The Untimely Demise of the Lawson Cypress

Some botanists have placed Chamaecyparis lawsoniana within the genus Cupressus, relegating the name Chamaecyparis to the rank of subgenus. For the horticulturist, however, its flat sprays of foliage and smaller cones (with fewer seeds per scale) are sufficient to distinguish Lawson cypress from those trees in the genus Cupressus, and we have chosen to retain the use of Chamaecyparis here.

Chamaecyparis lawsoniana is one of the most important conifers in ornamental horticulture. Lawson cypress or Port Orford cedar, as it is known in its native range and by foresters, is restricted in nature to a narrow strip of the Oregon-California coast and to the mountains of northern California. It was once an important timber tree for the region. Old growth stands were scattered in small groves, the trees potentially growing for 500 years and reaching a height of around 220 feet (70 meters). Its wood is light, hard, strong, and easily worked; the high quality lumber was once used extensively in shipbuilding and for the interior finish and flooring of buildings. Those days are over, not just because of changing tastes and the introduction of modern materials, but also because these cypress are dying in the forests.

Seeds of Port Orford cedar found their way to Peter Lawson’s Nursery in Edinburgh, Scotland, in 1854. Interesting foliage mutations were common among both the early seedlings there and their subsequent progeny elsewhere, despite the fact that, in habitat, trees are fairly uniform. Like other trees in the cypress family, Chamaecyparis lawsoniana is easy to propagate vegetatively—by simple cuttings or grafting onto seedling rootstocks. Besides a propensity for producing spontaneous branch-sports of various colors, textures, and forms, the species exhibits distinct seedling and adult growth stages. Juvenile foliage is mostly upright, but usually congested and prickly, while the adult foliage, made up of softer overlapping scale-like leaves, is borne on spreading branches. Some variants remain completely juvenile, and some get hung up halfway, showing both juvenile and intermediate leaves and branching. Given such inherent mutability, it is not surprising that there are some 200 registered cultivars. Several of the early selections still common in the Pacific Northwest include the steely blue ‘Alumii’ (1891) and the upright but softly weeping ‘Intertexta’ (1869). The golden columns of ‘Lutea’ (before 1872) and the grass green ‘Erecta’ (1870), with its tight, upswept growth and massive top-shaped physique, are unmistakable.

A Popular Conifer

As an ornamental, Lawson cypress is grown widely in New Zealand, western Europe, and Great Britain, where it is also planted as a timber species. In North America, there are ornamental plantings in the cities and towns of coastal Oregon and California and, to a lesser extent, in the Atlantic and southern states, but winters there are mostly too dry and summers overly hot and humid for good growth. The greatest concentration of cultivated Lawson cypress is in the maritime Pacific Northwest, in and around Seattle, Victoria, and Vancouver. Here, Lawsons were used during the housing boom following World War II, most planted as hedges or in the “buns-and-rockets” style of foundation planting common to residential front yards of the fifties and sixties. Some of these, notably the stiffly compact ‘Ellwoodii’ and the similar, but larger and greener ‘Fletcheri’ are comprised of only prickly juvenile or semi-juvenile shoots, their multiple upright branches forming distinctive, jagged crowns. Frequently broken apart by wet snow, such cultivars are often trussed up with baling wire or rope by well-meaning homeowners. These then develop the battered barrel look so familiar to Northwesterners. The truly shrubby cultivars often start life as dwarf conifers, or so they are listed in nursery catalogs. Few stay small, however, and with age, fewer still merit any distinction as garden plants, being no more interesting than any of the legions of formless Thuja occidentalis cultivars. One exception is the beautiful tamarisk cypress, C. lawsoniana ‘Tamariscifolia’, with its elegantly jumbled layers of sea green, fan-shaped foliage and spreading habit. This cultivar, introduced in the 1920s, is a favorite park subject in Vancouver.

