Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/1/2016
Publication Date: 1/1/2018
Citation: Boldt, J.K. 2018. Short-term reductions in irradiance and temperature minimally affect growth and development of five floriculture species. HortScience. 53:1-5.
Interpretive Summary: Energy inputs are a major cost associated with producing ornamental plants in greenhouses during winter or early spring. This is due to the combination of low outside temperatures and low light levels (due to short days and a low sun angle), which requires significant expenditures for heating and supplemental lighting. Less heating is needed on sunny days, compared to cloudy days, due to heating of the greenhouse by sunlight. One possible energy-efficient alternative is to optimize the growing environment on days that have a low heating demand (i.e., warmer outside temperatures, sunny, and/or low wind) and reduce heating and lighting on days that have a high heating demand (i.e. cold, cloudy, and/or windy). This technique could be effective if it does not cause an increase in crop production time, reduce plant quality, and result in higher overall energy costs. Therefore, the objective of this study was to evaluate the impact growing plants for one or more days per week in a reduced energy environment would have on quality and flowering characteristics. Five species of bedding plants were chosen in order to encompass a range of cold-sensitive and cold-tolerant crops: angelonia, lantana, dianthus, pansy, and petunia. Two greenhouse environments were set up. The “normal” greenhouse environment was typical for winter or early spring production – 72 °F day/65 °F night (22/18 °C) and supplemental lighting when cloudy. The “reduced energy” greenhouse environment had lower set points – 55 °F day/50 °F night (13/10 °C), no supplemental lighting except to extend the day length, and an energy curtain pulled closed. Plants were grown in the reduced energy greenhouse for 0, 1, 2, 4, or 7 days per week, every week, until harvest. As the exposure duration in reduced energy conditions increased, days to first flower, flower number, plant height, plant width, net photosynthetic rate, relative chlorophyll content, and shoot dry weight decreased. Days to flower is a primary determinant of a floricultural plant’s marketability and readiness for sale. Flowering was delayed by less than four days when plants were grown in reduced energy conditions for 2 days per week, except for angelonia which was delayed by seven days. Although plant quality metrics decreased as the exposure duration in reduced energy conditions increased, plants grown for up to 4 days per week in the reduced energy environment were still of marketable quality. This may be an added benefit, as growers would have less of a need to chemically control plant size. Heating costs were calculated from the start of the experiment until flowering. Even though days to flower was delayed for plants spending one or more days per week in reduced energy conditions, the cost per day was reduced. This resulted in energy savings of 2% to 10% when plants were grown for 1 day per week in reduced energy conditions, and up to 35% to 51% when plants were grown continuously in reduced energy conditions. In conclusion, many bedding plants can be grown for one or more days per week in reduced energy conditions without a marketable impact on plant quality or time to flower. Overall, results from this study indicate that growers in the northern US could grow plants in reduced energy conditions a few days per week as a way to reduce greenhouse energy costs during winter and spring production without dramatically increasing crop production time.
Technical Abstract: Energy inputs are a major production cost for greenhouse-grown plants, especially heating and supplemental lighting. A possible energy-efficient alternative is to optimize the growing environment on days that have a low heating demand and grow plants at reduced temperature and irradiance on days that have a high heating demand. The objectives of this study were to determine how many days per week annual bedding plants could be grown in a reduced-energy environment before delays in crop growth and flowering occurred and to calculate the potential energy savings. Angelonia (Angelonia angustifolia Benth. ‘Angelface Blue’), dianthus (Dianthus chinensis L. ‘Telstar Pink’), lantana (Lantana camara L. ‘Luscious Citrus Blend’), pansy (Viola ×wittrockiana Gams. ‘Matrix Blue Blotch’), petunia (Petunia ×hybrida Vilm. ‘Supertunia Mini Strawberry Pink Veined’), and petunia ‘Supertunia Vista Bubblegum’ were grown in a ‘winter normal’ (WN) greenhouse (22/18 °C day/night, supplemental lighting from high-pressure sodium (HPS) lamps, 14 h photoperiod) and transferred to a ‘winter low energy’ (WLE) greenhouse (13/10 °C day/night, energy curtain continuously closed, day-extension lighting with HPS lamps, 14 h photoperiod) for 0, 1, 2, 4, or 7 d per week. In general, days to first flower, flower number, plant height, plant width, net photosynthetic rate, relative chlorophyll content, and shoot dry weight decreased as exposure duration to WLE increased. Flowering on angelonia was delayed when grown 1 d per week in WLE, delayed on dianthus, petunia ‘Supertunia Mini Strawberry Pink Veined’, and lantana when grown 4 d per week in WLE, and delayed on petunia ‘Supertunia Vista Bubblegum’ when grown continuously in WLE. Macronutrient concentration increased or remained unchanged as exposure to WLE increased. Heating costs were estimated using Virtual Grower 3.0.9, and average cost savings of 1% to 8% occurred with 1 d per week in WLE, and increased to 32% to 51% when plants were grown continuously in WLE.