Culinary herbs 201

Departments - Hydroponic Production Primer

Growing herbs in a controlled environment requires precise strategy to achieve the best overall quality and flavor profile.

January 21, 2022

Fig. 1. A more flavorful herb can be grown by using supplemental light sources providing 10 to 30% blue light, such as the light-emitting diodes seen here.
Photos courtesy of Christopher J. Currey

Culinary herbs, including (but not limited to) basil and mint, are some of the most popular and valuable crops grown hydroponically in controlled environments. While culinary herbs can be and are grown outdoors in fields, herbs grown in controlled environments have added value due to the potential for superior flavor. This article will highlight a few strategies to further improve the quality and flavor of culinary herbs.

The road to Flavortown

The flavors in culinary herbs that we enjoy in our cuisine are ultimately a form of self-defense, whereby plants produce secondary metabolites to make them less palatable to animals. That is why the deer eat the hosta in your garden, but leave the mint and chives alone. Altering the greenhouse environment is one way to impart some mild stress on culinary herbs grown in controlled environments, but the strategy you choose should be selected carefully.

Modifying the air temperature can certainly affect plants and their flavor. However, the average daily temperature also controls the rate of growth and development. As a result, it may be best to look beyond temperature for modifying plant flavor.

While the growth and subsequent yield of food plants in controlled environments is strongly affected by photosynthetic light, the quality of light can impact flavors independently of the quantity of light. We have found that increasing the proportion of blue light in modest proportion (i.e. up to 30% of photosynthetic light) can enhance the production of flavor compounds in culinary herbs. High-pressure sodium (HPS) lamps are the most popular supplemental light source. However, HPS lamps have a very low proportion of blue light in them.

Many of the new high-intensity light-emitting diodes (LEDs) emit both red and blue light, with the proportion varying between different lamp types.

Even a low proportion (10-15%) of blue LED light can increase secondary metabolite production compared to plants grown under HPS lights. While the concentration of flavor compounds will increase with increasing blue light, 30 to 40% is about the maximum desired proportion, as any more may suppress leaf growth and result in a yield penalty.

Mineral nutrients may also be used to potentially alter flavor. In hydroponic water-culture, the nutrient concentration is measured as and reflected by the electrical conductivity (EC). From research at Iowa State University, we have found many culinary herbs do not require a high EC (i.e. 0.5 to 1.0 mS/cm) to maximize growth and, therefore, yields.

However, there is some research supporting the use of elevated ECs during the latter part of production to enhance the flavor of culinary herbs. By providing nutrient solutions with elevated ECs, the flavor is altered due to one or two reasons: 1) elevated EC impart stress on herbs and secondary metabolite production increases in response to the stress; and/or 2) cell and, therefore, leaf expansion is diminished because the osmotic stress from the elevated EC of the nutrient solution makes it more difficult for roots to take up water.

When using elevated nutrient solutions to improve flavor, using them at the end of production — such as during the last week or few days of the production cycle — is recommended. Using supraoptimal ECs for the whole production cycle will have a greater impact on yield than a briefer and truncated exposure to high ECs.

However, this temporary phase of elevated ECs can be a challenge to implement in a commercial setting. Most culinary herbs are grown on a continually staggered basis and each stage of growth, from recently transplanted seedlings to mature plants ready for harvest. Having a nutrient-film technique (NFT) or deep-water culture (DWC) system that channels or rafts, respectively, can be moved into or a separate reservoir that can be used to provide a higher EC nutrient solution to plants nearing harvest in an NFT system.

Fig. 2. These chives have already been harvested once, and are going to be harvested at least two additional times. Foliage harvested after an initial harvest can be more flavorful and have harder growth, which may be desirable traits for your customers.

Other aspects, such as texture, also contribute to crop quality

Foliage growth can be referred to as “soft” or “hard” growth. Soft growth is what some would call lush growth. Alternatively, “hard” growth is, as the name implies, harder in texture compared to soft growth. Physiologically, the difference between soft and hard growth rates is related to the growth rate, cuticle development, and stage of growth.

When plants, the environment, and culture are optimized for growing culinary herbs, recently transplanted young plants can grow quickly. Absent of much stress, the waxy cuticle does not develop too much, which imparts a softer feel to the foliage. Alternatively, when the growing environment and culture is less than ideal and /or plants are older, slower growth and more cuticle development results in harder growth.

There is no right or wrong answer when it comes to which texture is preferred by your customers, but you should have an idea of how you can tweak culture to produce the desired texture. Harder growth can be desirable for its stronger flavor and texture, which can stand up and out in cuisine, while soft growth may provide a milder and softer texture that may be more desirable.

Providing ideal air temperatures, light intensities and mineral nutrients will promote the most rapid growth and development, as well as a single harvest well before plants begin to form flowers, will produce soft growth. While imposing some stress — whether light, temperature or mineral nutrition — may help harden growth, be mindful that it may also negatively influence total yield.

A trick to getting harder growth without changing the environment and plant culture is to plan for two or three harvests off a crop before removing it. The second and third harvests will have harder foliage than the first harvest, since the crop is getting older and approaching flowering. Don’t push this strategy too much harder, as old crops tend to get too pungent or aromatic for fresh consumption.

Culinary herbs are a valuable crop, and the techniques outlined in this article can add even more value to your crops with your customers. As with any new production practice, do some trial runs first to see how you can successfully implement these strategies in your facility, as well as how your customers perceive the results.

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