The majority of seedlings and cultivars outgrow the shrubby character of youth and develop into dense, columnar or narrowly pyramidal trees, clothed to the ground in loose, overlapping sprays of gray, green, gold, or blue. Most Lawson cypress are of a size sympathetic to post-war architecture, which is fortunate, as they are exceedingly common. So much so, that neighborhoods throughout the Pacific Northwest are defined by them—in particular, by the cultivars ‘Alumii’ and ‘Erecta’, the regrettable ‘Fletcheri’, and the golden duo, ‘Stewartii’ (canary yellow overlying bright green) and ‘Lutea’ (narrower, with more burnished golden foliage). Area nurseries carried popular Lawson cypress cultivars until the 1970s, when it became obvious that something was killing them off.

Cypress Root Rot

The culprit was a soil-borne disease called cypress root rot caused by a species of Phytophthora, a genus of common fungus-like organisms that are part of a larger group of plant pathogens known as water molds. The lamentably familiar and widespread Phytophthora cinnamomi causes root rot and wilt of rhododendrons, boxwood, and yew and, in nurseries, root rot of a number of other plants. Research on sudden oak death syndrome, a disease wreaking havoc among Californian oaks, their relatives, and an alarmingly wide variety of other plants, implicates a poorly known Phytophthora species, P. ramorum. Another, P. infestans (late blight of potato), precipitated the infamous Irish potato famine, while P. cactorum causes various rots and cankers in plants as diverse as dogwood, maple, and pocketbook flower (Calceolaria). The biggest troublemaker for Lawson cypress, and the pathogen that is invariably fatal to it, is P. lateralis.

Cypress root rot was first recorded on planting stock of Lawson cypress in Seattle in 1923. The pathogen was identified and named in 1943 and by 1952, Phytophthora lateralis was found in natural forest stands, where it spread rapidly. That the disease found such a defenseless host suggests that the pathogen evolved well away from the Pacific Northwest, undoubtedly on Chamaecyparis, but on species that were able to match virulence with resistance at each evolutionary turn. Phytophthora lateralis probably arrived on the Seattle waterfront in infected wood products from Asia.

Similar scenarios have played out before. American chestnut blight, caused by the fungus Endothia parasitica, has its origins in oriental chestnut wood, for example. Because Asian chestnuts evolved alongside Endothia, natural selection gave them the means to develop a degree of tolerance to the pathogen. The American chestnut (Castanea dentata), having no such continuous exposure, was without immunity when infected lumber arrived early in the last century to ports along the east coast of North America. Chestnut blight tore through the eastern forests at an incredible rate, spread by birds and insects, windstorms and rain. Similarly, Dutch elm disease (Ceratocystis ulmi) was introduced to North America on elm logs in 1930 and spread catastrophically throughout the range of American elm (Ulmus americanus), destroying as well, the huge numbers of planted European species in cities and towns throughout the eastern states and provinces. The Dutch, having been the first to record this disease, are saddled with its common name, but its origin is likely Asia, as a number of elm species there are unaffected by the disease.

As pathogens go, water molds are at least equal to chestnut blight or elm disease in their destructive capacity. They are supremely well adapted to the mild climate of the Pacific Northwest, infecting weakened plants in wet conditions through damaged tissues; typical points of entry are the broken roots of recent transplants or root tips killed by summer drought or winter waterlogging. Under certain conditions, phytophthora also infect perfectly healthy roots, as in the case of Lawson cypress and Phytophthora lateralis. Propelled by a pair of whiplash tails, phytophthora spores swim through water from infected to uninfected plants. Poor drainage or a high water table are good indicators of phytophthora-prone soils, and areas down-slope of infected plants are at highest risk.

Symptoms and Controls

Wilt is one of the most obvious symptoms of phytophthora infection, as the pathogen kills by preventing roots from taking up water and by clogging the vascular system. The first symptom in drought-tolerant plants such as Lawson cypress, however, is usually a subtle change in color. Whereas healthy conifers generally exhibit foliage that is richly colored and robust, infected plants quickly become dull, their color fading until it is gradually replaced by the reddish brown of dead tissue. Neighboring trees quickly succumb, through either new water-borne infections or by movement of the disease through natural root grafts. Depending on the weather and severity of infection, death generally follows in two or three years. Phytophthora, once established, also produce thick-walled spores able to survive for years in infected wood, the roots of dead plants, and soil. Resistant spores are easily transported on tools and equipment, or even in the treads of car tires or hiking boots.

