Grevillea is a genus of evergreen shrubs in the family Proteaceae. Currently, 362 species are recognized, almost all of which are found in Australia; only a few are found in New Guinea, New Caledonia, and Sulaweisi (Makinson, 2000). Grevillea commonly grow in sclerophyll forests and heaths, with fewer occurring in rainforest or desert environments (Molyneux, 1978). The habit of these shrubs ranges from prostrate and wide-spreading to upright shrubs and tree forms. Grevillea also display great variations in leaf size and morphology, flower color, inflorescence size and presentation, as well as flowering season (Growns et al., 2013), making them popular and commercially successful ornamental plants in regions with a suitable climate.
Grevillea use as a commercial ornamental plant in North America is limited, principally because of a lack of cold hardiness. Almost all of Australia corresponds to the U.S. Department of Agriculture (USDA) hardiness zone 9 or higher (Dawson, 1991), and as a result, only the warmest regions of North America are suitable for outdoor cultivation of any species or hybrid. For landscape use, plants are almost entirely limited to warmer regions of California, Hawaii, and parts of the southeastern United States. Some of the most cold-hardy species or cultivars, most commonly royal grevillea (G. victoriae), are also cultivated to a limited extent in western Oregon and Washington, as well as in southwestern British Columbia.
Much of the information regarding the cold hardiness of grevillea is anecdotal. No comprehensive study of hardiness of the many cultivars or hybrids has been performed. Studies have assessed cold hardiness of individual hybrids like G. ×gaudichaudii (Stanley and Warrington, 1988) or compared the hardiness of a limited numbers of species (Mancuso et al., 2004), including field-grown grevillea being used for cut flowers (Heuer and Markovitz, 2011).
This study was performed to assess the suitability of cultivars and species of grevillea for landscape use in the Pacific Northwest United States USDA hardiness zones 8a (10 to 15 °F) through 9a (20 to 25 °F).
Australian Cultivar Registration Authority1974Grevillea juniperina ‘Molonglo’. 16 Apr. 2017. <https://www.anbg.gov.au/acra/descriptions/acc002.html>
MakinsonR.O.2000Grevillea p. 20–450. In: A. Wilson (ed.). Flora of Australia. Volume 17A. Proteaceae 2. Austral. Biol. Resources Study Canberra Australia
MancusoS.NiceseF.P.MasiE.AzzarelloE.2004Comparing fractal analysis, electrical impedance and electrolyte leakage for the assessment of cold tolerance in Callistemon and Grevillea sppJ. Hort. Sci. Biotechnol.79627632
OldeP.2007The Grevillea victoriae complex—New species in the flora. Grevillea Study Group Nwsl. 77. Assn. Soc. Growing Australian Plants Ulverstone Australia
Richardson-CalfeeL.HarrisJ.R.2005A review of the effects of transplant timing on landscape establishment of field-grown deciduous trees in temperate climatesHortTechnology15132135
StanleyC.J.WarringtonI.1988Seasonal frost tolerance of some ornamental evergreen broad-leaved and coniferous tree and shrub speciesN. Z. J. Expt. Agr.16239248
TullyP.1977The Poorinda grevilleas. Austral. Plants 9:213–215
U.S. Bureau of Reclamation2019Agimet. 7 Oct. 2019. <https://www.usbr.gov/pn/agrimet/>