Part of Greenhouse Management’s Lighting Virtual Conference Aug. 9 included presentations from Dr. Christopher Currey, assistant professor of horticulture at Iowa State University, Dr. Roberto Lopez, assistant professor and controlled environment and floriculture extension specialist at Michigan State University, and Dr. Ricardo Hernandez, assistant professor of horticultural sciences at North Carolina State University. The conference was sponsored by and included presentations from BLV, Heliospectra and PARsource.

The researchers provided a lighting primer, shed light on daily light integral requirements, and shared the economics of supplemental lighting. Here are some highlights from their presentations:

The what, when, and how of greenhouse lighting

In his presentation titled, “A Greenhouse Lighting Primer: What, When, and How,” Currey explained photoperiodic and supplemental lighting basics and what options are available to growers.

Location, location, location

According to Currey, light can be an issue based on where a grower lives. For example, growers living in the northern and southern extremes of the country are subject to less predictable light patterns throughout the year. Currey says that among the five factors that affect plant growth — light, water, mineral nutrition, gases and temperature — light is the one that is typically the hardest for growers to control. He says that this is especially true for growers who are not using any kind of supplemental lighting in their greenhouses.

Light is plant food.

Light matters for one reason: It drives plant growth. Light creates photosynthesis, Currey says, and photosynthesis in turn produces carbohydrates. Increased photosynthesis enhances overall plant growth, increases plant quality and improves yield in food crops such as tomatoes and eggplant. According to Currey, a one percent increase in photosynthesis leads to about a one percent increase in profit. For any greenhouse growers, proper lighting is essential to growing the best possible plants.

Dr. Christopher J. Currey
The limits of light

While increasing light does generally lead to improved growth, there is a limit on how much light can do, Currey says. In light intensity measured by micromoles, there is a light saturation point (LSP) where adding more light will no longer offer any added benefit to plants. At that point, a plant is at 95 percent of its maximum photosynthesis. 95 percent of the maximum marks the saturation point, Currey says, because the difference between 95 and 100 percent photosynthesis is statistically negligible. 

To calculate their plants’ LSP, growers can consult their local extension for assistance and collect the necessary information to make sure their lighting levels aren’t too high. If levels are too high, growers could be spending money on lighting that isn’t actually necessary. 

Shedding light on DLI requirements

In his presentation, “Shedding the Light on Crop-specific Daily Light Integral Requirements,” Lopez explained how to improve the production of various floriculture and produce crops using supplemental lighting to increase daily light integral (DLI). He advised growers to use quantum rather than photometric measurements to provide the most accurate measurements of both sunlight and electric light.

Incrementally increase DLI throughout cutting production.

For cuttings, growers should provide four to five moles of light per square meter per day during stages one and two, between the time cuttings are stuck to when calluses begin to form, Lopez says. In stage three, during root development, they should provide a DLI between eight to 12 moles per square meter per day. In stage four, they should increase the DLI to above 12 moles per square meter per day to produce a toned rooted cutting. Changing the DLI affects root biomass of various plant species differently. For instance, as DLI increases, the root dry mass of Argyranthemum and Diascia increase linearly, but the root dry mass of Angelonia levels off around seven moles per square meter per day.

Supplemental light influences plug production.

Researchers at Michigan State found that when light is below 10 moles per square meter per day, adding supplemental lighting provides the most benefit during the later stages of plug production, Lopez says. In their studies, the researchers found that the highest-quality plugs were those grown under constant supplemental lighting, rather than without receiving any supplemental light or only receiving supplemental light during select stages of production. Seedlings under high light typically flowered two to four days earlier than seedlings that didn’t receive any supplemental light. However, depending on the species and cultivar, flowering can occur up to two weeks faster under supplemental lighting than without.

Dr. Roberto Lopez
Daily light integral can interact with temperature.

With Tagetes patula ‘Bonanza Yellow,’ Lopez and his colleagues found that temperature and DLI affect flower size. Under cool temperatures and high light, flowers grew to the largest size, while under warm temperatures and low light, flowers grew to the smallest size. “As growers, this is really something important to think about,” he says. “If you’re really trying to accelerate the growth and development of your crop and you grow your plants in a really warm greenhouse but your light levels are low, you’re actually going to end up with a relatively poor-quality crop. A better thing is to really try to be more in the middle, where let’s say you have moderate temperatures and a moderate daily light integral.” 

The economics of supplemental lighting

Following a series of equations, growers can determine the capital expenditures supplemental lighting requires, says Hernandez, who delivered a passionate presentation titled, “The Economics of Supplemental Lighting.” In the presentation he explained, among other things, how to determine the number of lighting fixtures growers need in their greenhouse.

Follow an equation to design a light plan to determine the number of fixtures you need.

Growers need to know the following figures: supplemental light intensity (PPF [measured in µmol m-2s-1]), production area (A [measured in m2]), fixture efficiency (LPE [measured in µmol J-1 – µmol W-1s-1]), power draw (E [measured in watts]). By inserting these into an equation, growers can estimate N: the number of fixtures needed to achieve PPF. This is back-of-the envelope estimation.

Hernandez said growers should use the following equation: N = PPF x A / LPE x E

Dr. Ricardo Hernandez
Determine the values to plug into the equation.

Light power efficiency is calculated by emission rate (µmol s-1) / power draw (watts), which are figures that lighting companies provide. Lighting companies also calculate the utilization factor based on factors such as the height at which lamps will be installed, reflector capabilities of a lamp and light beam angles. The maintenance factor is related to the fixture itself, and pertains to factors such as how many hours the fixture is supposed to last before it breaks and how many hours should pass before the fixture reduces its light output.

Calculate the final number of fixtures based on greenhouse area.

The figure N represents the number of fixtures required per 1 m2. To determine the total number of fixtures they need, growers who have measured their greenhouse in acreage or ft.2 need to convert the acreage or ft.2 figure to m2. Then, they will need to multiply N by the total number of m2 in their greenhouse to determine how many lamps they need.

However, Hernandez says, growers need to take more factors into consideration and can plug in additional calculations to determine the costs and benefits of supplemental lighting. Such factors include the wholesale price of a crop and the gross margin on that crop. Weighing these factors will provide growers with a better idea of whether supplemental lighting is a feasible investment.

To purchase a link to view the full presentations, please contact Greenhouse Management at ktabor@gie.net. Webinar attendees should have received a link on Aug. 22.