The 2011 ASHS Annual Conference

Reducing greenhouse temperatures when ambient irradiance levels are low has long been a strategy to reduce energy costs and improve plant quality in greenhouse crop production.  A question arose as to the impact of periodic temperature reductions paired with periodic ambient low irradiance in winter and early spring in the northern US on crop growth.  Therefore, five species (Antirrhinum majus L. ‘Montego Violet’, Begonia semperflorens Hook ‘Cocktail Vodka’, Impatiens wallerana Hook. f. ‘Accent Red’, Petunia x hybrida Hort. ex. E. Vilm. ‘Supertunia Vista Bubblegum’, and Viola x wittrockiana Gams ‘Delta Premium White Blotch’) of herbaceous ornamentals were moved from “winter normal” (21.1/18.3 °C day/night leaf temperature, 300 µmol·m^-2·s^-1 irradiance, 10 h photoperiod) to “winter heat savings” (12.7/10 °C day/night leaf temperature, 50 µmol·m^-2·s^-1 irradiance, 10 h photoperiod) conditions for 0, 1, 2, 4, or 7 d per week to assess the impact of heat savings strategies on plant growth.  The “winter normal” environment represented typical winter greenhouse environmental conditions and “winter heat savings” represented cool, low light environments in the northern US, Canada and northern Europe.  Plant data were collected after 7 weeks.  Species and exposure day number interacted to affect plant height, plant width, flower number, and above-media dry weight.  P. x hybrida dry weight was lower after 2 d per week in “winter heat savings” conditions, while the other 4 species had reduced dry weight only after 7 d.  Flower size was unaffected, but total flower number decreased after 2 d in “winter heat savings” conditions on I. wallerana; after 4 d on A. majus and B. semperflorens; and was unaffected on P. x hybrida and V. x wittrockiana.  Time to first flower was delayed for all species after 4 d in “winter heat savings” conditions.  For instance, growing plants in “winter heat savings” conditions for 4 d per week delayed flowering by 7 d in V. x wittrockiana and 14 d in B. semperflorens (with delays of greater than 6 weeks observed in the 7 d per week treatment).  Depending upon species, plants can be grown for up to 4 d per week in the “winter heat savings” environment without decreased dry weight, flower number, or flower size, but flowering will be delayed.