Overview Cultivation Evaluation Results December 2013 References Online Resources

Grevillea is a genus of evergreen shrubs in the family Proteaceae. Currently 362 species are recognized, 357 of them found in Australia, 3 in New Guinea, 3 in New Caledonia and one in Sulawesi (see references, Makinson, 2000). Within their range Grevillea occur in most habitats from sea level to high altitudes. Some Grevillea are rain forest inhabitants, but in general they are more common in woodlands or open situations. Few of the species are alpine, or present in swampy areas, and few are salt-tolerant. 

Most grevilleas are small to medium shrubs.

 

Grevillea endlicheriana

but some are prostrate

Grevillea x gaudichaudii

and a few can become large shrubs or trees.

Grevillea ‘Mason’s Hybrid

Besides the diversity of size, Grevillea also differ greatly in foliage and flower effect. Foliage ranges from needle-like

 

Grevillea pinaster

to small, entire

Grevillea ‘Ruby Clusters’

or lobed

Grevillea montis-cole

 to large and fern-like

Grevillea ‘Moonlight’.

The foliage may in some cases be glaucous and young growth in particular may be covered with hairs or indumentum. It is also common for leaves to have a dense coating of hairs on their underside.

The individual flowers of Grevillea species are quite small but they occur in showy, dense inflorescences which display several forms. From a horticultural standpoint, they typically take one of three arrangements. These include the "spider" flower arrangement, in which the flower styles are presented in a pendant raceme, resembling the legs of a spider

Grevillea ‘Poorinda Constance’

In the "toothbrush" arrangement, the flowers are clustered on one side of the raceme 

Grevillea ‘Boongala Spinebill’

A third type of inflorescence, found particularly in cultivated plants, is the "brush" shape, where the flowers are present in large terminal cylindrical racemes, which are very showy

Grevillea ‘Mason’s Hybrid’

Flower color may be white

Grevillea ‘Mason’s Hybrid’

Flower color may be white

 Grevillea ‘Mason’s Hybrid’

Flower color may be white

Grevillea juniperina ssp. sulphurea

Or some variation on these colors, although in some species and cultivars, multiple colors may be present on flower parts, producing even greater impact.

Grevillea alpina.

In such a diverse group it is no surprise that the flower season varies greatly among the various species and cultivars. In mild climates it is possible to have one Grevillea or another in bloom at any time of the year. In addition, the length of the bloom period can be very long, as in the case of G. victoriae, which may remain in bloom from December through April. In Australia, the flowers are very attractive to various species of nectar-eating birds and these are considered important pollinators. Insects such as beetles, ants and bees are attracted to the flowers as are even some mammals. In California and the Pacific Northwest, hummingbirds are very attracted to the flowers as are both bumblebees and honeybees.

Grevillea ’Poorinda Constance’

Following flowering, seed pods develop, which may be a thick- or thin-walled follicle, each containing 1-2 seeds. In some cases, the seeds may have a papery wing to allow them to be distributed by the wind. Grevilleas do set viable seed in western Oregon.

Cultivation in North America

Use of Grevillea in North America is extremely limited, principally because of lack of cold hardiness. Most of Australia corresponds to USDA Hardiness Zone 9 or higher, and as a result only the warmest regions of North America are suitable for outdoor cultivation of any species or hybrids. For landscape use, the plants are almost entirely limited to warmer regions of California, Hawaii and parts of the southeast. Some species or cultivars are also cultivated in Oregon and Washington, west of the Cascades Mountains and southwestern British Columbia.

In California numerous Grevillea are popular landscape plants and many of them seem well-adapted to the Mediterranean climate parts of the state. An excellent collection of Grevillea as well as many other Australian plants is maintained by the University of California, Santa Cruz Arboretum and from this collection many cultivars have been introduced into the nursery trade. In both Hawaii and Florida, G. robusta was introduced, as in many other warm-climate regions of the world as a street tree, but has escaped from cultivation and is considered weedy. This species is also cultivated farther north, and is known to survive (with some foliar burn) at least as far north as Valdosta, GA. Other Grevillea which are grown in the southeast include ‘Noellii’, a hybrid which originated in California and ‘Canberra Gem’. Others such as G. victoriae have evidently been cultivated in North Carolina and the National Arboretum did have a plant of this species in an outdoor bed in 2013.

