Horticultural & ecophysiological evaluations of leatherwoods (Dirca spp.)

Norris, Kelly
Major Professor
William R. Graves
Committee Member
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Horticulturists have overlooked Dirca spp. despite the many ornamental characteristics of these shade-tolerant but difficult-to-propagate shrubs. The genus is characterized by slow growth, shade tolerance, flowers in late winter or spring, and mature growth approximately 1 to 2 meters tall. The first objective of the work undertaken for this thesis was to evaluate three species of Dirca in a common garden setting in central Iowa. As part of an on-going trial, I analyzed data collected from 2007-2010. Dirca mexicana had the greatest survival rate with 88.9% alive as of 2010, despite its southerly origins. Within Dirca palustris, plants from Thunder Bay survived better than plants from Florida but exhibited poor vigor and achieved low health ratings. Dirca mexicana and D. palustris from Florida were rated the healthiest, and their stems extended the most annually. Dirca mexicana was suggested as the best species for inclusion in landscapes as cold as USDA Hardiness Zones 4 and 5, whereas selections from different provenances, particularly those from Florida, evince a selection gradient for commercially viable traits. Dirca occidentalis will require further evaluation for hardier, more climate-suitable genotypes if it's to ever be promoted a landscape plant outside of its endemic distribution. The second objective was to document the edaphic responses of the same three species of Dirca under field conditions. Three treatments were imposed on an outdoor trial site; two acidic treatments and one based on the native soil pH of the site. To obtain the two treatments, we modified the soil (native pH 7.65) with 1-M sulfuric acid (H2SO4) to a pH between 4.5-5.0 and 6.0-6.5 using 200 mL/L medium and 70 mL/L medium, respectively. Stems of Dirca palustris extended the most in acidic root zones, but mean health ratings were highest in the slightly acidic and control treatments, suggesting a range of optimal growth and performance. Dirca occidentalis performed best for all response variables under slightly acidic conditions. Stems of Dirca mexicana extended the most in slightly basic root zones, and the species was rated the healthiest. Overall, the data suggest that the three species of Dirca perform well in a range of soils from pH 6.5 to 7.5, offering initial insights into the proper use of these shrubs in managed landscapes.