A new study has shown that cold-tolerant annuals in unheated
or minimally heated high tunnels can be a viable alternative for commercial
producers aiming to reduce energy costs.
Energy costs are the largest expenses in commercial greenhouse production of annual bedding plants and can account for 10-30% of total operating costs. High tunnels are traditionally used in cut flower production for season extension and frost protection. Daytime air temperatures inside high tunnels can range from 5-20 degree celsius warmer than outdoor air temperatures, but temperatures at night can be about the same or occasionally lower than outdoor air temperatures.
The study conducted by Christopher Currey, Roberto Lopez, and Neil Mattson was published in HortTechnology. They compared traditionally heated greenhouses (with an 18 degree C temperature ) with unheated high tunnels for growing 10 popular bedding plants (angelonia, vinca, celosia, dianthus, geranium, petunia, french marigold, viola, snapdragon, and osteospermum) at Cornell University and Purdue University.
At Purdue, several species were lost because of a cold temperature event (-6 degree C), necessitating a second planting a week later. Osteospermum was the only species that flowered significantly later in the high tunnel with a delay of 7 days compared to the greenhouse-grown plants. Thus delaying the high tunnel transplant date was an effective method for reducing plant mortality and reducing delays in days to flower because of the cold temperature environment of the high tunnel.
Thus, finishing cold-tolerant bedding plants in high tunnels saved energy costs, and will enable growers to meet their seasonal demand in a cheaper-to-build option while still providing some protection compared to the outside environment. Further improvements were noted in measures of plant quality for some crops, such as shorter, more compact plants than greenhouse-grown counterparts which could reduce the need for application of chemical plant growth regulators.
The authors said, "While we acknowledge the risk of finishing spring bedding plant crops in an unheated environment, there is a potential for gains in profitability for producers because of the lower capital and energy costs of high tunnels as compared with conventional greenhouses."