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.Pests: Aphids are the biggest pest in hydroponic lettuce production and negatively impact crops in several ways. Because of aphid feeding, growth rates may slow down due to diminished vigor, while the sooty mold that can develop on aphid honeydew can render plants unmarketable. Whiteflies are another pest of lettuce and greens that also secrete honeydew and, subsequently, sooty mold.
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.