Grevilleas for the Maritime Pacific Northwest
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Grevilleas for the Maritime Pacific Northwest PAUL BONINE Native shrubs from Down Under, grevilleas are proving to be cold hardy and adaptable to the maritime Pacific Northwest. Drought-tolerant and carefree, they provide months of color with exotic flowers that are attractive to hummingbirds and gardeners alike. Once relegated to the corners of botanical gardens in our region, more species and cultivars are poised to move from the shadows into the spotlight. (See page 35 for grevilleas in Southern California.) revillea is a diverse and enormous genus, forests of the southeast, and the highest moun- G *ranging from groundcovers to tall trees, tain zones. Most are shrubs, likely an adapta- that is native almost exclusively to Australia tion to drought and nutrient-poor soils. Such (with a few species in New Guinea). All are conditions also lead to an intriguing variation evergreen members of the protea family in leaf size and shape, from simple, entire (Proteaceae), an ancient group of plants cen- leaves, to fine needle-like leaves, to elaborately tered in the Southern Hemisphere. They have dissected feathery leaves—all of which make long been grown in California but have grevilleas as handsome for their foliage as for recently garnered the attention of Pacific their flowers. The fine coating of hair (indu- Northwest gardeners. Experiments with a few mentum) that covers all parts of the plant pro- alpine members of the genus have opened the vides further protection against the harsh door to a wider array of species and cultivars climate —modifying the temperature of the that expand our understanding and enjoyment plant surfaces and mitigating desiccation from of these plants. As more become available to sun and wind. gardeners, their charms, ease of culture, adapt- ability to summer drought, and extremely long flowering season are sure to propel their popu- Culture larity. They offer diverse foliage textures as well as unusual flower forms—and even fragrance. Australia’s typically poor, ancient soils have And, many grevilleas are shunned by deer. resulted in another distinct adaptation: special proteoid roots. Also known as cluster roots, these short-lived roots form large clumps just Highly Adapted to Their Environment below the soil surface, able to quickly soak up moisture from erratic rains. These fine roots are Australia is a vast continent and grevilleas also exceptionally efficient at extracting scarce have evolved to fill a wide range of niches. nutrients. As a result, grevilleas and other Three hundred and forty species reside in the members of the Proteaceae are highly sensitive deserts of the outback, coastal shrub communi- to overly rich soils; high levels of phosphorous ties, the tropical north, the temperate rain- in the soil can prove particularly toxic. Most 22 / Pacific Horticulture Oct/Nov/Dec 2011 grevilleas are adapted to soils low in organic temperate to tropical climate. Grevilleas found matter; too much organic matter can lead to in the highest and coldest regions will have the chlorosis and, in extreme cases, death of the greatest chance of success in our climates. plants. Grevilleas thrive on well-drained soils “Cold” is a relative term, however. In Australia, of average to poor fertility. In our native soils, it the coldest conditions result from a combina- is best to avoid all fertilizer and to limit or tion of elevation and radiational cooling; there exclude organic amendments. Many species is no source of arctic air, so the ultimate low are amenable to clay if planted on sloping sites. temperatures are more limited than in the Most grevilleas grown in the Northwest Northwest. Still, at and above tree line, up to thrive in full sun. They are supremely adapted ten species thrive; even plants at lower eleva- to drought, especially those with the smallest tions, where they may be subjected to cold air or narrowest leaves. Broad-leafed species seem drainage in valleys, can possess significant cold to appreciate more water during the driest hardiness. The provenance of individual plants months. Grevillea victoriae, for instance, often is important: plants growing at a higher eleva- begins to form its flower buds during the sum- tion will often show greater tolerance to cold mer. We have noted that this species will abort than will plants of the same species from lower buds if plants do not receive adequate moisture elevations. Therefore, it is helpful to know the during the driest months; a deep soaking once precise origins of each individual plant. a month is sufficient. Successful in the Maritime Northwest Exotic Flowers We have been collecting and testing grevil- Exotically shaped, vividly colored, and leas for cold tolerance at our nursery for the curiously arranged flowers are what attract past ten years. A gratifying number