Root cold tolerance of three temperate zone Viburnum species
Freeze damage to roots of container-grown nursery stock occurs when substrate temperatures dip below a root cold tolerance (RCT) threshold and plants may not recover if a large proportion of the roots are damaged. In this study, we used an electrolyte leakage test to compare damage to immature roots of three Viburnum species frequently used in landscapes in the northeastern USA. There were five test dates using the same source of container-grown plant material during the winter months between February 2014 and March 2016. Excised root samples in vials were submerged in a programmed recirculating glycol bath with minimum temperature treatments of 0, -5, -10, -15, -20, or -25°C. Distilled water was added to each vial when removed from the glycol bath and initial electrical conductivity (ECi) was measured on the solution after 24 h of storage at 2-3°C. Vials were then autoclaved and final electrical conductivity (ECf) was measured 24 h later. The ECi/ECf ratio represented the proportion of total electrolyte leakage caused by the minimum temperature treatments. The RCT is the predicted temperature at which 50% damage occurred. Previous researchers assigned thresholds of -7 and -9°C for immature roots of Viburnum plicatum var. tomentosum and Viburnum carlesii, respectively. Our results were similar for V. plicatum var. tomentosum (RCT -6°C) but showed V. carlesii roots could tolerate lower minimum temperatures (RCT -11.5°C). In fact, V. carlesii roots were nearly as cold tolerant as Viburnum opulus var. americanum (RCT -13.5°C), a plant hardy to USDA cold hardiness zone 2, for which RCT values had not previously been reported. Viburnum carlesii and V. plicatum var. tomentosum are zone 4-5 plants. This study confirms that root and shoot cold hardiness are not always well-correlated.
Neal, C.A. (2018). Root cold tolerance of three temperate zone Viburnum species. Acta Hortic. 1191, 121-128
cold hardiness, cold protection, container production, deciduous shrubs, nursery stock, electrolyte leakage