Phenology and fruit set of promising North American shrubs in the genus Dirca

The genus Dirca (Thymelaeaceae) comprises four species of deciduous shrubs noted for their unusual time of flowering (autumn through early spring), yellow autumnal leaf colour, and arborescent growth habit. Despite their attractiveness, these species are rarely marketed as horticultural plants due to their scarcity in nature and to difficulty with producing them in nurseries. Propagation from seed is possible, but fruit set (the percentage of flowers that lead to a fruit) of Dirca reportedly can be poor. My objective was to quantify fruit set of Dirca occidentalis at multiple sites within its native distribution in a small portion of California with a Mediterranean climate. Comparisons were made to Dirca palustris in the eastern United States, and the phenology of plants of both species was tracked as they broke dormancy to flower and form new stems and leaves. Data collected over five consecutive years showed variation in fruit set was more highly associated with the timing of individual flower formation than by site in the landscape. Fruit set increased from <5% for flowers open early in the blooming season to nearly 20% among flowers that opened later. When precipitation and temperature patterns led to atypically synchronous flowering, fruit set exceeded 35%, suggesting a critical role of insect pollinators. In a separate study, individual plants of both species differed markedly in fruit set, and phenological state within populations was more variable among plants of D. occidentalis than plants of D. palustris. Environmental data support inferences about the importance of annual variation in air temperature and precipitation. These data are being used in a comprehensive assessment of the potential value of Dirca species for use in ornamental horticulture.