Greenhouse bedding plant production begins in mid-to-late winter and early spring in temperate northern latitudes when ambient outdoor air temperatures and photosynthetic daily light integrals (DLI) are at seasonally low levels. During this time, GHs are heated to maintain desirable production temperatures. The energy for heating in northern climates accounts for 10% to 30% of the total operating costs of commercial GHs (Brumfield, 2007; Langton et al., 2006). Therefore, producers are seeking alternative bedding plant production strategies that reduce overall consumption of fuel.
Although HTs are not designed to be equivalent to GHs for year-round protected crop cultivation, there is potential for seasonal use in semiprotected crop cultivation. In floriculture, HTs are employed in cut flower production for season extension and frost protection (Armitage and Laushman, 2003; Byczynski, 2008; Lamont, 2009; Wien, 2009). HTs generally lack automated ventilation and are passively cooled through side- and/or end-wall and gable ventilation (Lamont, 2009). As a result, daytime air temperatures inside HTs are ≈5 to 20 °C warmer than ambient outdoor air temperatures (Lamont, 2009). However, night temperatures in HTs lacking heating systems are similar to, or can be even lower than, ambient outdoor air temperatures (Ogden and van Iersel, 2009). In addition to daily minimum and maximum temperatures, the large diurnal fluctuation in air temperature inside unheated HTs may impact bedding plant development and quality. Internode elongation increases as the difference between the day and night air temperatures (DIF) increases, and is suppressed as DIF decreases (Erwin and Heins, 1995). Excessive stem extension as a result of a large positive DIF usually warrants the application of chemical plant growth retardants.
The combination of low initial capital investment for an HT, reduction or elimination of fuel consumption for heating, and the relatively short crop time for many annuals, suggests that HTs may be a suitable alternative structure for seasonal spring bedding plant production in northern latitudes. Our research objectives were to quantify the growth and development of several bedding plant species in an unheated HT compared with a traditional GH at northern latitudes.
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