Oregon and Washington, as well as in Cold Hardiness of Grevillea in Western Oregon southwestern British Columbia. Much of the information regard- Neil Bell1, Heather Stoven2, James S. Owen Jr.3, ing the cold hardiness of grevillea is 4 anecdotal. No comprehensive study of and James E. Altland hardiness of the many cultivars or hybrids has been performed. Studies ADDITIONAL INDEX WORDS. landscape, ornamental plant, Pacific Northwest, shrub, have assessed cold hardiness of indi- spider flower, temperature vidual hybrids like G. ·gaudichaudii (Stanley and Warrington, 1988) or SUMMARY. A cold hardiness evaluation of 57 cultivars and species of grevillea Grevillea compared the hardiness of a limited ( ) was conducted from 2011 to 2014 in Aurora, OR, to assess landscape numbers of species (Mancuso et al., suitability in the Pacific Northwest United States. Plants were established using irrigation in 2011, but they received no supplemental water, mineral nutrients, or 2004), including field-grown grevil- pruning from 2012 to 2014. Plants were evaluated for injury in Mar. 2012 and Jan. lea being used for cut flowers (Heuer 2014 after winter cold events with minimum temperatures of L4 and L13 C, and Markovitz, 2011). respectively. Damage, at least on some level, occurred on most selections following This study was performed to their first winter after planting in 2011. During Winter 2013, further damage to, or assess the suitability of cultivars and death of, 33 grevillea cultivars or species occurred. The grevillea that exhibited the species of grevillea for landscape least cold damage and the most promise for landscape use and further evaluation in use in the Pacific Northwest United the Pacific Northwest United States were ‘Poorinda Elegance’ hybrid grevillea, G. australis G. juniperina States USDA hardiness zones 8a (10 southern grevillea ( ), cultivars of juniper-leaf grevillea ( ) to 15 F) through 9a (20 to 25 F). including Lava Cascade and Molonglo, and oval-leaf grevillea (G. miqueliana), all of which exhibited minor foliage damage. Materials and methods A total of 85 species and cultivars revillea is a genus of evergreen (Molyneux, 1978). The habit of these of grevillea were collected, primarily shrubs in the family Protea- shrubs ranges from prostrate and wide- from the Arboretum at the University Gceae. Currently, 362 species spreading to upright shrubs and tree of California, Santa Cruz, and three are recognized, almost all of which are forms. Grevillea also display great var- nursery sources in 2010. Cuttings were found in Australia; only a few are found iations in leaf size and morphology, collected in June, with the exception of in New Guinea, New Caledonia, and flower color, inflorescence size and royal grevillea (variegated), which was Sulaweisi (Makinson, 2000). Grevillea presentation, as well as flowering sea- rooted in Dec. 2010. Tip cuttings 4 commonly grow in sclerophyll forests son (Growns et al., 2013), making to 6 inches long were collected; how- and heaths, with fewer occurring them popular and commercially suc- ever, the length was dependent on the in rainforest or desert environments cessful ornamental plants in regions vigor and size of individual accessions. with a suitable climate. Cuttings were placed in individual bags Received for publication 17 May 2019. Accepted for Grevillea use as a commercial or- according to their accession and re- publication 8 Oct. 2019. namental plant in North America is frigerated. Cuttings were processed Published online 20 November 2019. limited, principally because of a lack of and stuck at Oregon State University 1Oregon State University Extension Service, Marion cold hardiness. Almost all of Australia North Willamette Research and Exten- County, 1320 Capitol Street NE Suite 110, Salem, OR 97301 corresponds to the U.S. Department sion Center (NWREC) in Aurora of Agriculture (USDA) hardiness zone within 3 d. Cuttings were quick-dipped 2Oregon State University Extension Service, Yamhill County, 2050 NE Lafayette Avenue, McMinnville, 9 or higher (Dawson, 1991), and as in 2500 ppm indole-3-butyric acid and OR 97128 a result, only the warmest regions of 1250 ppm 1-napthalenaecetic acid 3Virginia Polytechnic Institute and State University, North America are suitable for out- (Dip’N Grow, Clackamas, OR) and School of Plant and Environmental Sciences, Hamp- door cultivation of any species or stuck in 38-cell groove tube trays ton Roads Agricultural Research and Extension Cen- ter, 1444 Diamond Springs Road, Virginia Beach, VA hybrid. For landscape use, plants are (T.O. Plastics, Clearwater, MN) in 23455 almost entirely limited to warmer re- a 2:1 (v/v) mix of horticultural grade 4Research Horticulturist, Application Technology gions of California, Hawaii, and parts perlite and fine milled sphagnum peat. Research Unit, U.S. Department