Reduce Greenhouse Production Costs

Dealing with the high cost of energy for greenhouse operations can be a challenge, but close attention to detail and a few practical changes can help keep those costs in check.
By Joyce G. Latimer

Increased fuel costs and fluctuating winter temperatures make heating costs a significant burden on many greenhouse operations. So, how can growers deal with high energy costs in the greenhouse? The problem can be addressed in several different ways. Growers can conserve energy in the greenhouse, evaluate alternative or additional fuel sources or heating systems, evaluate growing temperatures and other production practices, consolidate operations into less space, critically evaluate when to bring the next greenhouse into production and, finally, streamline operations.

Regardless of what growers do to reduce energy use, they still have to examine how to pay for increased costs related to higher fuel prices. Growers have suffered increases in the prices of pots and plastic as well as peat, pesticides and fertilizers over the past few years. In addition to the increased direct costs of heating, growers will be faced with higher transportation costs as well - not just for the products they are delivering - but also for those they receive: It's been predicted that gasoline prices will continue to skyrocket this spring. So, in addition to reducing costs in the greenhouse, how can growers adjust their production and pricing to remain profitable?

The greenhouse structure

The first line of defense in efficient heating of a greenhouse is the structure itself. Losses vary depending on the greenhouse covering and the age of the structure. In general, newer structures will have better seals around the coverings and openings than older houses.

Maximize the insulation

Add a thermal blanket

Up to 85 percent of the heat loss from a greenhouse occurs at night. Using a thermal blanket to retain heat at night can be a cost efficient investment. These blankets are easier to install and create less shading in gutter-connected houses than in a Quonset house. Remember to use a porous curtain material so that condensation from the underside of the roof of the greenhouse will not pool above the plants. Use a flame-resistant material or, for growing structures, a thermal blanket that alternates flame-resistant and non-flame-resistant material. For greenhouse structures in which an internal curtain cannot be installed, external curtains are available that can reduce radiation loss from the greenhouse at night.

Heating system efficiency

Maintaining maximum heating efficiency of the existing heating system is critical to reducing heating costs in the greenhouse.

Warning: Do not inhibit the fresh air supply to the greenhouse heater. If you are using a heater that requires greenhouse air for combustion, be sure to leave about 1 square inch of opening for each 2,000 Btu/hr of heater output. If possible add an inlet pipe from outside air to serve the burner.

Add horizontal air flow

(HAF) fans

Reducing air leaks and heat loss in the greenhouse will make the house "tighter," which will also tend to increase the relative humidity. Regardless of the type of heating system used, it's important to install a sufficient number of horizontal air flow (HAF) fans to adequately circulate the air inside the greenhouse. Good air circulation will improve temperature and humidity uniformity in the greenhouse, which reduces the incidence of cold pockets in the greenhouse and improves plant quality and uniformity. Monitor the humidity level in the house, generally keeping it below 80 percent to minimize disease incidence, and be sure to vent when necessary.

For example, a 28-foot by 96-foot greenhouse requires an airflow of 5,376 cubic feet per minute (28 x 96 x 2 cubic feet per min per square foot = 5,376 cubic feet per minute). This greenhouse would require four HAF fans capable of moving air at 1,440 cubic feet per minute. This could be provided by four 16-inch fans with ¹/15-horsepower motors at 1,600 revolutions per minute. Horizontal air flow fans are generally available in two air flow capacities, but check the fan specifications to determine that they meet the calculated needs.

Environmental control

Alternative fuels

With significant increases in the per unit cost of natural and LP gases, many growers seek to evaluate alternative fuel sources. Some growers who had previously "upgraded" to natural or LP gas furnaces still had working oil, wood or coal-fired furnaces or boilers connected to their greenhouses, and switched back to those fuel sources.

Alternative heating systems

If costs are continuing to be a challenge, you may consider changing the primary heating system. Alternatively, growers should evaluate the efficiencies of different heating systems (Table 1) and consider using combinations of different types of heating systems for the greenhouse. Evaluate the newer, more energy efficient heating systems to determine the "pay-back" period for individual operations. It may be worth the investment.

Changing growing practices

It is logical that reducing the greenhouse temperature, especially at night, would reduce heating costs. In fact, reducing the night temperature by just 1°F can reduce a greenhouse heating bill by 2 percent to 3 percent. So, how low can we go? Greenhouse temperatures affect plant growth and flowering. In particular, they affect the time required to finish the crop. Be aware that some plants are more sensitive to lower temperatures and may cease to grow when a base temperature is met. This base temperature is lower for cool-season crops than for warm-season crops. In addition, growth is more strongly affected as the temperature approaches that base temperature.

Also, be aware that lowering the greenhouse temperatures can cause additional disease problems. You may want to run plants at optimum temperatures until the roots reach the edges of pots. Then lower the temperatures and run the plants drier to prevent root rot. Avoid overcrowding and provide horizontal air movement to ensure uniform temperatures and dry foliage. Use temperate irrigation water in the morning so the medium warms up faster and there is better nutrient uptake.

Recovering the costs

In many cases, growers are not comfortable adding a price increase sufficient to recover significant increases in fuel costs. In other cases, prices were set for many customers prior to fuel increases, and growers want to honor those commitments. In previous fuel crises, many growers across the country added fuel surcharges to their product prices. After doing the cost of production calculations, several growers found that adding a fuel surcharge to everything they sold during the spring would reverse their losses and restore a profit. They passed the fuel charges on to their customers, who in turn passed it on to the consumer.

Remember, you must determine your costs - and plan how to recover excessive costs due to high energy costs - so that you can afford to stay in business.

The costs of doing business in the green industry are ever changing, but it's unlikely we'll see a reduction in expenditures. Careful consideration of infrastructure and production practices can help keep those dollars in check - and put more of them in your pocket.

Joyce G. Latimer is an extension specialist in greenhouse crops at Virginia Tech, Blacksburg. She can be reached at jlatime@vt.edu.