Energy costs are a concern for every greenhouse grower, and it can be difficult to cut corners. Dr. Garrett Owen, assistant professor, sustainable greenhouse nursery systems, Ohio State, discussed energy saving options during a greenhouse management workshop sponsored by Ohio State.

Owen described the work he did in collaboration with USDA-ARS. The case study, which was based on accumulated simulation data, looked at spring flowering bedding plants to determine the water and carbon footprint of floriculture crops.

The study examined three different environments including an unheated, unlit high tunnel; an unlit, heated greenhouse; and a 68º F heated greenhouse with supplemental lighting to manipulate the photoperiod to maintain 16 hours of light.

The temperature in the high tunnel averaged about 12 to 15 degrees lower. “When we compared the environmental data to the greenhouses,” said Owen, “we saw a higher daily light integral – the amount of light plants received in 24 hours. Once we take environmental data into account, we can see how it significantly manipulated plant growth and development and marketability of two Coleus cultivars.”

Although theoretical simulation data is useful, a subsequent real-time study that measured energy and water use for snapdragons and petunias allowed researchers to put hard data behind carbon and water footprints. Data points began at the time of transplant and were collected until plants were marketable (about 21 days later).

Owen wanted to examine variations in greenhouse lighting. “The greenhouses were still heated to 68º,” he said, “but we wanted to simulate a grower that didn’t have supplemental day extension lighting.”

One house had industry standard high-pressure sodium lamps and two greenhouses had commercially available LEDs – one with a higher proportion of blue light, one with more red light.

“In general, all crops looked marketable,” said Owen. “However, under the high tunnel we had faded flowers, which is an indication of heat stress in an environment without fans to exhaust accumulated heat. Snapdragons grown under high tunnel conditions were not marketable compared to greenhouse-grown crops. Even with greenhouse-grown, we saw a delay in marketability or time to flower with or without supplemental lighting.”

He added that there were more superior plants under LEDs compared to those grown under sodium lamps. Petunias were all the same size under various growing conditions, but there were differences among snapdragons.

Horizontal airflow fans required the most energy compared to inflation blowers. “We understand horizontal airflow fans can be set to be on and off,” said Owen. “They needed to maintain a constant, and that’s where increased demand was seen compared to other components that help manipulate the environment.”

The study showed that horticultural lighting units accounted for 69% of total electricity. “This allows us to think about where to be more efficient in implementing strategies to help reduce electrical or heating costs associated with crop production,” said Owen.

“We’re looking at $2,800 to $3,200 to produce 4.5-inch snapdragons or petunias,” said Owen. “If you have a greenhouse that has about 78% ditching (space use) efficiency, then you will have about 38 cents per pot of energy and carbon costs associated with heating the greenhouse for producing snapdragons or petunias. Adding lighting adds about 43 [cents] per pot for 21 days. That excludes substrate, pot, labor and any chemicals.”

He described several energy-saving strategies for various production methods, including production delay if it isn’t worth starting a crop early. Plugs or rooted liners can be transplanted later, then greenhouses can be heated for seed production, plugs or cuttings.

Routine greenhouse repair and maintenance can help cut energy costs. Photo by Sally Colby

Consolidating production areas is another strategy. “It’s putting together crops that are cold temperate, cold intermediate or cold sensitive so you aren’t heating multiple greenhouses or bays,” said Owen. “Alternative growing areas can be low tunnels with retractable covers that provide cold protection and manipulate the photoperiod.”

In some cases, a high tunnel is a viable alternative. However, such structures may lead to heat stress which can affect plant growth, development and marketability. Heat stress can result in faded flowers, flower bud abortion and reduced aesthetic value.

Other strategies include repairing damaged parts of a greenhouse structure such as glazing material. Owen said this sounds simple, but glazing is often not intact. Growers should also address air infiltration under doors or through gaps where additions have been made, or air leakage around exhaust fans. Temporary plastic on doors can help growers manage greenhouse temperatures.

Cultural practices are influenced by reducing air temperature. Under cooler greenhouse conditions, applying fertilizer with excess ammoniacal nitrogen may induce ammoniacal nitrogen toxicity, resulting in plants that appear chlorotic.

Phosphorus deficiency can be induced at lower air temperatures because the substrate temperature is also cooler. Reduced root activity means roots are unable to take up sufficient phosphorus and deficiencies appear. However, some crops express different pigments or intensify foliage color or flower color under reduced air temperatures. It’s important to conduct nutrient/substrate testing to verify phosphorus deficiency related to air temperature.

Iron deficiency can occur under reduced air temperatures in relation to irrigation. Waterlogged substrate results in reduced root activity and induced iron deficiency. This can be overcome by increasing air temperature, reducing irrigation frequency and duration or providing fertilizer with iron chelate.

Thermal blankets over crops help create a microclimate to maintain a desirable temperature. Root zone heating provides heat precisely where it’s needed and can be done under benches or across the greenhouse floor. Root zone heating can also be done in high tunnels.

Root zone heating can improve plant quality and enhance marketability. In a cold tolerant genus such as Osteospermum, root zone heating had no positive effect. However, with a cold sensitive genus such as Vinca, the negative effect was overcome with 80º root zone temperature.

Air temperature has a significant effect on crop development and flowering. Owen explained that all plants have a base temperature at which crop development progresses, and as temperature increases, plant development increases to the optimal temperature.

“As we continue to increase temperatures, we’ll see a negative effect on the plant up until the plant stops developing,” he said. “That’s considered the maximum temperature.”

Plants can be grouped based on their base temperature: cold tolerant, cold temperate (intermediate) and cold sensitive. Grouping plants according to base temperature allows growers to make energy efficient decisions on reducing air temperature in different houses, such as for cold-sensitive tropicals.

“If we know how to group plants according to their base temperature,” said Owen, “we can make energy efficient decisions on reducing air temperature in different houses.”

by Sally Colby