of Agriculture- of the southeastern United States. Trays were placed in a mist bed with Agricultural Research Service, Agricultural Engineer- ing Building, 1680 Madison Avenue, Wooster, OH Some of the most cold-hardy species bottom heat ( 21 C) in a temperature- 44691 or cultivars, most commonly royal controlled greenhouse (20 C). We appreciate and acknowledge financial support grevillea (G. victoriae), are also culti- Rooted cuttings were transplanted from the Washington State Department of Agricul- vated to a limited extent in western to 6-inch square nursery pots in Nov. ture Nursery License Surcharge program (Olympia, WA), as well as the donation of cutting material from the University of California, Santa Cruz, Arboretum, and the assistance of the staff at the Arboretum. Thanks are also due to Oregon State University Units Marion County Master Gardeners for assistance with To convert U.S. to SI, To convert SI to U.S., plot establishment and data collection. multiply by U.S. unit SI unit multiply by N.B. is the corresponding author. E-mail: neil.bell@ 0.4047 acre(s) ha 2.4711 oregonstate.edu. 0.3048 ft m 3.2808 This is an open access article distributed under the CC 2.54 inch(es) cm 0.3937 BY-NC-ND license (https://creativecommons.org/ 28.3495 oz g 0.0353 licenses/by-nc-nd/4.0/). 1 ppm mgÁL–1 1 https://doi.org/10.21273/HORTTECH04377-19 ( F – 32) O 1.8 F C( C · 1.8) + 32
• February 2020 30(1) 117 2010 and topdressed with 5 g of 19N– power through greater replication. control was maintained through bi- 0P–15.8K controlled-release fertilizer Before planting, the site was lightly annual application of the pre-emer- (Florikote; Florikan, Sarasota, FL). cultivated by disking to break-up sur- gent herbicides isoxaben (Gallery 75; The exception was royal grevillea (var- face compaction in the top few centi- Dow Chemical Co., Indianapolis, IN) iegated), which was transplanted in meters. Four plant replicates from and oryzalin (Surflan A.S.; United Apr. 2011, and potted with the same each accession chosen for uniformity Phosphorus, King of Prussia, PA), protocol. Plants were grown in an of size, rooting, vigor, and health postemergent herbicide glyphosate unheated retractable roof greenhouse were planted on 19 Aug. 2011 in (Roundup; Bayer, Leverkusen, Ger- (Cravo, Brantford, ON) until planting rows 350 ft long, oriented north to many), and hand-hoeing. Plants were in the field site for evaluation. Despite south, and spaced 12 ft apart with an not sprayed with fungicide or insecti- the difference in the collection time of in-row spacing of 5 ft in a completely cide, and no significant disease or royal grevillea (variegated), the long randomized design. The planting pest issues were observed. Weather period of growth during Summer date was chosen to maximize plant data for the evaluation were provided 2011 resulted in a robust liner equiv- establishment before the drought pe- by a Bureau of Reclamation Agrimet alent to earlier accessions for field riod the following summer. Although weather station at NWREC (U.S. planting. A total of 57 grevillea were they worked with deciduous trees, it Bureau of Reclamation, 2019), which available in sufficient numbers for the was noted by Richardson-Calfee and is located 200 ft from the west edge of evaluation by the time of planting be- Harris (2005) that fall appears to be the planting (Fig. 1). cause not all collected accessions rooted the best time to plant in landscapes Cold injury data were collected or rooted in sufficient numbers. that are not well irrigated. on 12 Mar. 2012 and again on 17 Jan. ThesiteatNWREC(lat.45 16#N, After planting, grevillea were 2014. Plants were visually evaluated long. 122 45#W, 47 m elevation) is hand-watered and then overhead- for damage using a scale from 1 to 6 as located in USDA hardiness zone 8b irrigated as needed until reliable fall follows: 1 = no injury; 2 = minor leaf (15 to 20 F). The planting site was rains occurred in mid-Oct. 2011. injury; 3 = leaf/stem damage on outer limited by a 0.2-acre parcel in Latourell Water was not applied to the plants 30%; 4 = leaf/stem injury on outer loam soil. The limited plot size resulted beyond natural rainfall for the re- 60%; 5 = killed to ground, recovery in the authors choosing to evaluate mainder of the trial. Fertilizer was evident; and 6 = dead. Data were the maximum number of species and not applied at planting or thereafter, analyzed with an analysis of variance cultivars instead of minimally increasing and plants were not pruned. Weed (ANOVA) using SAS (version 9.4;
Fig. 1. Daily precipitation and mean air and soil [(5 cm D 20 cm depth) O 2] temperatures reported by the Agrimet weather station at the North Willamette Research and Extension Center in Aurora, OR, for the duration of the grevillea evaluations: (1.8 · C) D 32 = F; 1 cm = 0.3937 inch.
