Fig. 1. These peppers are grown in Dutch buckets filled with a lightweight expanded clay aggregate.
Photo: Christopher J. Currey

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.

Fig. 2. Most peppers grown commercially have two stems that are trained vertically, like these plants growing in stone wool slabs.
Photo: Christopher J. Currey

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.

Fig. 3. The damage on the “shoulders” of this pepper fruit is called sunscald and can occur when both air temperatures and light intensity are too high.
Photo: Christopher J. Currey

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.