Landscape Plant Evaluation Trials Woody Shrubs: Arctostaphylos and Grevillea
The evaluations of Arctostaphylos and Grevillea are part of an ongoing series of plant evaluation collectively referred to as. The goal of the program is to identify flowering shrubs suitable for use in landscapes of the Pacific Northwest, west of the Cascade Mountains. A priority of this program is to evaluate genera of shrubs thought not to be particularly hardy in the region, but which offer compelling ornamental characteristics. The initial criteria for the program, beginning with an evaluation of Hebe planted in 2000, were to assess growth, flowering, landscape quality and cold hardiness of as many cultivars or species of a given genus as could be acquired. Since 2000, the program has focused more specifically on evaluation of drought-tolerant genera that can be grown without summer irrigation. To date the program has completed evaluations of Hebe (2000-2009), Ceanothus (2001-2005), Cistus (2004-2009) and Halimium (2004-2009).
The evaluations of Arctostaphylos and Grevillea were planted in 2011 at NWREC. Arctostaphylos is a genus of woody, evergreen shrubs or small trees native primarily to western North America, from southern British Columbia into Mexico. One species, A. uva-ursi, is known as kinikinnick and is commonly grown as a groundcover in many areas of North America, including the Pacific Northwest. Unfortunately, kinnikinnick is not always a very vigorous or successful groundcover, as it often suffers, particularly west of the Cascades, with disease and pest problems. This species is also very low-growing, and as a result may not suppress weeds as effectively as a taller groundcover. In addition, the most commonly grown form, A. uva-ursi 'Massachusetts', originated from a population in the northeastern U.S. and is not adapted, as western species and selections are, to summer drought.
Because of the wide geographic distribution, the habit of manzanitas varies widely from prostrate, wide-spreading groundcovers to rounded shrubs to small trees. Beyond A. uva-ursi, none of the many other manzanitas are used to any extent in western Oregon landscapes except by enthusiasts. This even includes the manzanitas native to Oregon, of which there are quite a number (see the Oregon plant atlas at the Oregon Flora Project site). The reason why these plants are not used is because a near-complete lack of information about their growth, hardiness, pest resistance or other landscape characteristics in the PNW. As a result none are generally available in the wholesale trades, even though they may have been grown on a limited basis successfully by enthusiasts and retail nurseries.
Grevillea is a genus of about 360 woody, evergreen shrubs or trees native primarily to Australia. Their habit ranges from prostrate and wide-spreading to upright shrubs to tree forms. The common name "spider flower" refers to the unusual flowers of some of the species, which are long and slender and borne in dense bunches. The flowers are found in shades of red, yellow or white.
Grevillea have become popular in Mediterranean climates of the world for their evergreen habit, showy flowers and particularly their drought tolerance. Most species and hybrids bloom in late winter and spring. An additional ornamental characteristic of these plants is the foliage. When the plants are out of bloom, the soft green, silver or grayish foliage is still attractive. Because of their origin in drought-prone, warm areas, Grevillea are well adapted to summer drought, and are suitable for growing in coastal regions or in poor soil. They can be grown successfully along dry banks, parking lot islands and other locations where water is limiting. Grevillea perform reliably in these difficult situations, where many common landscape plants struggle.
Most of the many Grevillea varieties that exist are unknown to cultivation in the Pacific Northwest, or have been grown only by enthusiasts. Only rarely have they been used in landscaping and they are currently produced in only small numbers by specialty nurseries in the PNW. The Grevillea most commonly available in the PNW are G. victoriae, G. juniperina cultivars, and other cultivars like 'Canberra Gem'.
Cuttings of approximately 75 taxa of Arctostaphylos and 65 Grevillea were obtained in 2010 and rooted at NWREC. The rooted cuttings were grown on, and both the evaluations were planted in summer 2011. The plants were watered in until fall, after which they will receive no further irrigation. These evaluations also receive no fertilizer or pruning to replicate the treatment the plants would receive in a "low-maintenance" landscape. All plants will be evaluated over the next several years for growth, flowering, pests and diseases, landscape quality, and cold hardiness.
Neil Bell, Community Horticulturalist
OSU Extension Service - Marion & Polk Counties
3180 Center Street NE #161
Salem, OR 97301