Greenhouse Lighting Strategies: High-Quality Vegetable Transplants

Food

Different plant species have different light requirements at each stage of their development. In a greenhouse environment, one of the biggest challenges is ensuring that you are supplementing the natural light with the right light quality at every growth stage and in all seasons. In one of our By Growers for Growers webinars, Dr. Roberto Lopez of Michigan State University discussed supplemental lighting radiation quality as it influences high-wire transplant growth and development in a low-light greenhouse environment.  

Controlled environment and field vegetable production are dependent upon high-quality transplants. This leads to earlier and multiple harvests per growing season, stronger stem development, as well as increased profitability for growers. Dr. Lopez noted, “There had been a lack of published data in terms of spectral quality. Published studies basically only focused on red and blue radiation. We wanted to look at the addition of green as well as far-red radiation and how they would impact the growth of these transplants.”

Studying How Spectral Quality Impacts Transplant Growth

In terms of the plant material, Dr. Lopez and his team worked with cucumber ‘Elsie’, pepper ‘Kathia’ and tomato ‘Climstar’. The seeds were started in a growth chamber and then moved to a greenhouse. Dr Lopez commented, “We wanted to provide the same photoperiod across treatments, so we had a low intensity photoperiodic treatment from HPS lamps providing 25 micromoles of light. Our next treatment provided supplemental lighting from HPS lamps at 120 micromoles and LED grow light treatments also totaling 120 micromoles.”  

The LED lighting provided:  

  • 20% Blue, 10% Green, 75% Red and 15% Far-Red  
  • 25% Blue, 95% Red
  • 30% Blue, 30% Green, 60% Red

In terms of the daily light integral, under the low-light greenhouse conditions the control plants were provided approximately 6 moles of light, and the supplemental lighting treatments provided approximately 11 to 12 moles of light.

Results

In general, plant responses to radiation quality were found to be species-specific. For example, under the supplemental lighting treatments there was an incidence of leaf necrosis in tomato, but when far-red radiation was provided, that decreased. There were also findings that applied to all three plant species.

Dr Lopez commented, “What's the take home message from the study? We found that leaf area as well as both fresh and dry weight, so basically biomass, was greatest under a treatment providing blue, green and red light from LEDs (B30 G30 R60). We found that the stem diameter of cucumber, tomato and pepper was generally greatest when the transplants were grown under either HPS120 or LEDs providing blue, green and red radiation (B30 G30 R60). We also found that when green radiation replaced either red or blue radiation (B30 G30 R60 vs B25 R95), leaf area and fresh weight of all three were significantly greater.”  

LED grow lights provide growers with the ability to customize spectrums of light by specific crops to fit the needs of all plant growth applications. They can provide the right light at every growth stage to ensure high-quality plants year-round.  

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