Using energy-efficient technologies to produce bedding plants and microgreens in protected and controlled environments

Production of bedding plants in commercial greenhouses (GHs) located in northern latitudes begins in late winter and continues through late spring when low outdoor temperatures generally necessitate active heating to maintain temperatures suitable for growth and development. Meanwhile, year-round production of microgreens using multi-layer systems requires sole-source (SS) photosynthetic lighting for production. Energy used to provide active heating in commercial GHs or SS lighting in multi-layer systems is second only to labor as the most expensive indirect cost for specialty crop production in controlled environments. High-tunnels (HTs), root-zone heating (RZH), and light-emitting diodes (LEDs) are energy-efficient technologies used for protected and controlled environment production of specialty crops. However, limited research-based information is available regarding HTs or RZH for energy-efficient bedding plant production, or SS LEDs for microgreen production. The objectives of this study were to quantify the effects of: 1) three transplant dates in an unheated HT compared to a heated GH on growth and development of cold-tolerant bedding plants (Experiment 1); 2) five RZH temperatures in combination with reduced GH air temperatures compared to a commercial air temperature (control: CC) on growth and development of cold-tolerant, - intermediate, and -sensitive bedding plants (Experiment 2); and 3) LEDs of different light qualities and intensities on growth, morphology and phytochemical content of Brassica microgreens (Experiment 3). In Experiment 1, dianthus and petunia transplanted in week 13 were 33% and 47% shorter and had 51% and 31% more visible buds, respectively, when grown in a HT compared to a GH. However, there was a one week delay in time to flower (TTF) for dianthus and petunia in the HT, compared to the GH. In Experiment 2, as RZH temperature set-points increased, TTF of all cold-tolerant and -intermediate species decreased; however, there was a delay in TTF when compared to the CC. For example, compared to petunia and marigold grown with no RZH, TTF decreased by 10 and 6 d, respectively, when grown with a RZH set point of 27 °C. However, TTF of both species was delayed by 4 d when grown with a RZH set point of 27 °C and a reduced air temperature, compared to the CC. In Experiment 3, regardless of SS LED light quality, as daily light integral (DLI) increased from 6 to 18 mol*m-2*d-1 hypocotyl length decreased and percent dry weight increased for kohlrabi, mustard, and mizuna microgreens. Additionally, an increased DLI and light ratios (%) of red:blue 87:13 or red:far-red:blue 84:7:9, significantly increased total anthocyanins of kohlrabi compared with those grown under red:green:blue 74:18:8. Overall, the results obtained from these experiments indicate that HTs, RZH, and SS LEDs can be used for bedding plant or microgreen production and could reduce energy costs