Three Factors Affecting Perennial Plant Hardiness
Three studies were conducted during the winters of 2002-2003 and 2003-2004 for the thesis research of Andrea Luchini, looking at factors affecting cold hardiness of container-grown herbaceous perennials. The aim of the studies was to study the effect of various factors in order to provide more information to growers who overwinter perennial plants in containers. Plants in containers are generally less hardy than identical plants in the ground for a number of reasons. The factors, or treatments, we chose to look at during this study were the effect of date in the fall that plants were brought into a controlled, non-freezing, greenhouse setting (acclimation date study); age of plants (age study); and the effect of temperature cycles (above and below freezing) in both winter and spring (cycling study).
After these treatments, plants were frozen to about 28.5° (-2°C, control), 23° (-5°C), 17.5° (-8°C), 12° (-11°C), and 7°F (-14°C) in chest freezers, with computerized temperature controllers, at the UVM greenhouse. Temperatures in the freezer were also monitored with special temperature recording devices. The plants were allowed to re-grow throughout the spring at which time visual ratings as well as dry weight data was taken. All plants were potted in standard 4" pots with a soilless potting medium. This research expands upon and continues research done for several years by Leonard Perry and his graduate research assistants, and will continue in 2005.
For the two geranium cultivars, dianthus, and coreopsis in the acclimation study, there was no common date best for all to be brought into a cool, but controlled greenhouse from the cold. This confirms other previous studies that showed it is not crucial when perennials in pots outside are covered (protected from cold) in the fall.
For the plant age study, with dianthus and two geranium cultivars, the most pot-bound plants had less survival than those growing more vigorously. With only one year of data for each, though, plans are to repeat this study.
For the cycling study, with temperatures below then above freezing either one or two times, there were no effects on survival for two geranium cultivars, a Siberian iris, and a coreopsis. With another geranium, such cycles increased hardiness one year, but not the next. This factor is currently being investigated further by graduate student Sarah Kingsley Richards, and whether such cycles in fall, winter, or spring might results in differences. Previous studies by another grad student showed much larger cycles (about 15 degrees F above and below freezing, compared to the 6 degrees above and below here), often were detrimental.
For more on these studies, and to keep up with
future ones and other projects, check out the details linked on Perry’s
Perennial Pages under Freezing Studies on the Sustainable Research Page.
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