Some Silver-leafed Plants in California’s Central Valley

Artemisia tridentata. Photograph by T Elliot Weier

Artemisia tridentata. Photograph by T Elliot Weier

The author helps in our celebration of silver with thoughts on silvery foilage suitable for gardens in the interior parts of California.

Silvery plants with a bright metallic sheen that are cold hardy, too, are few and far between. Though some plants may be white and show up well at night, their leaves may not be truly argentine. A virtual silverness is the result of the arrangement and shape of plant hairs: either narrow but closely parallel and lying flat on the leaf, as on silverbush (Convolvulus cneorum); or flat, umbrella-like, and overlapping as on some species of Elaeagnus. These are effective adaptations for reducing transpiration by reflecting sunlight in harsh environments, giving the plant the appearance of brushed silver.

Here at the Davis Arboretum on the University of California campus (Sunset zone 14, USDA zone 9), we constantly experiment with plants to find those that will survive both our intensely hot and dry summers as well as our cold, often damp winters. While most silvery plants are well adapted to the summer conditions, they are less adapted to the winters, particularly the occasional seasons where temperatures drop into the teens. Following is a short list of the successes, and a few partial failures, among our silver selections, with recommendations on their use in the garden.

Silverbush is a shrublet with beautifully metallic foliage and white funnel-shaped flowers characteristic of the morning glory family (Convolvulaceae). Although it requires perfect drainage and doesn’t last long in most gardens, this Mediterranean coast native is well worth replacing regularly. Think of it as a short-lived perennial and replant it in a sunnier, drier place next time. Be sure to avoid using it in formal, linear, or massed plantings to avoid a “gap-toothed smile” or the appearance of true decimation. Convolvulus cneorum is best used as a replaceable accent not crucial to the framework or mass of a design, even though its appearance is striking. We usually replace our plants on the Davis campus every two years or so.

Hyalis argentea. Photograph by Ryan Deering

Hyalis argentea. Photograph by Ryan Deering

Just as metallic, but much easier to grow, is the still rare Hyalis argentea, a mustisioid in the sunflower family (Asteraceae). This shining example comes from foothills on the east side of the Andes in Argentina (appropriately enough). Varitas de plata or silver wand would be good names for this slowly spreading perennial. The narrow, arching stems and lance-shaped leaves are truly silver, especially in summer. Ours are now about eighteen inches tall, but have yet to flower. We are told that small violet colored flowers can appear at the appropriate age and time. This species looks like a graceful, miniature version of South Africa’s glorious Leucadendron argenteum (see Pacific Horticulture, January ’01), the cold-tender, temperamental silver tree, which is impossible this far away from the coast. (Even more impossible would be the famous silver sword (Argyroxiphium sandwicense) from the highest slopes of Hale’akala on Maui.) We look forward to growing lots of Hyalis—something much more than just a greeting for a certain visitor in Wonderland.

Some hardy shrubs in the oleaster family (Elaeagnaceae) have silvery leaves, but the shiniest species are not currently growing in the arboretum. The following are well adapted to our climate, however. The leaves of Russian olive or oleaster (Elaeagnus angustifolia) from central Asia are among the most metallic silver of any hardy tree, more so on the undersides of the leaves than on the tops. Even the tiny, fragrant flowers, fruits, and young twigs are covered in silvery scales. In winter, the leaves drop to reveal an unappealing, chaotically spiny branch structure, although old trees develop handsome, furrowed, dark rusty brown trunks. We would do well to try the even more silvery silver berry (E. commutata), a tough, deciduous shrub of northern interior North America. Buffalo berry (Shepherdia argentea) is a large, thorny, riparian shrub from high, dry, cold parts of our continent including California. Sea buckthorn (Hippophaë rhamnoides) from Europe and Asia, is similar but root-sprouts aggressively. The last two both have silvery, deciduous leaves and edible red fruit, but are rather too coarse for most gardens. Members of this family fix nitrogen symbiotically and are tough plants for tough climates.

Of all the grasses, perhaps Amur silver grass (Miscanthus sacchariflorus) bears the most shiny silvery of seed plumes—simply gorgeous! But beware: it can reach ten feet tall and could escape into our wetlands. Native to northern Asia, it has already invaded eastern North America. Although its beauty and grace are tempting, we’ll not likely be planting this lovely reed in the arboretum.

Bush lupine (Lupinus albifrons). Author's photograph

Bush lupine (Lupinus albifrons). Author’s photograph

Two silvery Californian shrubs have gently naturalized in the arboretum. Silver bush lupine (Lupinus albifrons), with a life span of about six years, is native to hillsides all around the Central Valley. Big sagebrush (Artemisia tridentata) is the state flower of Nevada and a symbol of the West. One of ours is still thriving after sixty-four years. Both species re-seed themselves appropriately. After the violet blue flower spikes of the lupine and the silvery plumes of the sagebrush have released their seeds, deadhead them for best appearance.

Some marginally cold hardy silver plants are worth growing in small numbers in the Central Valley or other colder regions of the West Coast because of their extraordinary beauty. The silver plectranthus (Plectranthus argentatus) from Queensland, Australia, is like a big, silvery coleus with spikes of small violet flowers. It reaches about sixteen inches tall before being struck down in all but the mildest winters. Think of this member of the mint family (Lamiaceae) as an annual except in mild areas along the coast. The low, spreading Mo’s gold everlasting (Helichrysum argyrophyllum) is like the Vatican flag: silver and gold. A member of the sunflower family and a native of South Africa’s eastern Cape region, it has silvery leaves and golden-bracted flowers, all wonderfully metallic. Cold, wet winters will eliminate it from the garden, but it is stunning while it lasts.

Helichrysum argyrophyllum. Photograph by RGT

Helichrysum argyrophyllum. Photograph by RGT

Another sunflower family member of similar metallicity is drumsticks (Craspedia globosa). This is native through much of eastern Australia on poorly drained soils. Tufts of silver leaves, six inches long and rather narrow, are upright “nests” for the round, hard yellow flower heads on stiff, sixteen-inch wands. They are wonderful for dried arrangements. These extraordinary plants will probably survive all but our coldest winters; clumps of these beauties have been thriving for us for several years.

We used to have a thriving plant of silver spear (Astelia chathamica), but it died in the 1990 freeze. Our plants came from the UC Botanical Garden in Berkeley (also the source of our Hyalis). Silver spear is appropriately named: open clumps of shining, silvery, broadly pleated spears for leaves. Ours got to fourteen inches in four years, propagated from a division. It originates on Chatham Island, east of New Zealand’s South Island, and is in its own family (Asteliaceae), once part of the lily family (see Pacific Horticulture, October ’00). We’ll be replanting silver spear, since it does survive most of our winters.

Weeping acacia (Acacia pendula). Photograph by RGT

Weeping acacia (Acacia pendula). Photograph by RGT

The same can be said of weeping acacia or weeping myall or boree (Acacia pendula), which we also lost in 1990. Except for its susceptibility to extraordinarily cold winters, it was one of our most appreciated and graceful small trees. Imagine a billowy head of weeping branches adorned with silver leaves on a picturesque gray trunk. The pale yellow flowers are an added, albeit quiet, decoration. Even if it were to be killed after fifty years of growth, the wood can be made into small boxes or cabinets that have the natural fragrance of raspberry jam. And then we could replant it again. This member of the pea family (Leguminosae) is from eastern Australia.

There are undoubtedly other truly silver plants that would be worth growing in our Central Valley gardens. We look forward to discovering and planting them to mirror the light of the sun and the moon—and for us to enjoy.