Greenhouse production of coleus generally occurs between the months of February and May in the United States to coincide with spring bedding plant sales. Mean outdoor daily light integrals (DLIs) at this time of year in the northeastern United States range from 15 to 40 mol·m−2·d−1 compared with 25 to 60 mol·m−2·d−1 in the southwestern United States (Korczynski et al., 2002). Mechanical impedance of light by glazing materials and greenhouse support structures reduces transmission of light into a greenhouse as much as 50% (Fisher and Runkle, 2004). Thus, the actual DLI in greenhouses ranges from 7.5 to 20 mol·m−2·d−1 in the northeastern United States to 12.5 to 30 mol·m−2·d−1 in southwestern United States. It is often necessary to provide supplemental lighting during the spring production season, particularly for growers in the northeastern United States. For example, minimum DLI recommendations for high light–adapted crops such as yarrow (Achillea millefolium), gaura (Gaura lindheimeri), and english lavender (Lavandula angustifolia) range from 15 to 20 mol·m−2·d−1 (Fausey et al., 2005).
Some bedding plants, such as coleus, may tolerate lower light levels compared with yarrow, gaura, and english lavender. Verkbert et al. (2004) recommend growing coleus at a minimum DLI of 9 mol·m−2·d−1 and no more than 18 mol·m−2·d−1 for high-quality plants. However, light tolerance varies among coleus cultivars; some cultivars are best grown in shade, while others would grow optimally in full sun (Armitage, 2001; Rogers and Hartlage, 2008; Stack, 2009). Stack (2009) conducted a field trial to assess the performance of 79 cultivars of coleus under northern New England full sun conditions and found that over half of the cultivars were good candidates for full sun, while the remaining cultivars were better suited for shaded conditions. Actual DLIs were not recorded in this study. The variation in sun tolerance among cultivars in the field suggests that there may also be a variation in light requirements in the greenhouse. Determining the lowest light levels needed for producing coleus may reduce the need for supplemental lighting, thus reducing energy use and increasing production sustainability.
Daily light integral has diverse effects on morphology, which would be important to consider when determining the lowest light levels appropriate for growing coleus. Shoot dry weight of cockscomb (Celosia argentea var. plumose ‘Gloria Mix’), impatiens (Impatiens walleriana ‘Accent Red’), and french marigold (Tagetes patula ‘Bonanza Yellow’), and total dry weight (shoot and root) of wax begonia (Begonia semperflorens-cultorum ‘Cocktail Vodka’) were greater when plants were grown under 14.4 mol·m−2·d−1 compared with those grown under 5.3 mol·m−2·d−1 (Nemali and van Iersel, 2004; Pramuk and Runkle, 2005). Fausey et al. (2005) reported that higher DLI resulted in improved visual quality (height, flower color, lateral branching, foliage color) of yarrow, gaura, and english lavender.
Daily light integral can influence leaf variegation in addition to overall crop growth. Coleus is widely used as a foliage plant, and it has uniquely colored foliage with leaves that are either monochromatic or a distinctly variegated combination of two or more colors, including burgundy, green, pink, coral, yellow, orange, and dark purple. Foliage crops including english ivy (Hedera helix ‘Gold Heart’), pothos (Epipremnum aureum var. pinnatum), and cast iron plant (Aspidistra elatior ‘Variegata’) developed less variegation when grown under lower amounts of light (Nam et al., 1997; Pennisi et al., 2005; Stamps, 1995). Conversely, the leaves of variegated cultivars of radiator plant (Peperomia obtusifolia) and dracaena (Dracaena sanderana) have more variegation when plants are grown under lower DLIs (Shen and Seeley, 1983; Vladimirova et al., 1997).
Successful coleus sales rely on a combination of dramatic coloration and dense growth habit. To our knowledge, the lowest light levels required to promote these characteristics in coleus have not been reported. Therefore, the objective of this study was to determine the lowest DLI necessary to produce the marketable coleus crop.
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