118 • February 2020 30(1) Table 1. Winter cold injury to grevillea grown during a landscape evaluation in Aurora, OR. Injury rating (1–6 scale)z Common name Scientific name 2012 2014 ‘Aromas’ Grevillea hybrid 3.0 3.8 ‘Boongala Spinebill’ Grevillea hybrid 5.0 6.0 ‘Canberra Gem’ G. juniperina · G. rosmarinifolia 1.3 6.0 ‘Clearview David’ Grevillea hybrid 4.3 6.0 ‘Fanfare’ G. gaudichaudii · G. longifolia 4.8 6.0 ‘Granya Glory’ Grevillea hybrid 5.0 6.0 ‘Mason’s Hybrid’ Grevillea hybrid 5.0 5.0 ‘Moonlight’ Grevillea hybrid 5.0 6.0 ‘Noelii’ Grevillea hybrid 2.3 6.0 ‘Olympic Flame’ Grevillea hybrid 2.5 5.5 ‘Pink Pearl’ Grevillea hybrid 2.0 6.0 ‘Pink Surprise’ Grevillea hybrid 5.0 3.8 ‘Poorinda Blondie’ Grevillea hybrid 5.0 6.0 ‘Poorinda Constance’ Grevillea hybrid 2.0 6.0 ‘Poorinda Elegance’ G. juniperina and 2.3 2.0 G. alpina · G. obtusiflora ‘Poorinda Queen’ Grevillea hybrid 1.5 5.3 ‘Poorinda Signet’ Grevillea hybrid 2.5 4.0 ’Red Glow’ Grevillea hybrid 3.3 4.5 ‘Robyn Gordon’ Grevillea hybrid 5.0 6.0 ‘Ruby Clusters’ Grevillea hybrid 3.3 5.3 ‘Scarlet Sprite’ Grevillea hybrid 2.0 6.0 ‘Wakiti Sunrise’ Grevillea hybrid 5.0 6.0 ‘White Wings’ Grevillea hybrid 5.0 6.0 Mountain grevillea (Axedale goldfields) G. alpina 2.5 6.0 Mountain grevillea (garden volunteer) G. alpina 3.5 5.3 Mountain grevillea (Wangratta, Victoria) G. alpina 4.3 6.0 Mountain grevillea (Warby Range) G. alpina 3.0 6.0 Holly grevillea G. aquifolium 3.5 6.0 Holly grevillea ‘Serra Road’ G. aquifolium 4.7 6.0 Holly grevillea (Grampians) G. aquifolium 3.5 6.0 Southern grevillea G. australis 1.8 1.0 Red silky oak G. banksii 5.0 6.0 Golden grevillea G. chrysophaea 5.0 2.3 G. delta 5.0 6.0 Spindly grevillea ‘Gosnel’s Quarry’ G. endlicheriana 5.0 6.0 Johnson’s grevillea G. johnsonii 4.8 6.0 Juniper-leaf grevillea ‘Lava Cascade’ G. juniperina 1.5 2.3 Juniper-leaf grevillea ‘Molonglo’ G. juniperina 1.8 2.0 Woolly grevillea ‘Coastal Gem’ G. lanigera 3.0 6.0 Laurel-leaf grevillea (Blue Mountains) G. laurifolia 3.3 6.0 Lavender grevillea ‘Penola’ G. lavandulacea 2.3 3.0 Lavender grevillea G. lavandulacea 4.5 5.8 Oval-leaf grevillea G. miqueliana 1.0 2.8 Red spider flower G. oleoides 5.0 6.0 G. paniculata 5.0 4.8 G. pinaster 5.0 6.0 Crimson grevillea G. polybractea 4.3 6.0 Carrington Falls grevillea G. rivularis 3.0 5.3 Rosemary grevillea (dwarf pink) G. rosmarinifolia 4.0 6.0 G. sericea 5.0 3.5 G. shiressi 5.0 6.0 Grevillea hybrid G. caleyi · G. hookeriana 5.0 6.0 G. vestita 5.0 6.0 Royal grevillea ‘Marshall Olbrich’ G. victoriae 1.3 4.0 Royal grevillea ‘Murray Valley Queen’ G. victoriae 1.8 5.8 Royal grevillea (variegated) G. victoriae 4.0 1.3 (Continued on next page)
• February 2020 30(1) 119 Table 1. (Continued) Winter cold injury to grevillea grown during a landscape evaluation in Aurora, OR.
