The Silver Tree

Silver trees (Leucadendron argenteum) on both sides of a hiking path, with Table Mountain in the background. Trees in flower have their terminal leaves splayed to display the flower heads. Author’s photographs

Silver trees (Leucadendron argenteum) on both sides of a hiking path, with Table Mountain in the background. Trees in flower have their terminal leaves splayed to display the flower heads. Author’s photographs

The genus is named after its most outstanding member, Leucadendron argenteum….known to botanists, gardeners and plant lovers throughout the world as the silver tree, but the early Dutch settlers dubbed it witteboom, literally ‘white tree’. In 1691 the botanist Plukenet used the name, translated into Latin, to describe the witteboom and related plants….Leucadendron, therefore, became the accepted name for this genus…

Marie Vogts, South Africa’s Proteaceae

On that projecting isthmus of land called the Cape Peninsula, at the bottom left hand corner of Africa, there can scarcely be anyone unfamiliar with the handsomest arboreal member of the protea family: silver tree or Witteboom, as it is known in high Dutch and its descendent language, Afrikaans. Always strikingly silver, it is transformed by the hot, drying, gale-force north winds of the cooler months of the year to a gleaming metallic lustre. The glory takes one’s breath away! Even the dry leaves of herbarium specimens retain their sheen, prompting Linnaeus, that first great modern plant namer, to exclaim at his desk in Uppsala, “this tree, the most shining and splendid of all plants.” I wonder if he was lucky enough to see a living specimen; the seeds were certainly available in Holland by then.

Silver tree (Leucadendron argenteum) is one of eighty species of this dioecious genus. It belongs to a family (Proteaceae) of great antiquity, fossils dating back to the time when the great southern continent Gondwanaland started splitting apart. Nowadays, the family is represented on all its daughter continents except Antarctica. In the winter-rainfall flora of the Cape Province, there are many genera and species of this ancient family, but Leucadendron probably reflects a relatively recent evolutionary direction for the family; only one other proteaceous genus in this region is dioecious.

Silver Trees at Home

In nature, silver tree is confined to the heavier, gravelly, well-drained soils derived from Cape granite on the lower slopes of Table Mountain, at the northern end of the Cape Peninsula. It is also found in two similar locations away from the Cape Peninsula on the Paarlberg and Simonsberg slopes, but these may not be natural populations. In all of these localities, it forms new sub-social stands after each bush fire. The initial burst of germination occurs in the autumn following a fire; in succeeding years, new seedlings continue to obtain a foothold within the parent population. Such mixed age stands are unusual for a species that reappears only from seed and not from stump sprouts after a fire. This process is probably facilitated by the rather open canopy of silver tree stands in nature.

The climate experienced here by silver trees is mild, and frost is absent. The maximum temperature seldom rises above 104° F and most often ranges between 55° and 86°F in the daytime. The rainfall totals twenty to twenty-five inches in winter, with little rain in the summer months (October to April in the Southern Hemisphere). The habitats are mostly in full sun; in summer, in at least some of the localities, the south-east trade winds blow at gale force for several days at a time without the least harm to the trees.

The tree grows from seed quite rapidly at first, reaching twenty-five feet in eight years or so and is eventually capable of reaching fifty feet and living for about as many years. It has rather coarse upright branches that, when young, give rise to a dense conical tree; the crown spreads and becomes more rounded when mature. The lance-shaped six- to eight-inch long, sessile, overlapping leaves ascend loosely up the branches obscuring the bark, which is only revealed by the abscission of the lowest leaves as the tree matures. Each leaf persists for several years, being evergreen—a rather incongruous description for so ever-silver a leaf! The thick undivided trunk has a distinctive pale, smooth, spongy-corky bark with horizontal leaf scars that stretch into conspicuous horizontal furrows as the trunk expands.

Male flower head on silver tree (Leucadendron argenteum)

Male flower head on silver tree (Leucadendron argenteum)

After only a few years, the first flower heads appear in early spring at the tips of the branches; they nestle in the uppermost leaves, which spread to form loose haloes around the heads as the flowers open. The trees at this time are particularly handsome and conspicuous, even from a distance. The male flower heads—the shape of a golf ball and of similar dimensions—are apricot yellow with a silvery pink sheen, the many minute flowers producing a delightful scent reminiscent of vanilla and violets, presumably to attract insect pollinators. The globose female flower heads, up to three or four inches across, are composed of sturdy overlapping bracts, each enclosing a diminutive and inconspicuous flower that, after fertilization, produces a nut the size of a small peanut. The woody, mature, conelike fruits are held on the tree for years, leading an early writer, Hendrick Claudius, to assign its first name, Pinus Africana, in the Codex Witsenius. He was quite mistaken about its affinities, as there are no native pines in Africa south of the equator.

