In response to rising energy costs over the past several years, greenhouse growers have implemented a variety of strategies to reduce costs, including lowering their air temperature set points, increasing insulation, starting production later in the season, consolidating production, installing thermal energy curtains, contracting fuel, purchasing energy-efficient heaters, or switching to alternative fuel sources (Hopkins, 2001). Today, energy for heating in Northern climates accounts for 10% to 30% of the total operating cost of commercial greenhouses (Brumfield, 2007; Langton et al., 2006). The cost to heat a greenhouse has increased in the past decade because the cost of fuel (e.g., natural gas, propane, and heating oil) has more than doubled. The average price of natural gas sold to commercial growers in the United States rose from $19.61 per 100 m3 in 1998 to $45.82 per 100 m3 in 2008 (Energy Information Administration, 2011). During the same time period, the average wholesale price received by U.S. greenhouse growers for 5″ or larger potted poinsettias has only increased by 14%, from $4.24 to $4.82 (U.S. Dept. of Agriculture, 1999, 2009).
The holiday poinsettia is the second most valuable potted flowering crop in the United States with a reported wholesale value of $146 million in 2010 for the 15-top producing states, representing 4% of the total wholesale value for floriculture crops (U.S. Dept. of Agriculture, 2011). Of the 36 million plants sold in 2010, the majority were red cultivars. Poinsettias are propagated from shoot-tip cuttings and grown in greenhouses from July to early December for sales in November through December. As energy costs continue to increase and poinsettia prices remain constant, poinsettia producers are lowering their greenhouse air temperatures (Faust and Kehoe, 2007; Lopez and Krug, 2009) without knowledge of cultivar-specific consequences.
Temperature controls the rate of plant development, including time to unfold a leaf and time to flower. In addition, if temperatures are at or below the species-specific base temperature (Tb, estimated to be 10 °C for poinsettia), developmental rate is zero (Berghage et al., 1990; Roberts and Summerfield, 1987). Research conducted by Liu and Heins (2002) on the moderate-vigor poinsettia ‘Freedom’ indicates that as the photothermal ratio [ratio of daily light integral (DLI) to temperature] decreases after the start of short days (SD), bract size and cyathia diameter decrease linearly. Therefore, it is imperative that the effects of RTF on scheduling and plant quality are quantified on moderate- to high-vigor and/or early-flowering poinsettia cultivars. Two red cultivars from each of the five major poinsettia breeding companies were selected for investigation based on their early response attributes (6-to 8-week response time), moderate to high vigor, and naturally large bracts because these attributes are essential for RTF.
RTF has the potential to reduce energy consumption and costs associated with production while increasing profitability (Faust and Kehoe, 2007). To our knowledge, no peer-reviewed studies have been published on the effects of RTF on a wide breeder selection of modern commercially available cultivars. The objectives of this study were to quantify how RTF influences time to anthesis, plant height, bract area index, and bract area-to-height ratio of eight modern poinsettia cultivars.
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