Cold Hardiness of the Rare Dirca occidentalis: Comparisons to Dirca palustris from Disparate Provenances

Abstract

Abstract We determined the cold hardiness of Dirca occidentalis (western leatherwood) within its restricted natural distribution in northern California and made comparisons to Dirca palustris (eastern leatherwood) from northern (Iowa) and southern (Florida) provenances. Stems of western leatherwood were sampled twice while all or some plants were dormant (October 27 and December 8, 2004, respectively) and again on January 19, 2005, when flowering and incipient vegetative shoot growth of most plants had begun. Stems of D. palustris were sampled on the same schedule and on March 2, 2005 (Iowa and Florida) and April 13, 2005 (Iowa) such that the last sampling date coincided with flowering and incipient vegetative shoot growth. Lowest survival temperature was determined based on visual assessments of the viability of cambium and phloem of stems exposed to progressively lower temperatures in a freezer. Lowest survival temperature of western leatherwood was −6 to −3C (21 to 27F) regardless of date. Eastern leatherwood accrued hardiness through mid-winter and then deacclimated; minimal lowest survival temperature of eastern leatherwood was −33C (−27F) in Iowa and −17C (1F) in Florida. The minimal cold hardiness of western leatherwood is consistent with its natural distribution in specialized niches within a Mediterranean climate. Unless western leatherwood accrues increased hardiness when planted where winters are harsh, its horticultural use will be restricted to areas with mild winters. In contrast, widespread use of eastern leatherwood is feasible if production challenges are resolved and selections from climatically appropriate provenances are used.