Cold hardiness of Grevillea in western Oregon

Authors: BELL, NEIL - Oregon State University; STOVEN, HEATHER - Oregon State University; OWEN, JAMES - Virginia Polytechnic Institution & State University; Altland, James

Submitted to: HortTechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/8/2019
Publication Date: 11/20/2019
Citation: Bell, N., Stoven, H., Owen, J.S., Altland, J.E. 2019. Cold hardiness of Grevillea in western Oregon. HortTechnology. 30(1):117-121. https://doi.org/10.21273/HORTTECH04377-19.
DOI: https://doi.org/10.21273/HORTTECH04377-19

Interpretive Summary: Grevillea is a genus of evergreen shrubs in the family Proteaceae. Its use as a commercial ornamental plant in North America is limited, principally due to a perceived lack of cold hardiness. Much of the information regarding the cold hardiness of grevillea is anecdotal. The objective of this research was 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). A total of 85 species and cultivars of grevillea were collected, primarily from the Arboretum at the University of California, Santa Cruz, and three nursery sources in 2010. The site at the North Willamette Research and Extension Center (lat. 45°16'N, long. 122°45'W, 47 m elevation) is located in USDA hardiness zone 8b (15 to 20 °F). Four plant replicates from each accession chosen for uniformity of size, rooting, vigor, and health were planted on 19 Aug. 2011 in rows 350 ft long, oriented north to south, and spaced 12 ft apart with an in-row spacing of 5 ft in a completely randomized design. Plants were evaluated for injury in Mar. 2012 and Jan. 2014 after winter cold events with minimum temperatures of -4 and -13 °C, respectively. The grevillea that exhibited the least cold damage and the most promise for landscape use in the Pacific Northwestern United States were ‘Poorinda Elegance’, southern grevillea, cultivars of juniper-leaf grevillea including Lava Cascade and Molonglo, and oval-leaf grevillea.

Technical Abstract: A cold hardiness evaluation of 57 cultivars and species of grevillea (Grevillea) was conducted from 2011 to 2014 in Aurora, OR, to assess landscape suitability in the Pacific Northwest United States. Plants were established using irrigation in 2011, but they received no supplemental water, mineral nutrients, or pruning from 2012 to 2014. Plants were evaluated for injury in Mar. 2012 and Jan. 2014 after winter cold events with minimum temperatures of -4 and -13 °C, respectively. Damage, at least on some level, occurred on most selections following their first winter after planting in 2011. During Winter 2013, further damage to, or death of, 33 grevillea cultivars or species occurred. The grevillea that exhibited the least cold damage and the most promise for landscape use and further evaluation in the Pacific Northwest United States were ‘Poorinda Elegance’ hybrid grevillea, southern grevillea (G. australis), cultivars of juniper-leaf grevillea (G. juniperina) including Lava Cascade and Molonglo, and oval-leaf grevillea (G. miqueliana), all of which exhibited minor foliage damage.