What is the “best” grow light for your controlled environment operation?

Growers who replace incandescent bulbs with screw-in LEDs to control photoperiod can reduce their energy consumption by as much as 85 percent. The LED bulbs could last up to 30 years. Photo courtesy of Southwest Perennials

Depending on whether you are looking to control photoperiod or increase plant growth will determine the grow lights you need to get the best results. 

Before growers even consider choosing a grow light for their controlled environment operation they need to have some basic information.

“One consideration is whether their objective is primarily to regulate flowering or to increase growth,” said Erik Runkle, horticulture professor at Michigan State University. “There is still some confusion among growers that they can screw in LEDs and expect to generate supplemental light that will increase plant biomass. Growers need to know whether they need low intensity light to regulate photoperiod or high intensity light to increase plant growth. This is one of the first things that needs to be clarified.”

Lighting to regulate flowering

Photoperiod lighting would be used to either inhibit or promote flowering.

“Usually photoperiod lighting is very low intensity so it’s a lot cheaper lighting solution,” Runkle said. “Where photoperiod lighting can go wrong is some growers will install these very low intensity lighting fixtures and say they are seeing a positive growth response similar to what would be expected with a high intensity fixture. What they are really seeing is a change in the leaf shape or the leaf habit that suggest there is more growth. If the biomass of the plants was measured it would show there is no increase in plant growth.”

Previously most growers used incandescent bulbs for photoperiod control. Today many growers are replacing incandescent bulbs with screw-in LED bulbs.

“If they screw the bulbs into a light socket that is a low intensity light,” Runkle said. “The cost for these LED bulbs has decreased, but they are still more expensive than incandescent bulbs.

“One advantage of LEDs is they are going to last at least 15 times longer than incandescent bulbs. Typically photoperiod bulbs are only operated four hours during the middle of the night. These LED bulbs are expected to last 15,000-25,000 hours. A grower might have to replace them in 30 years.”

The biggest advantage of LEDs over incandescent bulbs is energy efficiency.

“Energy consumption will drop significantly with LEDs,” Runkle said. “Growers are going to receive significant savings. Their energy usage will be cut at least 80 percent and maybe as high as 85 percent. This is a compelling reason to replace incandescent bulbs with LEDs.”

Lighting to increase growth

Growers who want to increase plant growth will need to know the light intensity they want to deliver to the crops they are producing.

“The first thing greenhouse growers will need to determine is how much light does their geographic location receive from the sun and how much do they want to increase the light intensity beyond the natural light,” Runkle said. “This might seem obvious, but sometimes growers say they want to deliver a specific light intensity, but they really don’t have a good basis for the intensity. The lighting needs of a grower in one state may be very different than a grower in another state. They may also be different for growers in the same state but producing different types of crops.”

Runkle, who works primarily with ornamental plant growers, said most of them want to provide high intensity supplemental light during the darkest months of the year.

“Flower growers are using supplemental light primarily for propagation and increasing plant growth during December, January and February,” he said. “Those are the three months growers try to key in on. Knowing how much light is delivered by the sun and the desired daily light integral (DLI) for the crops they are producing, growers can then determine the light intensity they need to deliver with grow lights.

“Most growers start lighting their young plants because that’s where they have a lot of plants per unit growing area. The cost of lighting on a per plant basis is very low because they are able to produce many plants in a given area.”

Runkle said growers may choose to light their finished crops depending on the crop and the economics.

“The target light intensity for growing ornamentals is going to be similar among the different stages of growth,” he said. “There is one exception during the propagation of unrooted cuttings. Growers may not need as a high an intensity to root cuttings vs. propagating seedlings or during later stages of production. In some cases, growers propagating unrooted cuttings in substrate under high intensity lighting may provide too much light resulting in a bleaching of the leaves. This seems to occur more often with high intensity LED fixtures than high pressure sodium lamps, but doesn’t occur in every crop. This phenomenon merits further investigation.”

Determining your crop lighting needs

The benefit of high intensity lights decreases as the solar light level increases.

“From an economical perspective usually once crop production moves into late March or early April there is little economic benefits to use lights for most ornamental crops,” Runkle said. “Greenhouse tomato growers may be lighting eight to nine months during the year because every photon of light could potentially lead to higher yields. It’s really a driver of whether growers are producing a plant as a unit or marketing a product from the plant like a tomato, pepper or cut flower.

