Specialty Leafy Greens: Improving on an opportunity

Photo courtesy: christopher J. currey

Specialty leafy greens are leafy vegetables providing appearances, tastes, and textures beyond lettuce, while sharing similar crop times and production methods. Arugula (Eruca sativa), kale (Brassica oleracea), pak choi (Brassica rapa var. chinensis), and swiss chard (Beta vulgaris) are some of the most commonly-grown specialty greens as a result of increasing consumer interest. This article aims to provide some of the fundamental considerations for producing leafy specialty greenhouse hydroponically in greenhouses and controlled environments.

Like lettuce, specialty leafy greens are characterized by their small size and short crop times when compared to vining, fruiting crops such as tomato, pepper, and cucumber. Therefore, specialty greens are best-suited to production in recirculating water culture systems such as nutrient-film technique (NFT) and deep-water culture (DWC) or raft systems. One consideration when debating between these two systems will depend on your approach to harvesting.

For crops harvested a single time, either will work well. However, if successive harvests are planned, or “cut-and-come again,” NFT systems may be more ergonomic for employees.

Specialty green seedlings can be grown in any of the substrates commonly used for recirculating water culture systems. This includes inorganic substrates such as phenolic foam and rockwool, as well as organic substrates such as coconut coir and peat moss that have a physical or chemical binder. Kale, pak choi, and Swiss chard seed should be sown singly in each cell, whereas arugula cells should be multi-seeded. A light covering of coarse vermiculite can be used over the seed to reduce seedlings “pushing” out of the cells. While up to 200 ppm N can be applied at each irrigation to seedlings during propagation, healthy seedlings can be grown with as little as 50 or 100 ppm N.

Once seedlings are transplanted into production systems for finishing, the recirculating nutrient solution pH and EC needs to be maintained for the species being grown. For specialty leafy greens, a pH between 5.5 and 6.0 will work well. If the pH starts to get too high and limit micronutrients, interveinal chlorosis from an iron deficiency may be visible on arugula. With respect to mineral nutrient, the electrical conductivity needs to be maintained sufficiently high to meet the growth requirements of plants and produce healthy-appearing foliage. Arugula and pak choi require less fertilizer than kale and Swiss chard, with an EC between 0.5 and 2.0 mS/cm most appropriate for these species; a higher EC can reduce yields. Swiss chard growth is optimal at 2.0 mS/cm, with lower yields at lower or higher ECs. Finally, although kale growth doesn’t increase with EC above 1.0 mS/cm, a lower EC will reduce growth and potentially induce nutrient deficiency symptoms. While these specialty green species vary in their nutrient requirements, they are frequently grown together in the same system. In this case, targeting an EC around 2.0 mS/cm will be the best option across all species.

The average daily temperature (ADT) controls the rate of crop growth and development and should be managed to promote quality crop growth and minimize production time. Arugula, kale, pak choi, and Swiss chard are all cold-tolerant crops. This can tempt producers into running a cooler ADT during production. While running a cooler air temperature certainly save heating costs compared to warmer temperatures on a day-to-day basis, the cool temperatures can extend the crop time. We have found the optimal temperatures of arugula, kale, pak choi, and Swiss chard to be in the low- to mid-70s Fahrenheit. By maintaining cool night temperatures in the low 60s, combine with warmer days in the temperature range mentioned, you can promote specialty green growth and development during the day, while saving on heating at night.

A common saying for controlled-environment food crop production is “a 1% increase in light equals a 1% increase in yield.” Managing light to maximize production specialty leafy green production can be an efficient proposition. Like lettuce, specialty greens are very effective at capturing light with their leafy canopy, and the growth promotion from light translates directly to increased sales since the entire shoot is harvested and marketed. We have found growth of arugula, kale, pak choi, and Swiss chard to be promoted as the daily light integral (DLI) increases up to 18 or 20 mol·m–2·d–1; while we have not found studies on these species above these DLIs, most greenhouses are shading for temperature control at those light intensities.

While increasing the light intensity for all these species will promote plant growth, the magnitude can vary with species. For example, as light intensity increases, growth promotion of pak choi shoots can be more than 10-fold greater than the growth promotion for arugula. Manage supplemental lighting inputs to get the greatest return on your inputs and investments.

Specialty greens provide opportunities for lettuce and herb growers to expand their leafy crop production and product portfolio. This article provides the key points for producing arugula, kale, pak choi and Swiss chard and, combined with in-house trialing, should get you off and growing in no time.

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