Lighting up culinary herbs

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Light is undoubtedly one of the most important factors influencing culinary herb production. When we think of light, we think of light intensity, or the amount of light our crops receive. But there is more to light than intensity. This article highlights how all three aspects of light — intensity, quality and duration — affect culinary herb production in controlled environments.

The impact of light on photosynthesis and subsequent growth of culinary herbs is usually the most popular aspect of light to talk about, and for good reason. The light at plant height inside a greenhouse or indoor facility translates directly to yield, and a common saying in CEA is “a 1% increase in light equals a 1% increase in yield.” Most greenhouses in North America will experience sub-optimal light intensities inside the greenhouse during some periods of the year. Even in indoor facilities using solely electrical lighting, crops may be grown under suboptimal light. How much is enough? Well, that depends on the species. For the most part, culinary herbs grow well with a photosynthetic daily light integral (DLI) up to and over 15 mol·m–2·d–1. Maximizing transmission in the greenhouse is key to efficient greenhouse production. But once the glazing material has been cleaned, excessive overhead superstructure minimized and the like, there is only one way to get more light into the greenhouse: use supplemental electrical lighting. Regardless of the type of light source, such as light-emitting diodes (LEDs) or high-pressure sodium (HPS) lamps, lighting is the only way to appreciably increase the amount of light herbs will receive.

Justify the light

Although all culinary herbs benefit from increased light intensity, it may not be economically justifiable to use supplemental lighting for all the herbs you grow. So which ones will you choose to light? There are a few ways to determine this. First, focus on your primary crops. Use lighting to improve yield first on those bread-and-butter crops. And in the case of culinary herbs, this is likely going to mean basil varieties. When looking at the rest of the culinary herbs regularly grown, we can segregate them into three different groups based on their photosynthetically active radiation (PAR) productivity (a measure of the increase in production per unit of light): “high” (dill), “moderate” (basil, parsley, mint, cilantro) and “low” (oregano and thyme). By lighting those crops with “moderate” and “high” PAR productivity, you are maximizing your returns on the investment in fixed (lamps) and variable (energy) costs related to supplemental lighting.

The spectrum, or quality, of light also influences culinary herb growth and development.

Before the introduction and wide-spread use of LEDs, there was minimal interest in modifying light quality to improve production of culinary herbs in controlled environments. However, LEDs have allowed us to find new ways of steering crop growth by using different light spectra. For culinary herbs, the primary wavelength of interest is going to include blue (~450 nm), red (~635 nm) and far-red (~725 nm) light. While all three of these wavelengths influence culinary herb growth, their influence on crops differ. Red and blue light are widely used as the basis for photosynthetic LEDs — lights intended to increase photosynthesis and subsequent growth. These colors are commonly used because of the absorption peaks by chlorophyll around these two wavelengths (though it should be noted that other colors can promote similar levels of photosynthesis as these two).

Red is the workhorse for promoting photosynthesis and growth. It is the most energetically efficient to produce by LEDs and most easily absorbed by plants. In a greenhouse situation where electric light is supplementing sunlight (that has other colors), you can emphasize the proportion of red light. In indoor environments without sunlight, additional spectra like blue and far-red should be added.

Although we don’t often think of far-red as a beneficial wavelength, it can benefit culinary herb production in several ways. First, it can enhance canopy photosynthesis by promoting leaf expansion and subsequent light capture. Additionally, far-red light has been shown to enhance photosynthesis when added to other spectra, something called the “Emerson Effect.” Additionally, increasing the proportion of blue light can increase the production of secondary metabolites that can enhance herb flavor, as well as increasing anthocyanin production for purple-leafed herbs, such as opal basil.

While we often think of lighting in static terms (i.e. “we light with HPS”), a dynamic approach may be used. Using LEDs with red and far-red light to promote leaf expansion of seedlings, followed by red-light-rich HPS light during production to increase fresh weight and ending production with three or four days of blue light to enhance flavor and/or coloration is an example of a dynamic lighting program for culinary herbs.

The daylength, or photoperiod, is the final aspect of light that can be managed in controlled environment production. Daylength can influence a wide variety of plant processes, from growth to flowering to dormancy. Among these processes, the most important would be flowering, as culinary herb flavor and foliage production can change when flowering occurs. There are a few culinary herbs that have a photoperiodic flowering response, like the long-day plant dill or the short-day plant stevia. Ideally, these herbs would be cultivated under non-inductive daylengths to inhibit flowering and promote foliage development. However, given that culinary herbs are generally grown as a polyculture with multiple species in the same production environment, this poses a challenge. Thankfully, culinary herbs are usually harvested before any flowers are able to form on plants. For many culinary herbs, a seedling time of two to three weeks and a finishing time of three to four weeks (and a total crop time of five to seven weeks) makes it possible for the majority of crops to be harvested in time. If you are finding flowers at harvest time, try increasing the number of seeds sown in transplants. By increasing the number of seedlings in an individual propagation cube, you will reach a harvestable size in an even shorter time.

Lighting in CEA provides a unique opportunity for producers to influence their crop. From boosting yields, enhancing growth and flavor and maximizing productivity, light management is a useful tool for every herb grown in controlled environments. Reflect on the successes you are currently having with your lighting and identify areas where you may be able to improve.

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