Growing high-quality petunias with energy efficient root-zone heating

Growers can grow petunias in a cooler greenhouse by using root-zone heating to save money and still produce a high-quality crop that flowers on time.

Three petunia varieties seven weeks after transplant, grown at 59 F and placed on bench without root-zone heating or with set points; or grown in a greenhouse without root-zone heating at 68 F. Photo: Roberto Lopez, MSU.

Three petunia varieties seven weeks after transplant, grown at 59 F and placed on bench without root-zone heating or with set points; or grown in a greenhouse without root-zone heating at 68 F. Photo: Roberto Lopez, MSU.

According to research conducted by Roberto Lopez, Michigan State University’s Department of Horticulture, and his graduate student Madeline Olberg at Purdue University, growers can reduce their day and night greenhouse air temperature set point down to 59 degrees Fahrenheit if they utilize root-zone heating and can produce a high-quality and compact petunia crop that flowers on time.

Seven commercially available and vegetatively-propagated petunia cultivars were all selected based on breeder input for cold-tolerance and vigorous growth: Petunia ‘Sun Spun Burgundy’, ‘Sun Spun Lavender Star’, ‘Sanguna Patio Red’, ‘Potunia Plus Red’, ‘Potunia Plus Purple’, ‘Supertunia Red’ and ‘Supertunia Bordeaux’. Additionally, Ryan Warner, MSU Department of Horticulture, provided two recombinant inbred lines (RILs) of petunia from his research program that were selected based on a relatively high development rate under cool (57 F) temperature, and a minimal reduction in development rate as temperature decreased from 68 to 57 F, from among the total population of about 150 RILs evaluated previously.

Plants were received from C. Raker and Sons as rooted cuttings or plugs (RILs), potted up, given a soft pinch and treated with Florel to reset the plant’s clock, thus eliminating any flowers or buds. They were then transplanted into 4.5-inch pots and grown under a 16-hour day consisting of natural daylight and day length extension lighting with high-pressure sodium (HPS) lamps. Plants were grown in either a greenhouse with an average daily temperature of 59 F, with or without root-zone heating set at either 70, 75 or 80 F or in a greenhouse with an average daily temperature of 68 F and no root-zone heating. The root-zone heat was supplied by hot water heat circulated through rubber tubing on the bench.

Upon completion of the experiment, plants were evaluated for time to flower and overall growth and quality. The data collected indicates that compact, high-quality petunias resulted when the greenhouse air temperature set was 59 F and the root-zone temperature set point was between 75 and 80 F (see photo). For more information about the study, read the July issue of Grower Talks.

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