Injury rating (1–6 scale)z Common name Scientific name 2012 2014 G. ·gaudichaudii G. acanthifolia · G. laurifolia 2.8 5.5 Statistics Tukey’s HSD 1.7 2.4 P value <0.0001 <0.0001 zVisually evaluated: 1 = no injury; 2 = minor leaf injury; 3 = leaf stem damage on outer 30%; 4 = leaf/stem injury on outer 60%; 5 = killed to ground, recovery evident; and 6 = dead. The 2012 rating was conducted on 12 Mar. 2012; the 2014 rating was conducted on 17 Jan. 2014.
2013. Winter 2013–14 featured an has addressed the causes of variation extended cold spell that began on 4 in hardiness of individual plants be- Dec., with a low temperature of tween growing seasons. The severity –3.8 C (Fig. 1). From that date of the cold weather and its duration, through 12 Dec., minimum temper- as well as the time of onset and atures were below freezing, culminat- physiological status of the plant, pre- ing in low temperatures of –13.3 and sumably influence the level of damage. –11.7 C on the nights of 8 and 9 From the standpoint of land- Dec., respectively. scape use, a plant that had an injury Although the temperatures ob- rating exceeding 2.5 during either served in 2011–12 were milder than winter would probably be unsuitable those of a typical zone 8b winter, this for landscape use in western Oregon. was enough to cause considerable In 2011, 68% of accessions did not foliage or stem damage to many ac- meet this threshold of cold hardiness cessions. Freeze-damaged grevillea (32% did). In 2014, 89% of accessions foliage typically turns brown or black, did not meet the threshold (11% did). and the most heavily damaged plants Among the least damaged plants dur- suffered foliage damage to the entire ing these cold events were cultivars canopy (Table 1). Similar conditions and selections of juniper-leaf grevil- in 2012–13 produced insufficient lea, oval-leaf grevillea, and southern symptoms to warrant an evaluation. grevillea. The last two of these are Whether this was the result of greater commonly found at higher elevations hardiness because the plants were in Australia. Oval-leaf grevillea grows Fig. 2. Some images of grevillea best older is unclear, although studies have in high-altitude forests in eastern Vic- suited for Pacific Northwest United demonstrated increasing hardiness toria, where it forms a large shrub up States landscapes: southern grevillea with the increasing age of tissue in to 8 ft tall (Makinson, 2000) (Fig. 2). (top) and oval-leaf grevillea (bottom). woody plants (McNamara and Pellett, During our evaluation, it formed a ro- 2000). The cold temperatures experi- bust, rounded shrub that was 5 ft tall enced in Winter 2013–14 exceeded by 2014, and it begun producing red SAS Institute, Cary, NC) and means the tolerance of most of the accessions flowers in late winter. Southern gre- were separated when appropriate us- and caused major damage throughout villea, sometimes also referred to ing Tukey’s honestly significant dif- the evaluation (Table 1). as alpine grevillea, is found in alpine ference test (a = 0.05) to compare the The level of injury to most of the to subalpine altitudes in the moun- injury ratings of individual accessions. accessions in the evaluation, especially tains of southeastern Australia, and following the cold weather of Dec. it is widely distributed in Tasmania Results and discussion 2013, was not surprising because the (Makinson, 2000) (Fig. 2). During Weather in Fall 2011 was mild majority of the accessions are native this evaluation, southern grevillea and allowed for the successful estab- to low-elevation areas of Western grew into a rounded shrub that was lishment of all plants. Between 1 Nov. Australia, New South Wales, and Vic- 2.5 ft tall and 2.5 ft wide by 2014, and 2011 and 1 Apr. 2012, minimum toria, which correspond to USDA it freely produced fragrant white temperatures dropped below freezing hardiness zone 9. Injury to specific