Trees of Hoyt Arboretum: Tanoak

By: Sean Hogan Madeleine Keeve
Sean with camera in Mexico 2006

Cistus Design & Nursery

http://www.cistus.com

Sean Hogan has been involved in horticulture since about 18 months of age and seriously since the age of three….

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To David Douglas goes the honor of having discovered this magnificent tree. On an exploring trip into central California he encountered the tan oak, and sent dried specimens back to England. It was Professor Sargent of Harvard University who introduced tan oak to the garden; plants of his introduction of 1874 are still in the oak collection at the Royal Botanical Gardens, Kew, England.

Arthur R Kruckerberg, Gardening with Native Plants of the Pacific Northwest

Tanoak (Lithocarpus densiflorus) in Hoyt Arboretum, Portland. Photograph by Sean Hogan

Tanoak (Lithocarpus densiflorus) in Hoyt Arboretum, Portland. Photograph by Sean Hogan

The five-by-seven accession cards are yellowed with age, each entitled “Portland Municipal Arboretum” and inscribed with fountain pen and a careful hand. Many years before computerized databases, arborists kept the planting records on these cards at the Hoyt Arboretum in the west hills of the city. The record-keepers and their fountain pens left us long ago, but the hundreds of old cards remain protected in the arboretum archives. More importantly, many of the trees they describe—dawn redwoods, monkey puzzle trees, Brewer’s weeping spruce—are still here. Among them, one can still find the tanoaks (Lithocarpus densiflorus), planted on a dry slope between 1933 and 1968.  These trees are possibly some of the finest representatives of the species in any public garden in the country.

Lithocarpus densiflorus, known both as tanoak and tanbark oak, is the only native species of a genus otherwise found in Southeast Asia and Indo-Malaysia, where 200–250 species exist. A broad-leaved evergreen forest tree, fifty to one hundred feet tall and a member of the oak family (Fagaceae), it closely resembles other evergreen oak relatives such as bush chinquapin (Chrysolepis sempervirens) and canyon live oak (Quercus chrysolepis). Its range covers Douglas County in southwestern Oregon south through the coastal mountains of central California, and east across the valleys to the Cascades and the Sierra National Forest south of Yosemite Valley in California, in habitats ranging from sea level to over 6000 feet.  It reaches its climax forest in southwestern Oregon and northwestern California, where it often forms pure stands. The tallest tanoak recorded is on the North Fork of the Little Sur River in Monterey County, at 208 feet (63.4 meters) tall and fifty-four inches (1.37 meters) diameter, measured at breast height.

Tolerant of temperatures as low as 0° F (-18° C) and as high as 100° F (45° C), tanoaks grow in a wide range of soils—from deep and loamy to shallow and stony—but require good drainage and little or no summer rain or humidity. In the hottest inland valleys, they favor a north-facing slope or an underplanting of groundcover to provide a somewhat cooler root run.

The shiny, blue green, leathery leaves, two to five inches (five to thirteen centimeters) long, have entire or slightly serrated edges and blue silver undersides. Unlike the dangling staminate catkins on members of the genus Quercus, Lithocarpus bear upright clusters of catkins. The genus name comes from the Greek words lithos (stone) and karpos (fruit), referring to the hard-shelled, acorn-like nuts.

In open, sunny areas, tanoaks may develop broad, rounded crowns and large, horizontal branches. The broadest specimen at Hoyt Arboretum, planted on a north-facing slope in 1946, embodies this habit perfectly, with elephantine limbs that sweep the ground; roughly thirty feet (7.62 meters) tall and twenty-five inches (63.5 centimeters) in diameter at breast height, it creates a thickly shaded dome under which one can stand and marvel at the furrowed gray beauty of its branches.

In closed stands, however, tanoaks (not surprisingly) compete for light by developing a tall central leader, narrow crown, and ascending branches. Planted among other evergreen oaks and their relatives (Quercus suber, Q. ilex, Q. chrysolepis, and others), all other mature tanoak specimens at the arboretum display this slender habit, the tallest exceeding fifty feet. Were the glossy evergreen leaves not obvious, they might be mistaken for their neighboring spire-shaped Douglas firs at a distance.

Foliage and developing catkins of tanoak (Lithocarpus densiflorus). Photograph by RGT

Foliage and developing catkins of tanoak (Lithocarpus densiflorus). Photograph by RGT

In cultivation, what would associate best with a tanoak? The obvious answer would be those plants from its various native habitats. Compatible trees include Pacific madrone (Arbutus menziesii), Oregon bay (Umbellularia californica), and ponderosa pine (Pinus ponderosa var. ponderosa). Good associate shrubs include blueblossom (Ceanothus thyrsiflorus), coast silk tassel (Garrya elliptica), salal (Gaultheria shallon), Pacific bayberry (Myrica californica), flowering current (Ribes sanguineum), thimbleberry (Rubus parviflorus) and evergreen huckleberry (Vaccinium ovatum). Western swordfern (Polystichum munitum) and many Oregon grapes (Mahonia spp.) appear to thrive under the dry shade of tanoaks. Given the promise of summer water restrictions throughout the West, a landscape featuring these handsome drought-tolerant plants might be a key component in the next generation of urban street plantings.

To show the public the great garden potential of Northwest native plants, an increasingly diverse collection of native cultivars has been planted around the entrance of the Arboretum Visitor Center. Selections of tanoak are being tried for variation in leaf form, habit, and size. Lithocarpus densiflorus var. echinoides, native to the Siskiyou country of southwestern Oregon and northwestern California, is a wonderful and variable, smaller shrubby form of tanoak that shows great horticultural promise. It ranges in size from under eighteen inches (forty-five centimeters) to over nine feet (2.75 meters) tall. The leaves are often proportionately smaller, and some possess a chalky blue cast; others have a golden coat of indumentum (fine matted hairs) on the undersides, similar to that of golden chinquapin (Chrysolepis chrysophylla). The selections planted at the Arboretum have been collected from geographically distinct areas in the Siskiyou region; like siblings in a large, attractive family, each young specimen displays its own particular handsomeness. Another choice form of tanoak, but hardly recognizable as one, was discovered in 1962 in Yuba County, California; it has narrow, deeply dentate leaves, and has been named L. densiflorus forma attenuato-dentatus.  Relatively fast-growing, it has an open, pyramidal habit. Cutting-grown plants of this lacy-leafed shrub are becoming increasingly available.


While the news of problematic dieback of tanoaks, coast live oak, and black oaks in areas of California is alarming, there is no known report of its spread to southern Oregon—yet. Pathologists believe that the dieback is caused by a form of phytophthora. A method for monitoring the spread and an understanding of its spread is still needed.