Improving Greenhouse Production with LED Lights

U.S. researchers are looking at the potential benefits to the propagation and production of greenhouse ornamental and vegetable crops using LED lights.

By David Kuack

Although U.S. researchers have started studying the effects of LED lights on the production of greenhouse ornamental and vegetable crops, much of the data being used by American growers comes from studies done in Europe. Purdue University horticulture professor Cary Mitchell said that studies currently being done in the United States will provide growers with information that is relevant to their production and climatic conditions.

Mitchell is leading a team of university researchers who have received a $4.9 million grant, including $2.4 million from USDA, to study LED lighting for greenhouse applications. Mitchell along with Purdue horticulture professor Roberto Lopez is working with scientists and engineers at the University of Arizona, Michigan State University, Rutgers University and Orbital Technologies Corp. Mitchell is working with graduate student Celina Gomez to study the impact of LED lights on the propagation and production of high-wire tomatoes. Lopez and graduate students Christopher Currey and Michael Ortiz are studying the use of LED lights on bedding plant cuttings and plugs.

Propagation trials

Due to limited greenhouse research space, Gomez is using one bench to compare the effect of providing supplemental light from a high intensity discharge lamp or from LED lights with control plants that receive only natural daylight. During the first year of the propagation study, Gomez is conducting an experiment every month. The experiment includes a control group of tomato seedlings that receive no supplemental light, an overhead HID lamp that provides the industry standard and overhead LED arrays that provide three different ratios of red to blue light.

“The propagation experiment is repeated for three weeks every month,” Mitchell said. “We are measuring the differences in plant growth from one month to the next. As we enter spring, the ambient light levels are increasing. Gomez will measure the daily light integral (DLI) that is occurring and the different red/blue ratios and what the plants prefer and determine what they need. In addition to the plant metrics being collected, we are also measuring the amount of electricity used for supplemental lighting.”

After the tomato seedlings reach the stage at which they would be grafted onto the rootstock, data is being collected including plant dry weight, height, stem diameter, leaf span and leaf area.

Mitchell said the propagation area that is equipped with the lights receives 5 moles per square meter per day of supplemental light in addition to the natural solar daily light integral that varies throughout the year.

“Since we have only done the experiment a couple of times so far this year, we’ve yet to see what kind of plant response pattern emerges,” he said. The supplemental light we are providing now might not be enough light during the dead of winter. Any benefits of supplemental light that occur during the winter should disappear as the trials move later into spring. Once we have obtained a full year profile of seedling response, we will be able to determine the optimum amount of supplemental light to apply each month.

“One of the best management practices that we hope comes of this long term study is to determine at what point it is important to use supplemental lighting, as well as when it is no longer useful to do so.”

For the propagation study the tomato seedlings are receiving supplemental light for 23 hours a day in order to achieve a daily light integral of 5 moles per square meter per day.

Tomato seeds are being germinated in a substrate called steadyGROWpro plugs. Six different tomato varieties are being tested: ‘Success’, ‘Komeett’, ‘Maxifort’, ‘Sheva-sheva’, ‘Liberty’ and ‘Felicity’. Seedlings of ‘Success’ and ‘Komeett’ are used for the production study after being grafted onto ‘Maxifort’. These varieties were recommended by Marco de Bruin at Bushel Boy Farms in Owatonna, Minn., because they have different growth habits.

Production trials

In the production experiments the grafted seedlings are being transplanted into Coco Agro coir slabs.

“The lighting treatments containing both test cultivars are blocked into separate half rows in order to determine if there are position effects within the greenhouse that could affect yields,” Mitchell said.

The plants are being provided with supplemental light twice a day. He said they are applying a daily light integral of 9 moles per square meter per day.

“In early March we were lighting for 12 hours per day,” Mitchell said. “Lighting usually starts well before sunrise and begins again before the sun goes down.”

The first production study in 2012 began at the end of January. Mitchell said the tomato plants that had received supplemental light treatments were already setting fruit in early March.

“The control plants that didn’t receive any supplemental light were way behind,” he said. “They were barely setting fruit. That’s what you would expect in a cloudy region like Indiana.”

The first production experiment of 2012 will be terminated after six months and a second will begin immediately. Mitchell said the second experiment will be the exact opposite of the first in terms of solar daily light integral changes.