Female flower head on silver tree (Leucadendron argenteum)

Female flower head on silver tree (Leucadendron argenteum)

The nuts mature in autumn, in time to germinate with the first rains, but they are seldom released except at the death of the parent tree. Then the cone scales part, and a feathery parachute (formed from the old flower parts) is sucked out by the wind, dragging with it the attached nut on a thread formed from the style and stigma, to whiz along for a surprising distance, considering its weight.

More Than a Pretty Leaf

Nowadays, the uses to which we put silver tree are mostly ornamental, but an architect who has restored many of the grand, old, thatched, Cape Dutch houses reports that those on the Cape Peninsula used silver tree beams and perlons in their roofs; the wood is particularly strong due to its distinctive cross-banded structure. In the time of the Dutch East India Company (prior to the 1800s), the nuts were harvested as a supplementary feed for pigs taken by the sailing fleet on its long return trip from Batavia to Holland. The leaves have been sold as bookmarks, as a base for the painting of little Cape scenes as souvenirs, or for incorporating into wreaths. The leaf trade became so excessive that, by the mid-twentieth century, a ban had been placed on their harvest, bringing to an end a pretty source of pocket money for local children. All these uses point to how common silver tree was in the early days of settlement at the Cape.

But the superabundance of silver trees is a thing of the past for, in the 1930s, a disease suddenly appeared, and trees of all ages and health started yellowing, wilting, and dying. A botanist employed at Kirstenbosch, the magnificent native plant garden in suburban Cape Town, discovered that a borer beetle seemed to be associated with the disease. By the 1940s, the problem had become so acute that a plant pathologist was called in to investigate the dying trees. He established the identity of the beetle, an indigenous buprestid that bored into the trunk, mostly near ground level. He concluded that the beetle was probably not the first cause, but perhaps only an agent of some recent pathogen not previously present at the Cape. Implicated in 1975 was the fungus Phytophthora cinnamomi, imported by accident some time before from the Far East to South Africa and now causing havoc in the avocado industry. The only practical advice to come out of these investigations was a suggestion to maintain a thick growth of shrubs underneath silver trees to hamper the beetle’s access to the base of the trunks—a council of despair, for there is still no satisfactory answer to the die-off, and silver trees are still prone to it; for this reason, they have now become scarce and vulnerable.

Silvery gray bark on the trunk of an old silver tree (Leucadendron argenteum)

Silvery gray bark on the trunk of an old silver tree (Leucadendron argenteum)

Silver Trees in Cultivation

Silver trees can be cultivated and are certainly worthy additions to the landscape where adapted. The nuts are sown in autumn in an acid, well-drained medium. The Royal Horticultural Society’s Dictionary of Gardening suggests a fifty-fifty mixture of peat and grit (sand) with added charcoal and a pH of 6.5 or below. They germinate in about four weeks and should be grown in individual pots for only one year, with perhaps a light feeding of seaweed or fish emulsion to carry them over. The RHS Dictionary also recommends a dilute application of magnesium sulfate (Epsom salts)and urea in spring and autumn; I caution that the latter could make the plants lanky and weak and may account for the RHS advice to prune after flowering—an idea quite extraordinary to the Cape gardener observing our chunky, sturdy, unpruned trees. Seedlings should be planted out in the second autumn, before their taproots have become constricted, to enable them to put down a deep root system before the hot summer weather sets in again. Plant in excellently drained, phosphorus-deficient, acid soil (silver trees, like most Cape plants, do not like lime), preferably on a rocky slope, and watered only sparingly in summer. Low phosphorus is essential; even residues of phosphorus in old agricultural lands or gardens can cause distortion of the branches into long, sinuous, weak growths scarcely able to support their own weight and giving rise to a floppy travesty of the genuine article. Without the borer beetle, gardeners outside of South Africa should have better luck with silver trees than we do. Above all, do not interfere with the roots of any Cape members of the Proteaceae, including silver tree; even walking over the roots too frequently can set the trees back by damaging the curious, superficial, coralline, proteoid roots that are put out each season. These roots are shallow and easily damaged; it is best to protect them with paving slabs and to avoid digging around them.

