling during the early stages and, as the infestation intensifies, leaves become brown and additional webbing is apparent.

Diseases: Diseases including Botrytis, Pythium and Rhizoctonia can all affect pepper crops. While pesticides can be applied, good greenhouse sanitation (removing dead and infected plant material), environmental conditions (adequate air movement, minimizing excessive humidity) and cultural practices (regular watering and fertilization) are the best way to prevent these diseases. In addition to bacteria and fungi, there are several viruses which can adversely affect peppers, including pepper mild mottle virus, tomato spotted wilt, and tobacco mosaic virus. Dipping hands in a milk solution can deactivate any virus that may be on your hands and can minimize spread between plants that are handled. Additionally, purchase seed from a reputable supplier that has been tested to be virus-free.

Physiological disorders: Flower abortion can be a common problem and can be the result of several problems. First, extreme warm temperatures can cause flowers to drop, and is most common during the summer months when outdoor temperatures are warmest. However, if the EC in the root zone gets too high, this can also cause flowers to abort. When fruits are exposed to bright and/or intense lights sunscald (Fig. 3) can occur. To avoid sunscald, steer plants in a vegetative direction to enhance leaf growth to shade the peppers. Also, leave an additional leaf or two when pruning suckers to provide more shade for fruits. Deformed fruits can result from poor pollination, especially when growing temperatures are cool. The use of bumblebees may help improve pollination and subsequent fruit growth. Finally, dark lesions that develop into necrotic tissue on the bottom of peppers results from blossom end rot, caused by a calcium deficiency. While the nutrient solution may contain insufficient calcium, it can also occur when there is sufficient calcium present, but uptake is diminished due to low light or high humidity.

Harvesting: Fruits are ready for harvesting once they have approximately 85-90 percent of their surface colored. Fruits should be removed from plants using a sharp knife; if you try to simply pull the fruit off the plants, both the fruit and plant can get damaged.

Postharvest care: Peppers should be cooled using forced air cooling. Fruits should not be hydrocooled, as the cooling solution can get inside the pepper fruit and cause a rot. Depending on how fruits are marketed, they may either be placed in cartons, clamshells, shrunk-wrapped polystyrene trays, or bags. Store peppers in a cooler at 45° F and maintain a relative humidity between 90-95 percent.

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