Landscape Potential of Rare Species of Dirca Assessed at Five Sites Across the United States

Dirca L. (leatherwood) species are aesthetically appealing North American shrubs, but their usefulness in horticulture will depend on the environmental conditions under which they thrive.  In this study, two narrowly endemic species of leatherwood were evaluated for survival, growth, and photosynthetic rate in common-garden trials for five years.  From March to May of 2010, 104 seedlings, 52 of Dirca mexicana Nesom & Mayfield and 52 of Dirca occidentalis A. Gray, were planted in completely randomized designs at each of five locations that spanned the United States within USDA plant hardiness zones 9b to 5a.  Plants were irrigated only during establishment and were not fertilized.  Survival of D. mexicana in mid 2015 was ≥ 85% at Kingston, Rhode Island; Haymarket, Virginia; Ames, Iowa; and Seattle, Washington, but was 0% at Stanford, California.  Survival of D. occidentalis ranged from 77% at Seattle to 0% at Haymarket.  Measures of annual stem extension and of canopy size after five years showed plants of both species were most vigorous at Kingston, and photosynthetic rates there were higher than at Seattle.  Variation among sites provides insights concerning the edaphic and aerial environmental conditions to which these species are adapted.  For example, although D. mexicana survived at higher rates than did D. occidentalis and persisted in hardiness zone 5a, D. mexicana was especially intolerant of the dry summers at Stanford.  Dirca occidentalis, which is indigenous only within relatively dry regions of hardiness zone 9b, responded well to the combination of comparatively high precipitation and well-drained soils at Kingston, in hardiness zone 6b, but did not tolerate the garden site in Haymarket, where summer temperatures were highest and soil drained poorly.  Both species demonstrated horticultural potential, but care should be taken to use these shrubs only where conditions of the natural environment are suitable or where supplemental irrigation can be provided.