Florinet Floriculture Newsletter - Greenhouse Energy Saving Tips

During the last two years, growers' worries and complaints have grown as fuel prices have climbed. Some growers have even told me that they would not be able to survive if energy costs keep rising. With present energy costs, growers may be have to adopt many of the energy saving techniques that were developed during the seventies and, to some extent, forgotten during times of bonanza.

The amount of fuel used to maintain a given temperature in the greenhouse depends on many internal and external factors. However, a "rule of thumb" indicates that for each degree Fahrenheit we reduce the greenhouse temperature, we reduce 3% the cost of heating. If this rule is true, even modest reductions in greenhouse temperatures can produce substantial energy savings. Growers are always tempted to lower the temperatures on their thermostats.

The question is: can we reduce greenhouse temperatures and still produce high quality crops? The answer to this question is yes, no and it depends. Very clear, right? Some crops like vincas and begonias suffer at lower temperatures and take too long to reach shipping size. Crops like nemesias and trailing snapdragons may actually reward lower temperatures with improved flowering. Other crops, like osteospermums, require low temperature for flowering by a process called vernalization. But, in addition to lowering temperatures, what other options do growers have? We'll discuss a few of them in this article. Before I proceed, I would like to call to the attention of the readers that I will provide information about the response of crops to low temperature as the literature has presented it through many years. Cultivar specific responses can be different to the general response of a crop. As a consequence, growers are advised to contact the breeders for cultural information before growing large quantities of plants at temperatures lower than those described in the literature.

During winter and spring in the US, it is very rare to enter a greenhouse and find it filled with a single crop. Most likely, in the greenhouses growers have a variety of crops all with different requirements of temperature, watering, pH, etc. Creating compartments in a large greenhouse or using different separate structures to house crops with different temperature requirements represents a way of saving fuel with crops that tolerate low temperatures and even increasing quality for crops that prefer it warmer. (There are reasons other than temperature that growers should compartmentalize their production facilities but that will be the topic for another article.)

Your snapdragons, osteospermums, cyclamens, dianthus, etc. can be grown in a separate house or section at a lower temperature. However, remember that "there is no free lunch". Temperature affects development (or the aging process of the plant): the lower the temperature, the longer it takes a plant to complete its life cycle. In other words, it will take longer for a plant to be ready for sale at lower temperatures. You have to take this into consideration when making the decision of lowering greenhouse temperature: you may be heating less for a longer time. Lowering the growing temperature of a warm crop (e.g. vinca), will have a greater impact on timing (take longer to selling stage) than the same temperature reduction on a cool crop (e.g. petunias).

Avoid reducing temperature at critical stages of plant development such as germination, floral initiation, and floral development. For example poinsettia bract growth is better at temperatures above 68 F. Usually, towards the end of the crop cycle temperature reductions are positive since they allow to maintain the crop in the greenhouse for a longer time and produce its hardening before is sent out. For example, marigolds can be grown at 60 F but can be hardened at 50 F; pansies can be grown at 55 F and hardened at 45 F. As a rule of thumb, hardening can be achieved at temperatures 10 F below growing normal temperatures for many crops. At least for a few days, before selling the crop, growers can have some energy savings.

Growers are tempted to lower the thermostat at night because is when they see the heaters work the most. The result is a larger difference of temperature between the day and night or in other words a larger +DIF which may result in stretching. To avoid stretched and low quality plants, more plant growth regulators will have to be used. If you plan to reduce greenhouse temperature, reduce BOTH day and night temperatures trying to maintain the smallest +DIF possible. Again, this still will result in longer growing times.

This advise is for all soils but specially for those that are unheated. If putting the crops on benches is not feasible (bedding plants growers usually have many flats directly on the floor), at the very least, flats should be raised 3 or 4 inches from the soil surface. Unheated soils are usually colder that the greenhouse air. It takes more energy to warm up the roots of plants that are sitting on the soil than those that are separated from it. In addition, warmer roots are less prone to diseases.