In the Pacific Northwest, Grevillea have been, and remain, essentially enthusiast plants and are available primarily through specialty vendors. However, several species or cultivars have proven well-adapted to the region, even performing well in a succession of cold winters in 2008-2010. The most commonly available Grevillea is most likely G. victoriae, which has been successfully grown as far north as Vancouver, B.C. It can be found as a number of different selections, including ‘Marshall Olbricht’ (a cultivar that originated at Western Hills Nursery in Occidental, CA and named for the original co-owner)

Grevillea ‘Marshall Olbricht’ in bloom in December

‘Murray Valley Queen’ and also a plant referred to generically as the “UBC form”, derived from the plant grown at the University of British Columbia Botanical Garden. Some forms or hybrids of G. juniperina have been cultivated successfully, including ‘Poorinda Constance’, ‘Molonglo’ and ‘Canberra Gem’. G. australis has also been grown to a limited extent. As in other areas, growing tender Grevillea in containers offers a way to expand the range of species that can be grown, assuming winter protection is available

Grevillea ‘Red Hooks’ in container, flowering in May in western Oregon.

OSU evaluation of Grevillea

An evaluation of Grevillea was begun in 2007 at the Oregon State University North Willamette Research and Extension Center (NWREC) in Aurora, OR. As with other Northwest Plant Evaluations, the goals were to evaluate growth, flowering, hardiness and overall landscape value of these plants (and to some degree available related genera such as Hakea and Lomatia). Cuttings and stock plants were received from Nurseries and Botanical Gardens locally and in California in 2007 and 2008. Many of the accessions were received from the University of California Santa Cruz Arboretum.

Grevillea at UCSC Arboretum

Rooted cuttings were potted into 4” pots in June 2009.

Potted Grevillea in June, 2009

A 0.5 acre field planting of 65 species and cultivars of Grevillea was established in September 2009 at NWREC adjacent to the Bureau of Reclamation Agrimet weather station. Soil on this site is a Latorell loam. Four replications of each taxa were planted in a Completely Randomized Design

Grevillea planting (#1), September, 2009

Unfortunately, severe cold weather in December 2009, with temperatures from Dec 9-11 near or below 10oF killed the entire planting.

Since this was not regarded as a fair test of the hardiness of these plants, coming as it did only three months after planting, a new effort was undertaken to establish this planting. Cuttings were once again obtained in June and December 2010 from area nurseries as well as from the UCSC Arboretum. Cuttings were rooted at NWREC, and were potted into 4” sleeves in spring 2011.

 

Grevillea cuttings, June 2010


Potted Grevillea, June, 2011

A total of 65 taxa were successfully propagated, and this included several HakeaLomatia and Banksia.

Plants were grown on during the summer, and a 0.5 acre field planting was once again established in late August 2011. Four replications of each taxa were planted in a Completely Randomized Design. The planting will consists of rows spaced 12’ apart with a grass alleyway between and an in-row spacing of 5’.

 

Planting the second Grevillea evaluation, August 2011

The data being collected starting in spring 2012 are:

  1. Plant height and width.
  2. Flowering data (collected monthly in season on a 1-6 scale).
  3. Winter injury data (collected each spring or as required).
  4. Pest and disease information.
  5. Landscape quality (a rating of foliage quality and overall appearance on a 1-6 scale).

Plants were watered for the remainder of the growing season until reliable rain began in October. After this time, no further supplemental water or fertilizer was provided. Plants are also not sprayed with any fungicide or insecticide, and they are not pruned. Weather data for the evaluation is provided by a Bureau of Reclamation Agrimet station at NWREC, located only 200’ from the west edge of the planting).

Results

Results of the cold hardiness evaluation were not long in coming. Low temperatures for the 2011-12 winter was 24oF, observed on December 13th and 22nd, 2011. Though this is mild by Willamette Valley standards (USDA Hardiness Zone 8b), it was sufficiently low to cause serious damage to a large swath of the evaluation.

 

Grevillea evaluation, August 2012. Note the many missing plants.

Plants were rated in spring 2012 on a 1-6 scale, with 1 being no visible damage and 6 being a plant that was dead and not exhibiting recovery. At the time of the rating plants were rated a maximum of 5, which was used for plants that were killed to the round but may subsequently exhibit some regrowth, as it was not clear whether affected plants would recover. As the growing season progressed however, it became clear that severely affected plants, those rated 3.5 or higher, did not recover but instead died. Injury to the plants and their relative hardiness are summarized in the following tables.

Table 1. Plants considered not hardy

y Cold injury visual rating: 1: no damage, 5= dead

 Table 2. Plants considered to be marginally hardy

y Cold injury visual rating: 1: no damage, 5= dead

Table 3. Plants considered to be hardy

 

yCold injury visual rating: 1: no damage, 5= dead

It remains to be seen whether those on the list in table 3 are actually hardy in this area over the long term, but the list does include those plants which have been in cultivation for a number of years.

