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 Cornell, all 10 species survived in the unheated high
tunnel environment where the lowest recorded temperature was -3.2 degree C. Cold-sensitive
species angelonia, celosia, and vinca survived this short-term cold exposure.
However, 7 of the species showed a significant delay in flowering when grown in
the high tunnel compared to when they were grown in the heated greenhouse.
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
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."