146 Cold Hardiness Evaluation of Deer-resistant Thuja plicata for Northern Latitudes

Abstract

The expansion of urban communities to rural areas is leading to an increase of the problem of deer damage. White-tailed deer (Odocoileus virginianus) damage to landscape plants in commercial nurseries, residential and public areas is very widespread. Thuja occidentalis (Arborvitae) is one of the most common landscape plants. It is widely produced by nurseries and used by homeowners in the landscape. However, it is also highly favored by deer for browsing. Thuja plicata (Arborvitae) the Western Cedars is a highly deer-resistant arborvitae. One of the principal limiting factors for new arborvitae for its success in nursery productionand its use in the landscape is cold hardiness (in northern climates). However, the cold hardiness of different Thuja plicata is not known. Deer-resistant Thuja plicata cultivars: `Atroviren', `Cancan', `Elegantissima', `Excelsa', `Gelderland', `George Washington', `Hilleri', `Sunshine', and `Virescens' planted in Sprintg 1998 at The Morton Arboretum research plot in Lisle, Ill. Branch cold hardiness was tested by artificial freezing in Jan. 1999 and 2000. Ice-nucleated samples were placed in an ultra-low temperature and kept at 2 °C overnight, and the temperature then lowered at 5 °C/h to –40 °C, at which time samples were taken out at each test temperature (at 4 °C intervals). After the freezing test, the samples were thawed at 4 °C for 24 h, then planted in a peat and perlite media and kept at 100% humidity in a greenhouse. Samples were evaluated after 2 weeks for visual browning and lowest survival temperature. There were significant differences in coldhardiness between the nine cultivars tested in Jan. 1999. `Elegantissima', `Excelsa'. and `Cancan' were the most hardy (–34 to 40 °C), followed by `Virescens', `Sunshine', and `Gelderland' (–27 to 32 °C), `Hilleri' and `Atrovirens' (–24 to 25 °C). `George Washington' ` was the least hardy (–20 °C) cultivar.