Effects of Adjustment to Spectral Ratios of Supplemental LED Lighting on Miniature Rose Growth and Morphology

The DLI during winter months in the northern California ranges from 10-20 moles/m²/day and is inadequate for optimal plant growth. High Intensity Discharge (HID) lamps are commonly used as supplemental lighting in greenhouses to offset the lack of natural light are gradually being phased out in favor of Light Emitting Diode (LED) lamps for their higher energy efficiency and longevity. In some cases, LED lights allow users to manipulate the quantity and quality of light emitted from the various diodes composing an array. This lets the user “tune the spectra,” to controlling several aspects of plant growth including those usually controlled through use of Plant Growth Regulators (PGRs).

Here we report use of spectral tuning during growth of miniature rose varieties “Renata” and “Karina” at Rocket Farms’ production greenhouse during winter months when supplemental lighting is a necessity. HPS lights were used to illuminate a control group with an average PPFD of 74.25 µmol m^-2 s^-1 , while three treatments consisted of a Full Power (FP) treatment at a PPFD of 61.5 µmol m^-2 s^-1 and spectral ratio of 65%Red:5%White:30%Blue, a High Red (HR) treatment for which blue light was eliminated for the final two weeks of growth (PPFD to 46.5 µmol m^-2 s^-1), and a High Blue/No PGR (HB/NPGR) treatment for which no PGRs were used. For the final two weeks of growth for this treatment red light was gradually reduced by 50% to a final PPFD of 33.5. Normal greenhouse conditions were maintained, and measurements were taken for height, width, and bud count weekly for four weeks.

Our objective with HR was to attain similar or higher bud count than HPS was achieved with both Karina and Ranata. With Karina, the HR treatment achieved the same bud count as the HPS control, at 16.3 buds, while the FP and HB/NPGR treatments had a lower bud count of 14.25 buds. However, Renata displayed no significant differences in bud count for HPS, FP, or HR with an average bud count of 4.7. HB/NPGR yielded a lower count of 3.3.

Our objective of maintaining a shorter plant without PGRs under the HB/NPGR was achieved for Karina, while Ranata yielded a taller plant.

These results confirmed that defined spectral ratios from supplemental LED lighting can influence plant growth similar to the action of PGRs, but that the degree of effectiveness is species or cultivar specific.