A Narrowly Endemic Dirca from Mexico Outperforms Its Broadly Distributed Congener in the Upper Midwest

Shrubs in the genus Dirca are rarely used despite their shade tolerance and beauty.  A trial of 81 plants, 27 of each of three species, was established in 2007 near the border of USDA hardiness zones 5a and 4b to evaluate survival and growth.  After four winters, survival of Dirca mexicana (89%) was greater than survival of Dirca palustris indigenous to Florida (44% survival) and Dirca occidentalis (41% survival).  Survival of Dirca palustris from Ontario, Canada, was intermediate (56%), but annual stem extension was only 60% that of Dirca mexicana and Dirca palustris from Florida.  Surviving plants of Dirca mexicana and Dirca palustris from Ontario showed minimal winter injury, but tips of some stems of Dirca palustris from Florida and Dirca occidentalis were killed.  Our data on survival, winter injury, plant heath, and stem extension of the California-endemic Dirca occidentalis suggest it will be especially challenging to identify genotypes adapted for use in the Upper Midwest.  In contrast, another narrowly endemic species, Dirca mexicana, has potential as a new shrub for horticulture.  Our results provide evidence for variation in cold hardiness and annual stem extension of Dirca palustris.  Although over half of plants from Florida had died after the first two winters, no additional mortality occurred over remaining years, and survivors were more vigorous than plants from Ontario.  This suggests that exploitable variation in cold-hardiness and vigor exists among and within populations of this broadly distributed species.