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Figure 1. Black-eyed Susan seedlings exposed to the same DLI as photoperiod increased from 12 to 21 hours. Plants receiving 21 hr of lighting had a 30% higher shoot dry weight than 12 hr plants.

Introduction

Supplemental lighting provides quicker crop turns, higher yields, and increased quality for the $6.5 billion a year greenhouse floriculture and vegetable industry. But it comes at a steep cost, especially when growers adopt LED technology. Electricity for lighting can account for 20-30% of operating costs and lighting has been estimated to cost the controlled environment agriculture (CEA) industry $600 million annually. Enter Project LAMP. They are a research and outreach team funded by a grant from the USDA Specialty Crops Research Initiative. The study’s mission is to help growers get more value out of their lighting systems by sharing horticultural and economical information, developing tools to manage lights for optimal crop growth and quality, and providing strategies to maximize their return on investment. The research team is led by Marc van Iersel at the University of Georgia along with colleagues at Colorado State University, Cornell University, Rutgers University, Texas A&M, Utah State University, and the USDA ARS in Toledo, OH. The team combines several disciplines (horticulture, economics, agricultural & electrical engineering, computer engineering, impact assessment, and information systems) to help us better understand the impacts and economics of plant lighting decisions.

Barriers to LED Adoption

LED technology has evolved greatly over the last five years. Some LED fixtures are now on the market that have double the efficacy (light output per unit of electricity) compared to traditional high-intensity discharge (HPS and MH) lamps. However, there are substantial barriers to the adoption of LED fixtures by growers. The group recently conducted a survey about the greenhouse industry’s lighting information needs. When asked why growers are not readily adopting LEDs, the most cited reasons were: initial capital costs too high, not enough knowledge on plant responses to LED lights, not sure which spectrum is right for me, and return on investment takes too long. Similarly, when asked what information needs growers have related to lighting the responses included: need for analyses of profitability of lighting for different crops (i.e., ROI, return on investment), determining how to choose the best lighting supplier, lighting requirements by crop species, and information on available rebates/incentives for LEDs. Essentially, the survey brought up two major points: growers are hungry for more information on the economics of LED adoption (or supplemental lighting in general) and more information is needed on crop responses to LED lighting (what is the best spectrum, intensity, and lighting control strategy for a given crop).

Research to Address Industry Needs

The core focus of project LAMP is to better understand how the complex capabilities of LEDs can translate into more profit for the grower. For example, HID fixtures are essentially on/off devices, they supply only one light intensity and the spectrum cannot be changed. Some LED fixtures allow users to select from different spectra, vary light spectrum over the day or crop cycle, and adjust light intensity in real-time. Here are a few examples of the research in action.

Spreading out the daily light integral (DLI): At lower light intensity (PPFD), plants are more efficient at using supplemental light. Put another way, when the light intensity provided by sunlight is already high, there is not much added benefit of supplemental light. Using dimmable LEDs, the team has tested the strategy of delivering the same DLI, but spreading it across more hours of the day. This strategy was tested for the leafy greens lettuce and mizuna. Spreading the same DLI over 20 hours vs. 10 hours per day led to an increase in fresh weight of 12% for lettuce and 20% for mizuna. For black-eyed Susan seedlings, increasing the photoperiod from 12 to 21 hours increased shoot dry weight by 30% and root dry weight by 24% (see Figure 1 above). These longer photoperiods do not increase the amount of supplemental lighting needed and increase crop yield without increasing electricity use.

Figure 2. Lettuce grown with far-red (FR) light, had a greater leaf surface area and canopy light interception.

Far-red increases light interception: When seedlings are young there can be a lot of wasted light, light that falls in-between leaves and is not absorbed by plants. Far-red light (wavelengths from 700-800 nm) has been previously undervalued and not included in the output of many LED fixtures. However, far-red light triggers plants to seek more light by either growing taller or developing larger leaves to capture more light.  The team has tested far-red light applications on the growth and development of lettuce and foxglove seedlings grown under LED light only. The increased far-red light intensity led to wider leaf surface area, and thus greater canopy light interception. Because the plants captured more light, this ultimately led to greater plant sizes (fresh and dry weight) of both lettuce (Figure 2) and foxglove (Figure 3, below). So far, the results demonstrate that far-red light should be included for seedling production in growth chambers. Growers often worry about plants becoming too tall/leggy when exposed to far-red light, but this has not been an issue so far in trials. Keep in mind that sunlight too contains far-red light, so plants are used to far-red as part of the light spectrum. Including the same fraction of far-red in LED light as what is present in sunlight (19%) can increase growth under sole-source lighting. And importantly, far-red light can stimulate growth more effectively than PPFD: lettuce seedlings grow faster with 81% white light and 19% far-red than with 100% white light. More work is needed to determine the potential benefits of far-red light in the greenhouse with a background of sunlight.

Figure 3. Fox-glove seedlings grown with far-red light had greater leaf surface area and size (fresh and dry weight).

Calculating lighting needs: Because of the diversity of lighting fixtures on the market, it is important to compare several different products before selecting one for your operation. Many of the questions we receive from growers relate to the upfront costs and annual operating costs of different fixtures. We developed a spreadsheet calculator that helps growers estimate the number of light fixtures needed for their operation and the annual electricity costs. The tool requires the user to enter information on the light output and fixture efficacy, information that is readily obtained from the specification sheets of reliable horticultural lighting suppliers. (The Design Lights Consortium also maintains a database of LED fixtures that meet their Qualified Product List criteria for efficacy, minimum product life, and warranty and is available at: https://www.designlights.org/horticultural-lighting/search/). Look for a major upgrade to the calculator tool in the coming months. The new tool will allow you to input your zip code to determine how much sunlight you receive throughout an average year and calculates the cost of providing supplemental lighting, based on your greenhouse conditions.

For more information about Project LAMP, including several 1-page fact-sheets on the research results and links to lighting videos, webinars, and articles, check out the project website at: https://www.hortlamp.org/. If you have suggestions or questions, please contact the research team at info@hortlamp.org.

Note all photos are courtesy of Marc van Iersel, University of Georgia
By Neil Mattson, Cornell University; Josh Craver, Colorado State University, A.J. Both, Rutgers University and Marc van Iersel, University of Georgia

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