“We want to see what challenges there are both with the propagation and the production starting in the summer and going into the winter,” he said. “If production is started in the greenhouse in July, the plants are going to be receiving a lot of sunlight. As the photoperiod starts to shorten going into fall that is when supplemental lighting will be more valuable.

“We are hoping to come up with recommendations for growers in this region or in any other northern region that has cloudy weather regarding when is the best time to start lighting their crops. We are also looking at timing the production so that growers are not competing with home-grown or field-grown tomatoes. That way the greenhouse growers are not competing with availability and price for what’s being grown in backyards or in the field.”

Priming the ornamentals propagation pump

Purdue horticulture professor Roberto Lopez and graduate students Christopher Currey and Michael Ortiz are studying the effect of supplemental light on the propagation of ornamental vegetative cuttings and plugs.

“We’re looking at the top three flowering crops that are produced from vegetative cuttings, which are geraniums, petunias and New Guinea impatiens,” Lopez said. Currey and Ortiz are comparing rooting, dry mass accumulation and other quality parameters under red and blue LED lights to high pressure sodium lamps. Initial trials with cuttings have shown that there are not a lot differences in terms of rooting time and quality between the two light sources. Additionally, preliminary data is showing no differences in the time to flower or quality of cuttings propagated under the various LED lights and high pressure sodium lamps for the three annual crops.

“Initially, the results are very similar for both rooted cuttings and finished plants,” Lopez said. “But this is very preliminary. There were really no differences seen for these three crops. What we were mainly trying to achieve was a certain daily light integral with both the high pressure sodium and red and blue LEDs. With the additional trials that we will be doing we will also be looking to quantify the amount of electricity used by the high pressure sodium lights and the LEDs.”

Best timing, amount of light

Lopez said none of the vegetative cuttings received supplemental light during the first seven days of propagation because that is when the cuttings are forming callus.

“A grower typically wouldn’t use lights during this period unless the light level was really low,” he said. “During that period the grower is trying to baby the cuttings to get them to form callus. If the light level is too high during this period the cuttings could be stressed. After a week the cuttings begin to form roots and start to photosynthesize. A grower can maximize photosynthesis during rooting by increasing the daily light integral.”

Currey’s research and studies Lopez performed at Michigan State University indicate growers should provide a daily light integral of between 8-10 moles per square meter per day to be able to increase rooting and the overall quality of the cutting.

Lopez and Ortiz are also testing LED lights during plug propagation of celosia, cosmos, impatiens, geranium, marigold, pansy and petunia.

“One of the biggest challenges with plug production of annual bedding plants is keeping the plugs compact,” Ortiz said. “Compact plugs ease transport in boxes and allow for a higher volume of plugs to be transported at one time. This is definitely something to consider as fuel prices continue to rise.

“Plugs are often grown in dense 288- or 504-cell trays that promote rapid stem elongation. We are using red and far red LEDs in end-of-day treatments in an attempt to manipulate the phytochrome-mediated genes that are responsible for stem elongation under dense planting conditions. If LEDs can be used to control seedling height, the industry can decrease its reliance on plant growth regulators.”

Lopez and Ortiz are also investigating red and blue LEDs as a supplemental lighting source during winter bedding plant plug production.

“The goal behind this experiment is to quantify root development under different ratios of red and blue LED light and high pressure sodium light,” Ortiz said. “We also are also trying to determine if supplemental light from LEDs can offer more rapid root development than light from high pressure sodium lamps. This can make a big impact on energy use in the industry.”

Lopez said producing cuttings is much different than producing plugs.

“With plugs a grower is starting out with plants that have roots,” Lopez said. “A grower may end up being able to delay the sowing of the plugs if he is using lights. We may find that the LEDs might prove to be even more beneficial with plugs than with cuttings.”

For more: Cary Mitchell, Purdue University, Department: Horticulture and Landscape Architecture, (765) 494-1347; cmitchel@purdue.edu. Roberto Lopez, Purdue University, Department of Horticulture and Landscape Architecture, (765) 496-3425; rglopez@purdue.edu;

https://sharepoint.agriculture.purdue.edu/agriculture/flowers/default.aspx

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