Supplemental Lighting Options for Bedding Plant Seedling Production

Happy spring!  With the season in full swing, we know that growers are busy this month.  I read an interesting article on the effects of supplemental LED lighting in bedding plants this week, and I’ve summarized it here with the key messages at the bottom of this post.  It seems like an appropriate article for the season, especially if you are starting to think ahead to how you might improve production for the 2017 spring season.

Much of the LED lighting research done in ornamental plants has focused on finding the best wavelengths for plant production.  This can vary based on the plant being grown, and the qualities desired.  Before we dive into the study, let’s review some of the reasons why you might consider LED lights from a plant production standpoint.

One of the benefits of LED lights is that the wavelengths can be customized for plant growth, in most cases red (664 nm) and blue (466 nm) are combined since they are the wavelengths best used by the plant.  In comparison, high pressure sodium (HPS) lights produce a large portion of light in the yellow and orange wavelengths, which are not used as well by the plant.

Having the right combination of red to blue light is important since combinations high in blue light are thought to cause an increase in leaf thickness, but produce more compact plants.  Combinations high in red light do the opposite – an increase in leaf area and a more elongated stem.  Good quality bedding plants can be produced under ratios of red:blue light, typically in the range of 85-70% red to 30-15% blue.  In this study ratios of 87:13 and 70:30 were used.  These seedlings are generally more compact, with a larger stem diameter and higher chlorophyll content.

In this study the researchers looked at the effect of red:blue light ratios as either supplemental lighting in a typical greenhouse setting or the only source of lighting (seedlings were grown in a chamber with no other light source).  Five species at plug size were looked at: periwinkle, impatiens, zonal geranium, petunia and French marigold.  Lighting was adjusted to give a photosynthetic daily light integral (DLI) of approximately 10 mol·m^-2·d^-1; a range of 10-12 DLI is important for producing high quality seedlings.

In general, all species showed an increased leaf area and increased biomass with the addition of supplemental light from either high pressure sodium or LED light sources.  All seedlings grown under supplemental lighting also showed increased chlorophyll levels as compared to those grown without supplemental lighting, but those grown under LED supplemented or just LED lighting showed a larger increase in chlorophyll content.

Differences in plant height were the most variable between plants.  Impatiens and periwinkle seedlings were taller with any type of supplemental light, LED or HPS.  Petunias, marigolds and geraniums on the other hand were taller under HPS supplemental lighting and showed no difference in height when grown under LED supplemental lighting as compared to plants grown with no supplemental lighting.

Interestingly, geraniums and marigolds grown under higher concentrations of blue light accumulated more anthocyanins than those grown under lower ratios or HPS lights.  The pattern was especially noticeable under the 70:30 ratio.  This wasn’t observed in the other species, so this may also be a plant specific trait.

What are the take away messages from this study?

• Supplemental lighting by HPS or LED lights is beneficial to increase plant biomass, leaf area and chlorophyll content
• Chlorophyll content in specific bedding plants can be increased by LED supplemental lighting
• The effect of LED lighting on plant height varies by bedding plant species
• LED lights including a higher blue ratio (70:30) may increase anthocyanin content in leaves depending on the species being grown.

Interested in hearing more about the effects of supplemental LED lighting on plant production?  Experts will be speaking at the Canadian Greenhouse Conference this October 5-6^th. Save the date!

References: Randall, W.C. and R. G. Lopez. Comparison of Bedding Plant Seedlings Grown Under Sole-source Light-emitting Diodes (LEDs) and Greenhouse Supplemental Lighting from LEDs and High-pressure Sodium Lamps (2015). HortScience (5):705–713. 2015.