of them most gardeners to Grevillea. Paired individual have proven hardy to cold. flowers are clustered into groups, or con- In December 2009, we recorded the coldest floresences, which are usually raceme-like. Each temperatures since 1990. For two nights, the individual flower is complex, with a strongly lows dipped to 7°F (-14°C), and to 10°F exserted style held above a pouch-like perianth, (-12°C) on the third night. This provided us which will often recurve as the flower opens. with an excellent test of the cold hardiness of The flowers offer nectar that is irresistible to our plants. Of the twenty selections in the hummingbirds. In their native environs, they are ground (we had chosen those from high eleva- natural attractants for honeyeaters and other tions or known to be tolerant of cold), sixteen nectar-feeding birds. survived with no damage. Our site is excep- Most grevilleas have an extremely long season tionally well drained in full sun, and none of of bloom. Some will be in flower for up to nine the plants were given any protection. All had months of the year—including the coldest and been in the ground for at least a year. darkest winter months. It is this ability to flower The winter of 2010-2011 was even more dam- through winter that has been most welcome in aging, beginning with a severe freeze arriving the Northwest. A curious trait among grevilleas unusually early in November. Some of the grevil- adapted to cold conditions is that their flower leas recommended here were damaged by this buds are protected by a dense coat of fine hairs. freeze, but all had fully recovered by summer; If the temperature drops below approximately not a single one was lost during the winter 26°F, open flowers will be destroyed, but the Established plants typically show a much unopened flower buds will be protected and will greater resistance to cold than the same species open when temperatures rise. newly planted. Thus, we recommend that all Hardiness to cold is the main limiting factor grevilleas in our climate be planted in early to for the cultivation of grevilleas in the Pacific mid-spring to give each plant the longest Northwest. Most of Australia enjoys a warm- possible time to establish before the first winter. Grevilleas for the Maritime Pacific Northwest / 23 Grevillea victoriae ‘Murray Queen’. Author’s photographs Two cultivars of Grevillea juniperina: apricot ‘Molonglo’ and ‘Low Red’ Grevillea juniperina ‘Molonglo’ Grevillea ‘Poorinda Queen’ Grevillea australis ance. This is a variable shrub, ranging from Alpine or southern grevillea is a handsome, upright rounded forms (four feet tall and as fine-textured shrub of cold, high elevations. wide) to ground-hugging, prostrate forms. In Small, olive green leaves are clad on the back the alpine highlands of mainland Australia with a fine brown indumentum that also covers and on Tasmania (where it is the only indige- the stems. This lends an overall tawny appear- nous species), it can often be seen growing 24 / Pacific Horticulture Oct/Nov/Dec 2011 Trailing Grevillea 5 gaudichaudii Grevillea juniperina ‘Xera Ember’ Grevillea ‘Marshall Olbrich’ Grevillea victoriae in bud against rocks that will radiate heat and flowers emit a sweet and powerful perfume, protect it from severe cold. Unlike most reminiscent of honey, that carries for quite a species, this is exclusively a spring bloomer. distance in the garden. This may be the hardi- In western Oregon, it opens its small white, est grevillea, enduring temperatures to 5°F floss-like flowers in March; they persist for without damage. It is adapted to fast-draining, weeks. Though not showy, these diminutive poor soils, including bare mineral soils and Grevilleas for the Maritime Pacific Northwest / 25 sand. At our nursery, one specimen happily average clusters; it has finer leaves than the other grows in clay soil on a slope. It is extremely selections we grow. ‘Orange Zest’ has an upright tolerant of dry conditions, as well, and can habit that results in an arching shrub to three feet thrive on little or no water in our dry summers. tall and six feet wide. A profuse bloomer, its Alpine grevillea is a shrub for full sun and will masses of clear orange flowers appear from late not perform in shady conditions. winter through fall. Both of these cultivars have endured 7°F without damage. Grevillea juniperina Juniper grevillea is a widely distributed Grevillea victoriae shrub of middle to high elevations in Australia. Royal grevillea is among the most regal of It is a highly variable species, with a broad shrubs that we can grow in the Pacific range in flower colors and growth habits. We Northwest. This tall species has large hand- have had great success with several forms of some gray leaves; the stems are clothed in this tough and drought-tolerant plant. Fine, gray as well, giving the entire shrub a soft needle-like leaves are grass green and reminis- quality.