Cold-hardiness in Accessions and Hybrids of Vitex

The genus Vitex contains several species that are widely used as ornamentals. Vitex species are distributed throughout tropical and sub-tropical areas ranging from Asia, India, Pakistan, and Sri Lanka to the Mediterranean and Southern Europe.  Cold-hardiness in hybrids and accessions in the University of Georgia (UGA) breeding program in Griffin, GA was assessed following the winters of 2014 and 2015 when mid-winter temperatures were unusually low. Minimum winter temperature  in 2014 was -14.2 °C and in 2015 was -12.2 °C. A rating scale of 0 to 5 was used to assess cold damage, with 0 indicating no damage and 5 indicating severe damage. Species was found to have an effect on cold-hardiness (p < 0.0001), with accessions of V. negundo L. var. heterophylla showing the least damage and accessions of V. trifolia L. var. purpurea having the most severe damage. Accessions and hybrids of V. agnus-castus L. sustained an intermediate amount of damage. V. trifolia occurs in Vietnam and other areas of Asia. V. negundo is native to Pakistan, India, and Sri Lanka. V. agnus-castus is indigenous to Southern Europe, the Mediterranean, and Central Asia. No interaction between species and year was found (p = 0.7535). Accessions and hybrids were also assessed to determine if the form of the plant had an effect on cold-hardiness. Replicated accessions were either pruned to tree form or left to grow in a shrub form. Pruning had an effect on some accessions. We will use the insights gained from this study to select accessions as parents in the UGA breeding program to produce Vitex cultivars with increased cold-hardiness.