Here in the Cape, we all want to grow a silver tree in our gardens, but we seldom do so. Certainly, it is a beauty, its small size fits into our small modern gardens, and it is just the tree for the street or the patio. So the motivation is there, but we have become discouraged by its response, first to the loose wind-blown sand that most of us have for our gardens; it seems to hate sand here. Or maybe it succumbs to the high water table that prevails in winter in many of our younger suburban areas. Even if we garden on a slope of granitic soils, and our tree takes off splendidly for a while, we are still faced with the unpredictable death of our treasure by phytophthora, perhaps just as it is becoming tall enough to make an effective statement in the garden. The frustration and disappointment are just too great to bear.

But in California, who knows? They are grown there, though not frequently and not always well. Maybe the soils are too alkaline, or too high in phosphorous. The unpredictable frosts do not help. The coastal fogs are not a problem, as silver trees grow excellently in the mist-belt of the Eastern Transvaal mountains, despite much summer rain and quite different soils. The only drawback of California’s fogs will be to reduce the brightness of the metallic glitter to a gray pallor until the leaves dry out.

Now I am starting to ask myself why I do not have another go at growing a silver tree in my own garden: my soil is right and I have a well-drained rockery. Maybe the time has come to try once more. Or, maybe I’ll try silver tree’s little brother, Leucadendron uliginosum, so silvery that walking through stands of this seven-foot-tall shrub on the mountain slopes of the southern Cape can be almost blinding. It may suit California gardens as well. It is tolerant of summer watering and of being sown in summer, grows in mineral-deficient sandy soil, and, because it often experiences episodes of snow in winter, might tolerate California with more grace and dependability.


Silver Trees in California

Silver trees were first grown in California in the mid- to late 1880s. They have been a popular feature of collections at such public gardens as The Arboretum of Los Angeles County, Huntington Botanical Gardens in San Marino, UC Santa Cruz Arboretum, UC Botanical Gardens in Berkeley, and Strybing Arboretum & Botanical Gardens in San Francisco. Other trees have been grown in the private gardens of plant enthusiasts throughout the milder coastal regions of the state.

In the 1930s, Elizabeth and Lockwood de Forest commented on a number of silver trees in the Santa Barbara area, bemoaning the fact that more were not planted; they believed the region offered the best opportunity for their cultivation outside of Table Mountain. Today a number of silver trees noted by the de Forests remain on a private estate, part of the old Hope Ranch on the coast near Goleta.

Eric Walther, the first director of Strybing Arboretum, planted several silver trees in the gardens sometime prior to his retirement in 1957. The sandy soils provided superb drainage for the trees, and allowed them to thrive even with fairly regular summer irrigation. One tree survived until 1998, when it succumbed to old age at more than forty years of age, among the oldest silver trees in the state.

Silver tree was selected as one of the signature plants for Strybing’s new South African garden in the mid-1980s; approximately two dozen young saplings were planted. The trees grew quickly until the big freeze of December 1990, when temperatures dropped several degrees below freezing, killing most of them. This same cold period killed established trees at UC Botanical Garden, where temperatures fell to the mid-teens, and seriously reduced the nursery availability of replacement trees.

The Arboretum at UC Santa Cruz has included silver trees in its South African collection, which is dominated by members of the protea family. The staff plants a few trees each year, but finds that the heavier soils, gophers, and phythophthera tend to lower the survival rate during the first few years.

Full sun, well-drained, somewhat acidic soils, occasional summer irrigation, and an open site free of frost seem to be the primary requirements for the success of silver trees in California.

RGT


A Silver Tree Resource Guide

Sources of Seed:

Silverhill Seeds
PO Box 53108, Kenilworth, 7745 South Africa
www.silverhillseeds.co.za

Sources of Plants:

Australian Native Plants Nursery (wholesale and retail)
Casitas Springs (between Ventura and Ojai), CA
805/649-3362

Sierra Azul Nursery (retail)
2660 East Lake Avenue, Watsonville, CA 95076
831/763-0939
www.gardenliving.com/sierraazul.html

San Marcos Growers (wholesale)
PO Box 6827, Santa Barbara, CA 93160
805/683-1561
www.smgrowers.com

Check also at annual plant sales of UC Santa Cruz Arboretum, Strybing Arboretum, and other public gardens.