Seasonal Change in Cold Hardiness of Field-grown Ash Trees

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  • 1 1Washington State Univ. Puyallup, Puyallup, WA 98371-4998
  • 2 2Oregon State Univ. North Willamette Research and Extension Center, Aurora, OR 97002

Cold hardiness of Fraxinus americana `Autumn Purple', Fraxinus oxycarpa `Raywood' and Fraxinus pennsylvanica `Summit' was measured in laboratory tests. Current season stem growth was collected from trees in Willamette Valley nurseries at 3 to 6 week intervals from November 1994 to February 1995 and from October 1995 to March 1996. Replicated 9-cm stem samples with two buds each were placed in tubes and immersed in an ethylene glycol bath. Samples were nucleated with crushed ice, held overnight at –2°C and then frozen at 3°C/hour. After freezing, samples were thawed overnight, incubated at room temperature and 100% relative humidity for 10 to 14 days, then sample viability was determined by visual browning. A Tk50, the temperature at which 50% tissue injury occurred, was calculated for buds and stems. Buds were generally less hardy than stems. `Raywood' was slower to cold acclimate in the fall and did not become as cold hardy in midwinter as `Summit' and `Autumn Purple'. Cold acclimation and midwinter hardiness of `Summit' and `Autumn Purple' was similar; however, `Summit' deacclimated more rapidly. Between the 11 Dec. 1995 and 9 Jan. 1996 freeze tests, `Summit' stems lost about 9 °C of freeze tolerance. In both the 1995 and 1996 February freeze tests, `Summit' stems were less hardy than `Raywood' stems.

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