Intracanopy lighting 101

Light can be one of the most limiting factors for hydroponic crops, and this is especially true in the greenhouse where seasonal variation and limitations inhibit crop productivity. However, even during brighter times of the year, there can be unwanted variations in light intensity within some crops. In the planting densities typically used in greenhouses and controlled environments, there is a point when plants will be shading each other, though the degree of mutual shade varies with crops. For short-statured crops like leafy greens and herbs, the effect of mutual shade on yield during production is minimal. Alternatively, for tall, vining crops like tomato, cucumber and pepper, light intensity decreases within the canopy. As light is absorbed by younger leaves and the shading effect reduces the light available to lower leaves, productivity is less than the plant's full potential. At this point in the growing process, the leaves are still capable of photosynthesizing and contributing to growth.

Traditionally, supplemental light to increase photosynthesis and, ultimately, productivity and yield is provided from lights suspended overhead, above crops. However, the penetration of supplemental light down into canopies is limited in high-wire vine crops that are trellised during production. Placing traditional high-intensity discharge (or HID) lights like high-pressure sodium (HPS) lamps closer to the canopy would damage plants from the intense radiant energy that can burn foliage, flowers and fruits. However, the development of high-intensity light-emitting diodes (LEDs) has paved the way for light to now be placed closer to the older leaves through intracanopy lighting.

Intracanopy lights are designed to be placed within the canopy of vine crops (Fig. 1). Most frequently, they are placed parallel in between the two canopies in a double row or between bifurcated plants. By using intracanopy lighting, the light intensity within the canopy increases, allowing older, but still fully functional and productive leaves to photosynthesize more and increase subsequent fruit yield. Generally, intracanopy lights alone are not used to provide supplemental light to greenhouse crops. Rather, intracanopy lights are most frequently used to supplement traditional overhead lighting sources. However, sometimes the amount of supplemental lighting from overhead lights or the number of overhead fixtures can be reduced when both are used.

Intracanopy lights are available from several different manufacturers, so it’s best to review the manufacturer’s track record. Like many of the LED lights available today, there is a wide range of spectral compositions that can be selected, with red as the primary spectra and additional blue and/or white light as well. One thing to keep in mind is employee comfort. Depending on the mounting height of the intracanopy lights relative to your employee’s average eye level, there can be more direct exposure to blue light. The mounting height and orientation of intracanopy lights will likely bring them into close proximity to employee lines of sight, while blue light can sometimes cause discomfort for people working closely to them without the proper eye protection.

The economics behind intracanopy lighting is a more challenging one and you will need to determine its affordability for yourself. Several studies report the energy costs associated with operating costs of intracanopy lights can be offset by the improvement of yields with the additional light. Therefore, the bigger question is the initial capital investment of the lights themselves and the costs associated with installation. This is a harder question to answer because the price tag for your intracanopy lights are dependent upon your model preference, who you are purchasing from, how many you are purchasing at once and other factors.

With the development of intracanopy LEDs, greenhouse lighting for hydroponic vine crops is changing. The opportunity for increased productivity and quality during low-light times of the year is more possible with intracanopy lights.

The author (ccurrey@iastate.edu) is an assistant professor in the Department of Horticulture at Iowa State University.