“For most structures, determining the lighting needs is going to be comparable whether it’s for a greenhouse, vertical farm or indoor grow. Regardless of the structure there is a target DLI, whether it is an indoor crop or greenhouse crop. The only complicating factor is how much light is delivered by the sun. In an indoor production facility all of the light is coming from the installed lights. There is an added complexity to determining the light need for a greenhouse. Once the desired DLI is determined, it is a matter of how to deliver the intensity that is needed for how many hours per day.”

Runkle said he prefers to use the term target DLI rather than optimum DLI.

“Optimum DLI is a subjective term,” he said. “Optimum DLI can depend on other environmental conditions and the grower’s market. Also, an optimal DLI may or may not be economical.”

Choosing a light spectrum

There is a variety of light spectrum commercially available. Runkle said light spectrum affects plant growth, efficiency of the fixture and human vision capabilities.

“The light spectrum affects plant growth, but it also affects the color of light and how it influences people working in a production facility,” he said. “Some workers may complain about working under purple or pink LED lights.

“From the plant’s perspective, the light spectrum has a big effect on the shape of the plant. More blue light typically results in more compact plants. When I talk with growers about lighting their crops I ask if height control or more height control is one of their goals. Red LEDs are the most efficient type of LEDs. That is one of the major reasons why more red LEDs are used in grow light fixtures. The most efficient LED fixtures have more red LEDs than white or blue.”

Lighting companies experienced with working with ornamental, vegetable and cannabis growers should be able to suggest what spectrum growers should use and justify why they support it.

“Light companies should be able to assist growers by providing them with the lights to match their crop needs,” Runkle said. “I don’t usually give a specific recommendation of a certain spectrum because it is subjective and situational.”

Choosing a light fixture

Greenhouse growers are going to connect grow lights to their greenhouse environmental control computer and based on the weather, the lights will automatically turn on or off.

“One of the advantages of LEDs is they can be turned on and off without influencing the light’s longevity,” Runkle said. “With HPS fixtures the start-and-stop cycles can negatively impact the bulb’s longevity.

“In a greenhouse the lights can be operated based on the current natural light level. There is new control software and hardware available that considers not only the current light level, but also the target DLI and how much natural light the plants receive during the course of a day. This new technology enables growers to potentially achieve a target DLI each day. In the case of an indoor facility, the lights can be operated on a timer making it much easier from a control standpoint.”

Looking at grow lights from an energy efficiency perspective, the Design Light Consortium (DLC), offers a qualified products list. Runkle said if a light is a DLC-listed product growers have some assurances not only about its efficiency, but also eligibility for rebates and durability.

“The DLC list ties into energy efficiency and potential grow light rebates,” Runkle said. “Usually if a light meets the criteria to be on the DLC Qualified Products List, there is good chance it will be eligible for some type of rebate. There are other criteria necessary for a grow light to be a DLC-listed fixture. These include having at least a five-year warranty and having a tolerance to damp conditions.

“Generally the relative humidity conditions are going to be higher in a greenhouse than in an indoor facility. For propagation there might be 100 percent relative humidity. There may be some indoor farms that don’t have adequate HVAC systems to provide sufficient air conditioning capacity resulting in high humidity that can cause major problems. Under these environmental conditions growers are going to need grow lights that can tolerate high humidity.”

Choosing a lighting company

Runkle advises growers to consider a lighting company’s reputation and reliability.

“There are so many different LED lighting companies marketing their products to the horticulture industry,” he said. “Unfortunately some have gone out of business. There could be a company that offers a five-year warranty, but if it goes out of business the company is not in a position to support that warranty. Having a warranty is good, but equally important is whether the company will be around to support that warranty if the fixture doesn’t perform.

“We’re all looking for a good deal. But if a good deal seems too good to be true, it probably is. Talk to other growers who are using the products to find out about performance and company reliability, especially if you haven’t heard of the company before.”

For more: Erik Runkle, Michigan State University, Department of Horticulture, runkleer@msu.edu; https://www.canr.msu.edu/people/dr_erik_runkle.

This article is property of Urban Ag News and was written by David Kuack, a freelance technical writer in Fort Worth, Texas.