Growing Trends: Temperature

Take precautions when growing at cool temperatures

High energy costs have led many growers to try to reduce their fuel costs by growing crops at cooler temperatures. Below are some things to think about if you are planning to lower your greenhouse temperature to reduce fuel costs.

Seed germination
Do not try to be frugal on heat here. Too-cool temperatures during seed germination will delay germination, reduce percent germination and decrease the uniformity. For most species, germination media temperature should be 72°F to 76°F. After a crop is established, most plants will better tolerate less than optimal temperatures.

Scheduling
Lower greenhouse temperatures will increase production time and flowering time. The increased crop production time may eliminate fuel savings gained by reducing the greenhouse temperature. Compare the total fuel cost of producing a crop under the temperature you would normally use with the increased production time of cooler temperatures with the lower cost per day.
 

Warm and Cool Crops
The genetic cold tolerance of plants varies depending on where they originate. Tropical and subtropical warm crops such as alternanthera, angelonia, begonia, browallia, cuphea, dahlia, fuchsia, heliotrope, impatiens (New Guinea and garden), lantana, vinca, celosia, cleome, coleus, cosmos, gomphrena, ipomoea, melampodium, portulaca, scaevola, sunflowers, torenia, zinnias and many vegetables are naturally sensitive to cool temperatures. Some warm crops should not be grown at less than optimum temperatures (less than 65 to 68°F nights for some, 68 to 72° for others) or quality will be compromised.

Some crops will finish into good quality crops when grown cool (62 to 66°F) including: argyranthemum, bidens, calibrachoa, diascia, lobelia, nemesia, osteospermum, annual phlox, bacopa, verbena, petunias, snapdragon, alyssum, dianthus and pansies.

Some crops are flexible and will perform well under a wide range of temperatures including bidens, calibrachoa, geranium, lobelia, annual phlox and bacopa.

Growing cool
Plants adapt best to cooler temperatures with a well established root system. Grow plants at their optimum temperatures to get root systems established quickly. Once plants are rooted out and starting to fill out, then lower the temperature.

It’s best to avoid growing pots directly on unheated ground. Ground temperatures can be 10 to 20°F cooler than air temperatures (unless root zone heated). This problem can be compounded when plants are placed on the floor of hoop houses just opened up after being unheated for most of the winter. Under some conditions, fifferences of more than 10°F in air temperature can also occur between the benches and the eave height.
 

Pests and diseases
Generally we think of aphids having more offspring and developing faster at warmer temperatures, which is one of the reasons why aphids such as green peach aphid and melon aphid are usually more of a problem in the summer months. However, recent studies at Cornell University showed that foxglove aphid actually has almost twice as many babies at 50° to 60°F than at 77°F. Foxglove aphid also develops faster than either green peach or melon aphid at 60°F ­­- meaning you will likely see foxglove aphids.

Some pathogens are more active at cool temperatures. Plants, greenhouse floors and benches stay wet longer after watering, creating favorable conditions for pathogen spores to germinate. Cool media temperatures increase the risk of root rots caused by Pythium, Rhizoctonia and Thielaviopsis. Pythium is favored by temperatures at 53 to 68°F and although Thielaviopsis can be a problem under cool or warm temperatures, 62°F is optimum.

When plants are grown cool, they require less water and the media stays wet longer, resulting in high relative humidity around the plants. Like other fungi, Botrytis germinates, infects and develops in a specific range of temperatures and relative humidity. Generally, spore germination and infection depends on a film of moisture for eight to 12 hours, relative humidity of 93 percent or greater and temperatures of 55 to 65°F. Following infection colonization of plants occurs at temperatures up to 70°F. Warm days followed by cool nights result in condensation on plant surfaces and favorable conditions for Botrytis.

Regularly monitor foliage and roots for disease and apply fungicides early. Fungicides work more slowly in cool growing media, so it may take longer to see results or an application may be less effective.

Try to keep floors and benches as dry as possible and keep air circulating using horizontal air flow.
 

Plant nutrition
In addition to air temperature, cold irrigation water also drops the temperature of the growing media. In cold growing media, the uptake of water and nutrients will be slower. Plants grown cool may require less water and less frequent irrigation, so plants tend to be fertilized less often. Although plants are being grown cool, it’s important to continue to fertilize and maintain proper fertility.

Have a question? You can write Tina at tsmith@umext.umass.edu.