Greenhouse - More on Lowering Heating Costs

The following is another short article on reducing greenhouse heating costs from Michigan State University.

Fuel costs for many greenhouses will be substantially higher than they’ve been in recent history, and perhaps ever. I have compiled a list of suggestions for how greenhouse operations can reduce energy consumption, accelerate crop development or improve space efficiency. Some of these suggestions require capital investment, but I believe in most situations, the return on investment can be significant.

Don’t cheat on heat

Crop timing increases as temperature decreases. If you plan to market the plants on the same date as in the past, then you need to begin growing the crop earlier, meaning you will have to start heating your greenhouse earlier. Energy consumption per crop grown in the spring can be higher when crops are grown cool because you have to heat the greenhouse for a longer period of time.

Use a retractable energy/shade curtain

Most of the energy consumed by a greenhouse is used for heating, and most (perhaps 80%) heating occurs at night. Deploying a retractable shade/energy curtain at night can significantly reduce heat loss by providing another insulating layer to the greenhouse.

Provide supplemental lighting to plugs

A majority of seedlings and cuttings are produced in late winter and early spring, when natural light levels in the northern half of the United States and Canada are low. Increasing the light level can accelerate crop development by increasing plant temperature and by reducing the number of leaves formed before the first flower develops.

Provide long days to long-day plants

Many bedding plants and perennials are long-day plants, meaning that they flower earlier when grown under a long photoperiod. Common examples of long-day plants include ageratum, blue salvia, dianthus, pansy, petunia, rudbeckia, snapdragon, and tuberous begonia. During the spring, the photoperiod is naturally short until April, so flowering of early long-day crops is delayed unless artificial long days are provided.

Improve insulation

Look for gaps near fans, pads and doors; make sure there are no holes or gaps in your roof; and consider adding an extra layer of insulation to your north wall. Just be mindful not to reduce incoming light too much – or the quality of your crops might be compromised.

Grow cold-tolerant and cold-sensitive crops separately

Not all plants respond to temperature the same way. Plants like vinca and celosia grow very slowly at 60°F (16°C) while other plants such as ageratum, pansy and ivy geranium continue to grow moderately well at this low temperature. Whenever possible, grow cold-tolerant crops in one greenhouse and cold-sensitive crops in a separate greenhouse.

Only open up a greenhouse when it can be filled

Once a greenhouse is opened for use in the early spring, heat needs to be used whether it is full of crops or not. If you have multiple ranges, try to schedule your spring crops so that you fill each greenhouse when first opened.

Use a larger plug size to reduce final crop timing

During the finish stage, there are fewer plants per square foot of greenhouse space compared to during the plug stage. Thus, heat and lighting costs per plant are lower when plants are grown at the higher plant density during the plug stage. By using a larger plug size, you could increase your plug stage duration and reduce the time of your finish stage.

Install horizontal air flow fans

Horizontal air flow fans not only mix warm air with cool air, but also improve the uniformity of temperature within the greenhouse. If you already have horizontal air flow fans installed, make sure they are all operating and are well positioned (i.e., not angled down towards crops or upwards towards the roof).

Increase your heat/vent deadband

You want to avoid frequent cooling/heating cycles, and the best way to do this is to increase the temperature deadband during the winter and spring. A slightly larger increase in the venting setpoint will allow you to take advantage of heating from the sun. Remember to monitor your average daily temperature and make adjustments as necessary to stay on track with your production schedule.

Reprinted from "Lower your spring heating bill" by Erik Runkle in the October, 2005 edition of the Greenhouse Alert Newsletter from Michigan State University.