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Commercial bedding plant production in northern latitudes often begins in late winter and continues through spring, when average outdoor temperatures require growers to actively heat their greenhouses (GHs). High tunnels (HTs) offer energy savings as they are passively heated and cooled structures that have a low initial cost. As a result, they have been used in northern latitudes to advance and extend the growing season and improve the quality of high-value horticultural crops. However, there is limited published information on growing bedding plants inHTs in northern latitudes.Our objectiveswere to quantify the effects of transplant date in an HT with or without a rowcover (RC) compared with a traditional heatedGHon the growth andmorphology of three cold-tolerant bedding plant species at two northern latitude locations, Purdue University (Purdue) and Cornell University (Cornell). Seedlings of snapdragon (Antirrhinum majus L. ‘Liberty Classic Yellow’), dianthus (Dianthus chinensis L. ‘Telstar Crimson’), and petunia (Petunia 3hybrida Vilm.-Andr. ‘Wave Pink’) were transplanted on weeks 13, 14, and 15 in 2012 (Purdue) and 2013 (both locations) andmoved to either a glass-glazedGHor anHTwithout (HT) or with a rowcover (HT+RC). Several quality measurements increased when plants were grown in the HT compared with those grown in the GH. Dianthus and petunia transplanted at Purdue during week 13 in the HT and HT+RCwere 33% and 47% shorter and had 51% and 31% more visible buds, respectively, compared with those grown in the GH. Similarly, petunia transplanted at Cornell during week 13 in theHT andHT+RCwere 45% and 43% shorter, respectively, than their GH counterparts. The shoot dry mass of dianthus and snapdragon at Purdue was significantly higher when grown in the HT compared with the GH, regardless of transplant week or the use of RC likely because of increased daily light integral (DLI) in theHT environment. There was about a 1-week delay from transplant to first open flower for week 13 dianthus (at Purdue) and petunia (at both locations) when finished in the HT or HT+RC vs. their GH counterparts. Such a delay would be acceptable to growers who want to reduce the use of chemical growth regulators and heating costs. However, at both locations snapdragon transplanted on week 13 to the HT orHT+RC environmentswere delayed by 22 to 26 days comparedwith theGH. A delay of over 3 weeks could interfere with a grower’s production schedule, possibly making this crop unsuitable for production in northern latitude HTs. 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. AHT typically is a single-layer, polyethylenecovered 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 andRunkle, 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 formarket is not possible in anHT 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 coldintermediate 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 830 HORTSCIENCE VOL. 50(6) JUNE 2015 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. Materials and Methods Expt. 1 Plant material and culture. Seedlings of snapdragon (A. majus L. ‘Liberty Classic Yellow’), dianthus (D. chinensis L. ‘Telstar Crimson’), and petunia (Petunia ·hybrida Vilm.-Andr. ‘Wave Pink’) in 288-cell (6-ml individual cell volume) plug trays were obtained from a commercial GH propagator (C. Raker and Sons, Litchfield, MI). The plant material was received at Purdue University (Purdue) in West Lafayette, IN (40 N latitude) in 2012 on 27 Mar. (week 13), 3 Apr. (week 14), and 10 Apr. (week 15) and in 2013 at both Purdue and Cornell University [Cornell, Ithaca, NY (42 N latitude)] on 28 Mar. (week 13), 4 Apr. (week 14), and 11 Apr. (week 15). The experiments at Purdue and Cornell are denoted as Expts. 1A and 1B, respectively. On each receiving date, 21 seedlings of each species were transplanted into 10-cm-diameter (480 mL) round containers filled with a commercial soilless medium comprised of (by vol.) 65% peat, 20% perlite, and 15% vermiculite (Fafard 2; Fafard, Inc., Agawam, MA). Plants were hand irrigated as necessary with water supplemented with water-soluble fertilizer (Peters Excel 21–5– 20; Everris NA Inc., Marysville, OH) to provide (in mg·L) 200 nitrogen (N), 26 phosphorus (P), 163 potassium (K), 1.0 iron (Fe), 0.5 manganese (Mn) and zinc (Zn), 0.24 boron (B) and copper (Cu), and 0.1 molybdenum (Mo). At Purdue irrigation water was supplemented with 93% sulfuric acid (Ulrich Chemical, Indianapolis, IN) at 0.08mg·L to reduce alkalinity to 100 mg·L. At Cornell, no acid supplementation was used, tap water alkalinity was 115 mg·L. Greenhouse environment. Seven plants of each species were randomly selected during each transplant week, spaced equally in trays, and placed on benches located in a glassglazed GH under natural photoperiods and a constant air temperature set point of 21 C. Temperature was maintained with exhaust fan and evaporative-pad cooling, radiant hotwater heating, and retractable shade curtains controlled by environmental computers (Maximizer Precision 10; Priva Computers Inc., Vineland Station, Ontario, Canada) and (Operator Program; Argus Control Systems, White Rock, British Colombia, Canada) at Purdue and Cornell, respectively. High tunnel environment. Seven plants of each species were randomly selected during each transplant week, spaced equally in trays, and placed in an HT on top of a layer of landscape fabric. At Purdue, the east-westoriented HT (14.6 · 7.9 · 3.7 m high) had a triple galvanized structural steel frame (FarmTek, Dyersville, IA) and 6-mm SunMaster polye

hortscience

High Tunnel versus Climate-controlled Greenhouse: Transplant Time and Production Environment Impact Growth and Morphology of Cold-tolerant Bedding Plants