By the end of 2013 the planting was well established and growth of the remaining plants has been rapid, for the most part. Besides G. australis and G. victoriae, the species most commonly found in cultivation in the PNW, cultivars such as G. juniperina ‘Molonglo’, ‘Poorinda Constance’ and ‘Canberra Gem’ were growing well. The surprises of this evaluation have so far been several lesser-known cultivars or selections. These include ‘Aromas’, G. lanigera ‘Coastal Gem’, G. lavandulacea ‘Penola’, ‘Olympic Flame’, ‘Pink Pearl’ and ‘Scarlet Sprite’.

Both Lomatia myricoides and L. tinctoria were growing well and both had flowered. Of the Banksia planted out, B. marginata and B. integrifolia ssp. monticola were growing very vigorously although neither had produced flowers.

Considering how mild the winter temperatures have been since the establishment of the evaluation, it remains to be seen how well these plants will fare in a more typical winter.

Grevillea evaluation, October 2013.


G.lanigera ‘Coastal Gem’, October 2013

 

G. lavandulacea ‘Penola’, May 2013.

 

G. ‘Olympic Flame’, May 2013.

G. ‘Scarlet Sprite’, May 2013.


Banksia marginata, May 2013

Grevillea evaluation: the cold spell of December 2013

Unlike the previous two winters, which had seen very mild conditions, the weather turned sharply cold in western Oregon in early December, 2013. Low temperatures at the North Willamette Research and Extension Center in Aurora dropped to 10°F on Dec 8th and 11°F on December 9th.  Prior to this there had been numerous nights in late November where temperatures dropped into the high to mid-twenties and given this, and the occurrence of the two cold nights in the middle of the dormant period, as well as the fact that the remaining plants in the evaluation were now well-established, it provided a legitimate opportunity to assess comparative hardiness of the remaining cultivars. A visual estimate of damage to the plants was done on January 17th, 2014, 5 weeks after the cold event. Weather during the period after the cold event had been fairly mild, which allowed for good symptom development. Plants were evaluated on a 1-5 scale, with 1 equaling no damage; 2 equaling minor foliar discoloration or damage; 3 equaling leaf or stem injury to outer 30% of the canopy; 4 equaling leaf or stem damage to outer 60% of the canopy; and 5 equaling damage to the leaves or stems on the entire plant.

Data are summarized in the following table. The table is arranged arbitrarily into two columns, with lightly damaged plants in one column and heavily damaged plants from which good recovery will be less likely, or take longer, or is not expected at all. These are only averages of damage to the 4 plants of each cultivar in the evaluation; no statistics were performed on the data. In some cases there are less than 4 plants per cultivar remaining in the evaluation.

 

The results in some cases are expected, as in the relative lack of damage to G. australis and various cultivars and selections of G. victoriae, whose hardiness is known and has made them among the more widely cultivated forms of Grevillea in the Pacific Northwest. Other cultivars that fared well also have a track record of good hardiness including ‘Canberra Gem’ and ‘Poorinda Constance’. Some other, lesser-known cultivars performed well, however, such as ‘Poorinda Queen’, ‘Scarlet Sprite’, ‘Pink Pearl’ and ‘Poorinda Signet’. Although the last two did receive some shoot tip damage, it was not severe enough that good recovery would not be expected. On the other hand, other cultivars which had grown and flowered well and appeared promising, including ‘Aromas’ and ‘Olympic Flame’, suffered considerable injury and at least based on these results, cannot be considered hardy enough for landscape use, except in the most protected locations.

Grevillea References

Burke, D (1983), Growing Grevilleas in Australia and New Zealand, Kangaroo Press (out of print).

Elliot W.R. and D.L. Jones. Encyclopedia of Australian Plants suitable for cultivation. Vol. 5. 1990. Lothian Publishing Company, Melbourne.

Grant, W.A. 1991. Grevilleas in my garden. Pacific Horticulture. 52(1):27-31.

Judd, C.T. 1982. Grevilleas of the Grampians. Australian Plants. 11:312-314

Makinson, R.O.. 2000. Grevillea. In: Flora of Australia. Volume 17A. Proteaceae 2. Australian Biological Resources Study/CSIRO. ISBN:9780643059696. Melbourne. 524pp.

McGillivray, D., assisted by Makinson, R (1993), Grevillea, Melbourne University Press. (out of print)

Olde, P and Marriott, N (1994), The Grevillea Book, Vols.1,2,3, Kangaroo Press. (out of print)

Online Resources 

Australian Native Plants Society

Australian National Botanic Gardens

Flora of Australia online