flowers in April. on 36 occasions. The coldest temper- cultivars varied between the two cold Two cultivars of juniper-leaf gre- atures, –4.4 and –3.3 C, were recorded spells evaluated in this study; in some villea also displayed relative hardiness. on 13 and 22 Dec., respectively (Fig. 1). cases, they varied considerably. This ‘Molonglo’ and ‘Lava Cascade’ form Winter 2012–13 was similarly cold, was seen to some extent in previous prostrate, wide-spreading, fairly dense with minimum temperatures below landscape studies of woody plants groundcovers up to 1.5 ft tall that have freezing on 27 occasions during the (Bell, 2009; Bell and Altland 2010). apricot and red flowers, respectively. period from 11 to 23 Jan. The lowest Although there is considerable litera- ‘Molonglo’ was developed in the temperatures were near –4.5 C, ture about the variations in the cold early 1960s at the Australian National which occurred on four occasions, hardiness of woody plants between University, and it established a repu- 2 Jan., 12 Jan., 21 Jan., and 22 Jan. seasons within a given year, no work tation for high frost tolerance in the
120 • February 2020 30(1) landscapes of the Canberra region distinguish, and landscape specimens Heuer, B. and T. Markovitz. 2011. In- (Australian Cultivar Registration Au- have exhibited injury in severe win- teractive effects of frost and effluent irri- thority, 1974). In this evaluation, it ters. ‘Murray Valley Queen’ is con- gation on Grevillea species. Isr. J. Plant displayed only minor leaf damage sidered to be a selection of G. Sci. 59:197–206. during the Dec. 2011 and Dec. victoriae ssp. nivalis (Olde, 2007). McNamara, S. and H. Pellett. 2000. Cold 2013 freezes. ‘Lava Cascade’, also ‘Marshall Olbrich’ is of undocu- hardiness of Phellodendron sachalinense marketed as ‘Low Red’, is a ground- mented origin, but it apparently arose Friedr. Schmidt seedlings increases with cover selection of uncertain origin as a seedling at Western Hills Nursery age. HortScience 35:304–305. that displays hardiness comparable in Occidental, CA. Despite the dem- Makinson, R.O. 2000. Grevillea, p. 20– to ‘Molonglo’. onstrated cold injury, the showy win- 450. In: A. Wilson (ed.). Flora of Aus- Another accession that displayed ter blooms of this species make it tralia. Volume 17A. Proteaceae 2. Austral. good hardiness was ‘Poorinda Ele- a desirable landscape plant for places Biol. Resources Study, Canberra, Aus- gance’. This cultivar is a hybrid of where it can survive. tralia. a juniper-leaf grevillea of New South The grevillea that exhibited the Mancuso, S., F.P. Nicese, E. Masi, and E. Wales origin and another hybrid of least cold damage and the most prom- Azzarello. 2004. Comparing fractal anal- mountain grevillea (G. alpina) and ise for landscape use in the Pacific ysis, electrical impedance and electrolyte obtuse-leaf grevillea (G. obtusifolia); Northwestern United States were leakage for the assessment of cold toler- it forms a rounded, spreading shrub ‘Poorinda Elegance’, southern grevil- ance in Callistemon and Grevillea spp. J. up to 6 ft tall with yellow flowers lea, cultivars of juniper-leaf grevillea Hort. Sci. Biotechnol. 79:627–632. (Tully, 1977). The various accessions including Lava Cascade and Molonglo, Molyneux, W. 1978. Grevillea in horti- of mountain grevillea did not ex- and oval-leaf grevillea. culture. Austral. Plants 75:287–298. hibit noticeable cold tolerance, and obtuse-leaf grevillea is native to Olde, P. 2007. The Grevillea victoriae coastal Western Australia, where frost Literature cited complex—New species in the flora. Gre- is rare. Therefore, the cold hardiness Australian Cultivar Registration Author- villea Study Group Nwsl. 77. Assn. Soc. Growing Australian Plants, Ulverstone, of this hybrid is likely derived from ity. 1974. Grevillea juniperina ‘Molonglo’. 16 Apr. 2017.
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