Fig. 1. Tomatoes grown in the winter benefit from supplemental lighting to increase greenhouse light levels. Note the snow on the polyethylene film.
Photo: Christopher J. Currey

One of the most important aspects of hydroponic food crop production is light. There are several reasons for this. Light impacts plant growth and development in several different ways. There are three different aspects of light that should be considered: 1) light quantity, 2) light quality and 3) duration of light or day length. All three of these characteristics of light affect growth and development of food crops, though in different ways and in varying magnitudes.

Light quantity

For hydroponic food crop production, the quantity of light can dramatically affect the yield of fruiting and leafy crops. Light quantity can refer to the instantaneous quantity of light, light intensity or the cumulative amount of light — the daily light integral (DLI). Light intensity drives photosynthesis which, in turn, produces the carbohydrates which serve as the building blocks for plant growth. Since the majority of food crops are sold on a per-unit-weight basis, it is advantageous to increase plant growth as it results in increased yields.

While light intensity is important, DLI is another effective means of monitoring photosynthetic light in the greenhouse. Light intensity is a snapshot in time; throughout the course of the day in greenhouses, it can change with the time of day, position of the sun in the sky and cloud cover. Therefore, the cumulative amount of photosynthetic light a crop receives during the day can be a more meaningful way to measure and manage light in greenhouses. Many food crops grown hydroponically may be considered high-light plants, with growth increasing with light.

One of the big advantages to greenhouse hydroponic production is that these facilities can produce crops during the winter in temperate climates, where outdoor production is not possible. However, ambient DLIs during the winter are usually low and sub-optimal. The only way to appreciably increase the light intensity or DLI inside the greenhouse above ambient light levels is to use supplemental lighting.

This has traditionally been accomplished by using high-pressure sodium (HPS) lamps (Fig. 1). Some of the advantages of HPS lamps include their broad spectrum and ability to illuminate large areas in greenhouses. However, HPS lamps are ~25 percent efficient in converting electrical energy into light for plants, with some energy turning into heat. Additionally, HPS have a lifetime of around 20,000 luminous hours.

Recently, the development of high-intensity light-emitting diodes (LEDs) has resulted in new lighting technology for food crop producers. Some of the benefits of LEDs include the ability to tailor light quality, up to ~50 percent efficiency in converting energy into light with some wavelengths, and an expected luminous life of 50,000-plus hours. While the energy consumption of LEDs can be a big boon for those looking to minimize energy expenditures, LED light does not scatter like HPS light, meaning more lamps may be required to light your growing area. Additionally, the up-front cost for LEDs may turn off some growers.

Fig. 2. The blue light reflecting off of the leaves is used to enhance the red leaf color of this lettuce grown hydroponically during the winter. In winter, light levels are low, which can diminish foliage pigmentation.
Photo: Christopher J. Currey

If you are producing hydroponic food crops, start measuring your light intensity or DLI. You may be surprised that, while the greenhouse may seem well-lit enough for you, it is below the levels that would boost crop productivity. Give serious consideration to using supplemental lighting for your greenhouse food crop production, especially if you are in more northerly latitudes. Investments in supplemental lighting, while not trivial, may pay off with increased yields and revenue from increased crop productivity.

Light quality

Another factor of light that should be considered for hydroponically producing food crops is light quality. Light quality refers to the wavelength or “color” of light. Different light spectra can have different effects on plant growth and development. Light between the wavelengths of 400 and 700 nanometers is referred to as photosynthetically active radiation (PAR). This light is the most useful for photosynthesis and, as a result, is important when evaluating supplemental lighting in your greenhouse.

In addition to affecting photosynthesis and therefore, growth, different spectra can affect the development of hydroponic crops. The effects of different spectra are numerous. However, red, far-red and blue light have the most pronounced effect on plants. Red and far-red light, and their abundance relative to one another, affect stem length, flowering and leaf expansion.

Stem elongation is enhanced when the proportion of far-red to red light increases. Far-red-rich environments promote stem and internode elongation. In ornamental plant production, stem elongation is undesirable because it can diminish the aesthetic quality of containerized plants. For hydroponically grown crops, stem length is generally not important.

One exception is for seedlings intended for grafting. Some greenhouse vine crops, including tomatoes and cucumbers, use seedlings that are grafted onto rootstocks that increase productivity. Grafting small seedlings can be a challenge. However, increasing far-red light during seedling growth can increase stem elongation and make handling seedlings and grafting more accessible.

Fig. 3. Ever-bearing strawberry cultivars produce more flowers under long days. Note the light-emitting diodes suspended above the crop to provide low-intensity photoperiodic lighting.
Photo: Christopher J. Currey

Blue light also can affect plant appearances, including leaf size and leaf color. Many LED arrays include blue light in proportions of 0 percent up to ~30 percent relative to other wavelengths. Blue light can inhibit leaf expansion and reduce the amount of leaf area able to capture light and photosynthesize. Blue light is generally beneficial for plant growth, but excessive levels may result in excessively compact seedlings. Additionally, blue light enhances anthocyanin production, which is the reddish-purple pigment found in leaves. In the winter, when light levels are low, red-leafed plants such as red lettuce and purple basil may have diminished coloration and appear greener. Increasing blue light intensities can help increase anthocyanin and produce more marketable crops (Fig. 2).

Day length

In addition to light quality and quantity, the length of the day (photoperiod) can affect the development of some crops. Most importantly, photoperiod can affect flowering. For greenhouse floriculture crop production, managing photoperiod is very common. For instance, poinsettias, chrysanthemums and kalanchoe all initiate flowers under short days and may be classified as short-day plants, whereas many spring annuals and perennials flower under long days and may be classified as long-day plants.

For many greenhouse food crops, it is not necessary to control flowering using day length. For vine crops in which flowers must be produced in order for fruit to develop (tomatoes, peppers or cucumbers, for example), flower formation is not affected by day length and plants produce flowers regardless of the photoperiod. Lettuce and some culinary herbs are induced to flower under long days. Though flowering of these leafy crops is undesirable, the relatively short crop time for leafy greens and herbs is short and plants are harvested prior to flowering. Therefore, measures to inhibit flowering are usually not needed.

One exception to this is strawberries. Strawberry cultivars are generally classified as “June-bearing” or “ever-bearing.” June-bearing cultivars are short-day plants and flowers are formed under short days. Ever-bearing cultivars are classified as facultative long-day plants and, though flowers may be formed under any photoperiod, more flowers are formed under long days (Fig. 3).

Take-home message

Light can have a large impact on the productivity of your hydroponically grown food crops depending on your location, greenhouse structure and the time of year. However, the effects of light are not limited to photosynthesis and growth alone. Different wavelengths of light can elicit specific responses that hydroponic crop producers can take advantage of, while day length can control flowering and subsequent fruit production. Take some time to review how you can start managing light to improve productivity and quality.

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