Annual bedding plants are the most valuable sector of the commercial floriculture industry, accounting for 62% of the reported wholesale value of $5.9 billion in the United States (USDA, 2014). Commercial GH production in northern latitudes begins in late winter and continues through spring, when the crops are marketed to consumers. In temperate climates, outdoor temperatures during production necessitate protected cultivation with active heating to prevent crops from freezing and to ensure that growers meet specific market dates. However, with the relatively volatile prices for propane, heating oil, and natural gas during the last decade, heating now accounts for 10% to 30% of the total operating costs for commercial GHs (Brumfield, 2009; EIA, 2014; Langton et al., 2006). To reduce costs associated with heating, growers have installed thermal energy curtains, increased insulation, switched to alternative fuel sources, and purchased energy-efficient heaters (Blanchard and Runkle, 2011a). Some growers in northern latitudes are starting to grow bedding plants in HTs to further reduce or eliminate heating costs (Steve Hood, personal communication). Additionally, HTs can provide warmer day temperatures and protection from rain as compared with plants growing in an outdoor environment. However, there is limited published information regarding bedding plant production in HTs.
A HT typically is a single-layer, polyethylene-covered structure that lacks automated ventilation, is heated by solar radiation, and is cooled through side or end walls that are manually opened and closed (Lamont, 2009). They are primarily used in temperate northern latitudes to extend the production season and improve the quality of high-value horticultural crops, including vegetables, fruits, and cut flowers (Hunter et al., 2012; Knewtson et al., 2010; Lamont, 2005; Ortiz et al., 2012; Rowley et al., 2010). Additionally, they are used in temperate and tropical regions of the world to exclude rain from crops, which reduces disease pressure and crop loss (Lamont, 2009). Recent research has shown that growers can use HTs to reduce or eliminate heating costs associated with finishing cold-tolerant bedding plants in northern latitudes (Currey et al., 2014).
Greenhouse growers use average daily temperature (ADT) to predict when crops will be marketable (Blanchard and Runkle, 2011a). It is well documented that temperature controls the rate of plant development, including time to unfold a leaf and time to first open flower (Adams et al., 1998; Kaczperski et al., 1991; Roberts and Summerfield, 1987). Plant development is zero at or below a species-specific base temperature (Tb). As temperatures increase above Tb, the rate of development increases until the optimum temperature (To) is reached. For many crops, the development rate increases nearly linearly with ADT between Tb and To (Blanchard and Runkle, 2011a; Roberts and Summerfield, 1987). This linear relationship enables growers to predict when crops will be marketable based on the ADT. Consequently, a grower’s ability to predict when their crops will be ready for market is not possible in an HT due to lack of temperature control. Notwithstanding this limitation, in some situations, the energy savings of reduced or no heating associated with HT bedding plant production can still outweigh the ability to schedule crops for specific market dates (Currey et al., 2014).
A comparison of finishing spring bedding plants transplanted during week 14 in HTs to a GH revealed that dianthus (D. chinensis), petunia (Petunia ×hybrida), and pansy (Viola ×cornuta) could be produced in an HT with little to no delay in time to flower. For example, dianthus, petunia, and pansy grown in an HT were delayed by as few as 4, 4, and 0 d, respectively, compared with a GH (Currey et al., 2014). However, a −6 °C night resulted in the death of several cold-sensitive and cold-intermediate species. This revealed the potential risk associated with the production of spring bedding plants in HTs. Since several cold-tolerant species survived the cold night and were only slightly delayed in flowering time, we investigated the effects of transplant week to determine if earlier transplant times were possible. To our knowledge, no work has been performed to determine the effects of early-season transplant (weeks 13 to 15) of cold-tolerant bedding plants in unheated HTs located in temperate northern latitudes. Also, we postulated that a RC could reduce the impact of low temperatures, as demonstrated by Currey et al. (2014). Therefore, the objectives of this study were to quantify the effect of three transplant dates in two northern latitudes, the use of a RC, and holding plants in a heated GH before moving them to an HT on the growth and development of three cold-tolerant bedding plant species.
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