Light Manipulation for Ornamental Crop Production

Have you ever heard of an ornamental kale or pepper plant? Rather than being grown for consumption, these ornamentals are cherished for their aesthetic appeal: showcasing leaves, fruits, and flowers of interesting colors, shapes, and aromas, thus contributing significantly to ornamental crop production systems (Figure 1). According to the most recent USDA census, the U.S. ornamental production industry, which is a key segment of crop growth strategies, is valued at $6.2 billion per year. This industry encompasses over 17,000 growers who utilize glass or other controlled environments for their crops, demonstrating the importance of advanced production systems in enhancing the aesthetic and economic value of ornamental plants (USDA NASS, 2020).

These controlled environment growers adeptly manipulate light to alter plant shape, increase cutting quantity, and, in the case of flowering ornamentals, control flowering time. Such light manipulation practices are essential for crop scheduling and optimization in ornamental crop production systems (Runkle and Heins, 2006). Greenhouse growers employ various products to manipulate light, including shade/blackout cloths, exterior applied greenhouse coatings, and supplemental lighting. These techniques are pivotal in enhancing crop growth, ensuring the year-round production and supply of high-quality ornamental plants.

Figure 1. Bolivian rainbow pepper, medusa pepper, and ornamental kale showing eye-catching fruits and leaves.

To understand the applications of these different products, two adjustable characteristics of light are important to know: light quality and light quantity. Light quality pertains to the spectral wavelength composition of the light source; for example, sunlight quality at Earth’s surface is approximately 43% photosynthetically active radiation (PAR) (400-700nm) and 4% ultraviolet (UV) (300-400 nm) (NationalRenewable Energy Laboratory, n.d.).

PAR and UV light, as well as far-red (700-750 nm), contribute to plant growth and development and are called biologically active radiation (Zhen et Figure 1. Bolivian rainbow pepper, medusa pepper, and ornamental kale showing eye-catching fruits and leaves.al., 2021). PAR, however, is the most critical to plant growth because it contributes directly to photosynthesis. 

Light quantity is the amount of PAR light received, expressed as daily light integral, (DLI). Two variables contribute to DLI. One is instantaneous light intensity, or the number of PAR photons received during one second in a given area, expressed as photosynthetic photon flux density ,(PPFD). 

Second is the duration of light received during a day, called photoperiod. Photoperiod is crucial for plant development, most notably inducing flowers in many crops. Commercial growers primarily manipulate light quantity; however, other light characteristics (including quality) change. 

Decreasing PPFD – Shade Cloths

One reason ornamental growers reduce PPFD is to grow plants that naturally thrive in shadier environments, such as coleus, begonia, and impatiens. Shade cloths, called shade nets or screens, are commonly used to decrease PPFD. They contain materials that reflect or absorb light, resulting in more diffuse light that can penetrate deeper into the crop canopy (Shahak, 2006). 

This diffuse light provides side-lighting to hanging basket plants (Figure 2), which had total sales of $ 67.2 million in 2020 (USDA, 2020). Hanging baskets require even weight dispersion on all sides to avoid tipping, including evenly dispersed lighting to obtain the desired plant structure with light diffusion.

One of the most popular hanging basket ferns grown in controlled environments is the Boston fern. Seltsam and Owen (2022) used shade cloths to apply five different levels of DLI to identify an optimal range to grow the species. A DLI range of 10-12 mol m-2d-1 Figure 2. 

Production greenhouses are growing hanging baskets above rows to maximize their space use efficiency. They maximized the frond number in 7 of 11 cultivars. Based on height and diameter, plant compaction was also optimized for ‘Compacta’ and ‘Super’ within this range. 

Figure 2. Production greenhouses growing hanging baskets above rows to maximize their space use efficiency.

One of the most popular hanging basket ferns grown in controlled environments is the Boston fern. Seltsam and Owen (2022) used shade cloths to apply five different levels of DLI to identify an optimal range to grow the species. A DLI range of 10-12 mol m2 d1 Maximized frond number in 7 of 11 cultivars. Based on height and diameter, plant compaction was also optimized for ‘Compacta’ and ‘Super’ within this range.

Shade cloths improve plant structure not only by decreasing PPFD and diffusing light but also by decreasing the amount of heat a crop receives, which impacts stem elongation and leaf expansion (Erwin and Heins, 1995; Dale, 1964, 1965; Milthorpe, 1959). As a result, shade cloths provide ideal conditions for crops that prefer cooler temperatures, such as marigolds, chrysanthemums, and pansies. Austerman et al. (2023) grew pansies for six weeks under red, black, pearl, and aluminized shade cloths to see which color cloth improved plant compaction. 

Pearl-colored shade cloths decreased the height the plant had to reach to flower and grow true leaves, indicating a more compact growth habit, which makes damage during shipment less likely. These examples demonstrate the usefulness of shade cloths to improve plant growth uniformity and compaction by providing more diffuse light and decreasing heat the plants receive.

Decreasing Photoperiod – Blackout Cloths

Figure 4. Supplemental HPS lighting in Greenhouse. 

The light quality of  LEDs can be customized, which allows for the application of specific wavelengths to a crop.   In a study by Collado and Hernández (2022), geranium ‘Maverick Red’ and petunia ‘Dreams Midnight ’ were grown in greenhouses under supplemental red LEDs (670 nm) or HPS lamps. Compared to the HPSlight quality, the red LEDs doubled the red light PPFD, increasing the red:far-red light ratio tenfold.   

When the same DLI, the morphology of the geraniums and petunias was more compact under LEDs compared to HPS lamps. Petunia also showed increased chlorophyll and anthocyanin concentrations, contributing to leaf color and ornamental value (Gazula et al., 2007). Figure 5. 

LEDs in controlled environments. (Figure 4). Supplemental HPS lighting over orchids. Additionally, the study included a paclobutrazol (PBZ) application to compare plant compactness resulting from light quality to that from PBZ. 

PBZ is a commonly used ornamental plant growth regulator (PGR) that increases plant compaction but is highly toxic to aquatic life(Syngenta, 2021). The compactness of petunia ‘Dreams Midnight’ was significantly greater under LED treatment than HPS, and the PBZ application did not intensify this effect. These results suggest that light-quality manipulation without PGR compacts petunias. 

Figure 5. LEDs in controlled environments.

Another benefit of increased red light is that mother plants can produce many cuttings. In Kobori et at. (2022), researchers grew two cultivars of impatiens undersupplemental LED lights supplying 150 μmol m-2s-1 of various red: blue light ratios for 12 hours daily. Two hundred two days after transplant, the treatment receiving the most significant proportion of red:blue light had produced 35% more cuttings than the control, which received no supplemental light. 

The response differed between cultivars, suspected to result partially from higher leaf anthocyanin concentration in ‘Sunpatiens Compact Royal Magenta’ than ‘Sunpatiens Compact White.’ Responding to a question posed by Currey and Lopez (2013), whether the presence of leaf pigments in mother plants affects cutting quality. 

Passive Luminescent Greenhouse Layers

Luminescent layers allow for spectral modification like LEDs, but unlike LEDs, they do not require electricity. They convert portions of solar energy from less to more photosynthetically efficient wavelengths (UV and blue [200-500 nm] to red [600-700 nm]). 

Unlike cloths, the energy is transferred to a different wavelength instead of filtering out, resulting in greater PAR intensities and crop yields. While the documentation of benefits of luminescent layers on edible crop growth exists, benefits to ornamental crops do not; however, fresh-cut flowers that prefer high light intensities, such as gladiolus and chrysanthemum, could benefit.