Bottom heat puts the heat near the plants. It keeps roots and foliage warm without heating the large volume of air of the greenhouse. There are different systems using this principle. Initial installation cost may be an issue for some growers. Additional benefits of this heating method is that it keeps the plants dry even when the greenhouse temperature is reduced. avoiding some diseases such as Botrytis.

Thermal blankets are one of the most effective tools of greenhouse environmental control. They can be used to reduce temperatures during the day and keep the heat inside the greenhouse at night. This tool also has a high installation cost.

Although limited, growers have the option of selecting shorter cycle crops (fewer weeks from start to finish) and/or crops that grow at lower temperatures. Crops that grow faster can be sown and planted later requiring less number of days of heating. These faster crops/cultivars will make use of the higher levels of light and possible higher temperatures later in the growing season. Scheduling, can become a tool for energy saving.

Keep the crop in a small area when the crop is small and plants can be kept pot to pot and then, progressively, open and heat new areas (greenhouses or sections of a greenhouse) when the crop has to be spaced because of its growth. In this case it is very important to make efficient use of greenhouse space. If benches have to be used, movable benches are probably the best. Also, how plants are distributed on benches is very important: staggering pots is more efficient that placing them in rows. It is possible to put 15% more pots by staggering than by standard row spacing.

If greenhouse temperatures will be lowered to conserve fuel, preheating irrigation water becomes very important. Depending on the crop, low temperatures may reduce root activity and absorption of nutrients may stop. Furthermore, low temperatures may stress roots and make them more susceptible to diseases.

Assuming that crops that can be grown cool can also be sold profitably, switching to them should make sense. However, proper consideration of the length of the growing cycle (crop timing) and quality of the crop grown at lower temperatures should be taken into consideration before deciding. It may not be an advantage to heat less for a longer time! Many bedding plant growers have multiple cycles of a given crop. Growing crops colder may interfere with how many crop cycles they can obtain from a given area. These type of decisions require growers to have a good idea of the costs of production of each one of their crops. Unfortunately not all growers have a handle on this issue.

The following is a list of crops that can be grown at cooler temperatures (below 60 F). A few of those can be grown at temperatures in the lower fifties. Some are "old" and well known crops. Others have become popular more recently.

Alstroemeria	Delphinium	Osteospermum
Alyssum	Dianthus	Pansy
Anemone	Diascia	Primula
Argyranthemum	Dill	Ranunculus
Aquilegia	Freesia	Schizanthus
Bracteantha	Fuchsia (some cv.)	Snapdragons
Calceolaria	Gazania	Statice
Calendula	Iris	Torenia
Centaurea	Matthiola	Tulip
Coreopsis	Nemesia	Veronica
		Viola

A typical greenouse structure contains heat as efficiently as a colander contains water. Of the many ways heat escapes a greenhouse, some can be either eliminated or substantially reduced. It is imperative for growers to seal holes in the polyethylene film and cracks in the walls. Doors and vents that do not shut properly should be repaired. Walls that are not made of translucent materials should be insulated. Metals conduct heat so all metal parts having contact between the greenhouse interior and exterior should be insulated. For example, metal gutters of gutter-connected greenhouses have one surface on the (warm) interior and the other surface on the (cold) exterior. Some growers insulate the interior surface of gutters with foam to reduce heat loss. One word of caution before improving insulation: growers located in climates with heavy snow falls benefit from their heat inefficiency because snow melts fast on their greenhouse roofs. If snow does not melt and accumulates on the roof, the greenhouse can collapse due to snow weight.

There are good sources of information on how to weatherize greenhouses, for example the website of the National Greenhouse Manufacturers Association (www.NGMA.com).

Growers should be careful when reducing greenhouse growing temperatures in order to save fuel � especially night temperatures. The two most reasonable approaches are to compartmentalize and to make an effort to reduce heat losses by greenhouse weatherization. In addition, changing/adopting some of the crop management practices described above should help growers conserve energy without sacrificing crop quality.

A few thoughts on how to deal with high heating costs

A few thoughts on how to deal with
high heating costs