Typical annual bedding plant production in northern latitudes (≥40°N) occurs from midwinter to spring in heated greenhouses. Unfortunately, greenhouse heating can account for 10% to 30% of operating costs for these greenhouse operations (Lopez and Runkle, 2014). The high cost and input required for greenhouse production has therefore given way to the consideration and implementation of alternative, low-cost production methods. One alternative is growing or finishing annual bedding plants in an unheated high tunnel. High tunnels consist of a frame, generally made of pipes or galvanized tubing, covered by a single or double layer of polyethylene (Lamont, 2009). High tunnels typically do not have any type of automated heating, cooling, or ventilation system, and thus are passively solar heated and cooled via sidewall, endwall, and/or gable vents (Lamont, 2009). Previous research at Purdue University and Cornell University has shown that unheated high tunnel production is a plausible method for some cold-tolerant bedding crops, but development is often delayed (Currey et al., 2014; Gerovac et al., 2015).
Rate of plant development is zero at or below the species-specific base temperature (Tb), increases linearly up the optimum temperature (Topt), after which it rapidly decreases and ceases upon reaching the maximum temperature (Tmax) (Blanchard and Runkle, 2011). Plants can be categorized by their Tb, such that when Tb ≤ 4 °C, 4 °C < Tb < 7 °C, or Tb ≥ 7 °C, plants are categorized as cold-tolerant, cold-intermediate, or cold-sensitive, respectively (Blanchard and Runkle, 2011). The lack of active heating in high tunnels results in a decreased mean daily temperature (MDT) compared with greenhouse production in northern latitudes during the traditional bedding plant season. For example, average MDT has been reported to be 6.3 to 7.5 °C and 1.4 to 3.6 °C colder, in April and May, respectively, in a high tunnel compared with in a greenhouse with an MDT set point of 18 to 21 °C in Tippecanoe, IN (Currey et al., 2014; Gerovac et al., 2015). Consequently, time to flower (TTF) of ‘Telstar Crimson’ dianthus (Dianthus chinensis), ‘Wave Pink’ petunia, and ‘Liberty Classic Yellow’ snapdragon (Antirrhinum majus) transplanted during week 13 were delayed by 8, 8, and 26 d, respectively, when grown in an unheated high tunnel compared with a heated greenhouse; although, at later planting dates, this delay was minimal or nonexistent (Gerovac et al., 2015). Currey et al. (2014) also found that ‘Dreams Midnight’ petunia, ‘Super Parfait Raspberry’ dianthus, and ‘Penny Lane Mix’ viola (Viola ×cornuta), planted during week 14, experienced no delay in development when grown in an unheated high tunnel compared with a heated greenhouse.
As greenhouse space becomes limited in the spring, some growers in northern latitudes begin growing outdoors (Crum, 2008; M.W. Olberg, personal observation). However, the morphological and developmental effects of unprotected outdoor bedding plant production are not well documented. MDT is often lower in outdoor production than in high tunnels during the traditional bedding plant production period, and therefore, further developmental delays could be expected (Wien, 2009). Developmental delays in greenhouse production are extremely undesirable due to the high costs of production per square foot, but crop delays, if planned for, are not as deleterious in low-cost production systems, such as high tunnel or outdoor production. Additionally, the DLI can be significantly higher in high tunnels compared with greenhouses and would be even higher outdoors (Currey et al., 2014; Gerovac et al., 2015). High DLIs have been reported to hasten flowering and improve overall crop quality, in terms of increased biomass accumulation, increased branching, more compact growth, and increased number of visible buds (Adams et al., 1998; Blanchard et al., 2011a, 2011b; Faust et al., 2005; Heins et al., 2000; Kaczperski et al., 1991; Oh et al., 2010). The production of more compact plants due to high DLI and increased air movement in outdoor production could also limit the need for chemical plant growth regulator (PGR) applications (Crum, 2008; Faust et al., 2005; Latimer, 1998; Liu and Heins, 2002; Moccaldi and Runkle, 2007). These benefits could consequently make high tunnel or outdoor production systems suitable for cold-tolerant and cold-intermediate bedding plant crops, although the risks of crop loss due to weather extremes must also be considered. High tunnel or outdoor production may also benefit the ultimate consumer, as plants become acclimated to outdoor conditions during production, before transplant into the landscape.
Although there is published information on the growth and development of various bedding plant species in unheated high tunnels, to our knowledge, there is no published information on the effects of unprotected outdoor production on cold-tolerant and cold-intermediate bedding plants, especially in northern latitudes. Therefore, the objectives of this study were to 1) compare the growth and development of 10 annual bedding crop species grown in an unheated high tunnel or outdoors, 2) evaluate the effect of a 1-week initial acclimation period in the high tunnel before outdoor production, and 3) quantify the effectiveness of these production methods for producing high-quality bedding crops. This information could be valuable for growers to determine what crops can be produced outdoors and what they can expect in terms of crop timing, morphology, and freezing injury, and for exemplifying any effects of an acclimation period before outdoor production.
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