Options for controlling cypress root rot are limited; we have distributed the spores of Phytophthora lateralis far too widely to effectively keep a lid on its spread. Even in the Lawson’s native habitat, Forest Service personnel restrict the movement of vehicles and equipment to prevent the spread of the disease. Closed to logging, these areas are now under serious threat from off-road vehicles and mountain bikes. As for chemical controls, few can kill the pathogen outright, and no synthetic chemical has been shown to work once the pathogen has entered the host. Controlling groundwater movement and surface runoff does limit infection by reducing the advance of spores, but this is seldom an option in the landscape. Because Chamaecyparis lawsoniana is adapted to the West Coast’s virtually rain-free climate from about April to November, summer watering is not only unnecessary, it may actually weaken the trees during hot weather dormancy. Stress on trees from heavy foot traffic, crowding, or excessive irrigation and fertilizer applications can be prevented; doing so may play a role in disease resistance, but these measures merely postpone the inevitable.

Some success in the battle against cypress root rot has been achieved with the application of biological agents—microbes specifically antagonistic to water molds, for example—but this is a costly and complicated procedure and entails repeated injections into the wood of infected plants. A more promising approach is the application of mycorrhizae to the soil around trees. Mycorrhizae are fungi that form mutualistic associations with trees and other plants (many common forest and woodland mushrooms are mycorrhizal species). Typically, these fungi infect a tree’s roots and form a mantle of thread-like conducting tissues into the surrounding soil. From the tree, the fungus extracts sugars—the products of photosynthesis—while the fungus provides an extensive and efficient water and nutrient collecting network for the tree. But beyond even those significant advantages, it appears that mycorrhizae also impart some degree of disease resistance to their hosts. Staff at the University of Washington report encouraging results on a mature grove of infected Lawson cypress at the Washington Park Arboretum. Two years following treatment with a commercial mycorrhizal preparation, treated trees are all showing renewed vigor and improved color. Perhaps, if applied before an infection has progressed too far through the tree, mycorrhizae may be part of an effective control strategy. One wonders, though, why the native mycorrhizal species haven not saved the natural stands in Oregon and California.

A Sad Trend

Meanwhile, a disturbing patchwork of dead and declining Lawson cypress creeps across the landscape. A cure would not only be of significant benefit to those whose gardens are affected, but also a boon to the landscape industry. Nurseries are constantly searching for replacements. Texturally, Lawson cypress is distinctive. The majority exhibit cascading sprays of bold, aromatic foliage reminiscent of western red cedar (Thuja plicata) or Nootka cypress (Chamaecyparis nootkatensis), though the Lawson is always more densely furnished with foliage and never as massive or coarse as either. The blue green and gray tones of some Lawson cypress cultivars are occasionally matched with upright conifers such as the blue selections of Rocky Mountain juniper (Juniperus scopulorum), or Arizona cypress (Cupressus arizonica), but in the damp of the Pacific Northwest, these are never as attractive nor as long lived. The Japanese Sawara and Hinoki cypresses (Chamaecyparis pisifera and C. obtusa) come close to matching the diversity in Lawson cypress, but plants derived from these species are always stockier and their foliage more tufted. Chamaecyparis obtusa ‘Crippsii’ is sometimes used as a stand-in for the golden selections and C. pisifera ‘Boulevard’ is admittedly a good blue, but they remain stout and stodgy in comparison. Thuja plicata ‘Hogan’ is a fine replacement for the tall, narrow green Lawsons, and the pendulous forms of Nootka cypress are considered by some to be superior as weepers, but excessive weeping can be gloomy, and this is seldom a characteristic of Lawson cypress.

In the end, there are few substitutes of similar merit. The essential Lawson cypress (leaving aside the shrubby forms) presents its perfect, tapered crown in a hundred shades of green and blue and gold. The trees are fast enough to be used for screening, sufficiently shade tolerant for use in hedges, and narrow enough, even in age, to suit our urban lack of elbow-room. Lawson cypress is beautifully adapted to the climate of the Pacific Northwest, requiring no supplemental moisture from irrigation once established, shedding heavy, wet snow with aplomb, and looking splendid in the winter rain. They are glorious in their middle age in our gardens, and we’re losing them, all because of an accident of geography and evolution, and the soil in the tread of a tire or on the sole of a gardener’s shoe.