Resurrecting Lawson Cypress for the 21st Century

By: Tanya DeMarsh-Dodson

Tanya DeMarsh-Dodson has gardened since she could walk and has worked in the field of horticulture for more than twenty…

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A young plant of Chamaecyparis lawsoniana ‘Minima Glauca’ grafted unto a rootstock of moss sawara cypress (C. pisifera f. squarrosa); the enlarged base of the  topgrowth could eventually create problems for the grafted plant. Author’s photographs

A young plant of Chamaecyparis lawsoniana ‘Minima Glauca’ grafted unto a rootstock of moss sawara cypress (C. pisifera f. squarrosa); the enlarged base of the topgrowth could eventually create problems for the grafted plant. Author’s photographs

Anyone raised in the maritime Pacific Northwest, from Portland to Vancouver or Victoria, is familiar with Lawson cypress in residential gardens and public parks, as it is here that the greatest concentration of cultivated trees is found. Accounting for its popularity as a garden plant are its gracious habit, the range of blue, yellow, green and gray green tones in its leaves, the densely held cascading foliage typical of many forms, its tolerance of some degree of shade, and its suitability for this region’s dry summers. There are more than two hundred cultivars in cultivation worldwide.

Not long ago, here in Pacific Horticulture (October ’02), Douglas Justice lamented the demise of Lawson cypress or Port Orford cedar (Chamaecyparis lawsoniana), one of the more important conifers in ornamental horticulture here in the Pacific Northwest. Its nemesis has been a water-born mold, the fungus Phytophthora lateralis that attacks the roots of the cypress and kills the plant. Since its introduction in the 1920s, this fungus has spread so widely that it is now regarded as endemic in the Pacific Northwest. It now affects Lawson cypress not only in our gardens but also in their relatively remote native range (the mountains of Northern California and along a narrow strip of the Oregon-California coast). As Justice indicated, attempts to control the disease have met with limited success: chemical controls are ineffective, and controlling ground water and surface runoff to curtail movement of the fungal spores into unaffected areas is not possible in most landscapes. The experimental use of biological agents, both mycorrhizae and microbes antagonistic to water molds, has been somewhat effective, but is too complicated to be a practical treatment for use by most gardeners. It may be, however, that the nurserymen who propagate the plants have outwitted the disease by the choice of rootstocks onto which the trees are grafted.

Nurseryman Gordon Hallgren grew up in Everett, Washington, appreciating the beauty of Lawson cypress in his neighborhood and in the parks where he worked for a decade. Like many in the nursery business, Hallgren grew various selections of Lawson cypress for many years at Peacedale Nursery, for sale to retail nurseries in the Puget Sound Basin. He propagated the cultivars from rooted cuttings, growing them first in small containers and then planting them in the ground and growing them on until they reached a size appropriate for retail sales. In the 1980s, he found that the plants he sold did not always thrive in the gardens where they were planted, and, as the decade progressed, he began to lose more and more plants in production. The cuttings he made grew, but the plants failed when set out in the field. Because he valued their contribution to the landscape, Hallgren continued growing a few popular cultivars, ‘Wisselii’ and ‘Ellwoodii’ among them, into the 1990s. When plants of ‘Ellwoodii’ began to fail in his fields in the mid-1990s, he phased the Port Orford cedars out of production, convinced that Phytophthora lateralis was an inextricable presence in his soil.

A twenty-year-old specimen of Chamaecyparis lawsoniana “Minima Glauca’ grafted onto C. pisifera ‘Plumosa Vera’

A twenty-year-old specimen of Chamaecyparis lawsoniana “Minima Glauca’ grafted onto C. pisifera ‘Plumosa Vera’

A Rootstock for Grafting

For more than twenty years, Hallgren had enjoyed a twenty-five foot tall pair of Chamaecyparis lawsoniana ‘Wisselii’ growing in front of his home on the nursery grounds; he had used these plants as a source of cuttings for the ‘Wisselii’ he grew for sale. Because of its dark green foliage and its structured but whimsical form, ‘Wisselii’ remained one of Hallgren’s favorite cultivars. In 2000, he noticed branches on the two ‘Wisselii’ in front of his house had began to discolor, becoming a dull dark green. Realizing he was seeing the effects of Phytophthora lateralis attacking their roots, Hallgren tried to save the plants he loved by grafting scions from their more vital branches onto the rootstock of moss sawara cypress (Chamaecyparis pisifera f. squarrosa). The grafts took and the plants grew well; it seemed his strategy was successful.

After a year or so, however, Hallgren noticed that there had been some overgrowth on these grafted plants of ‘Wisselii’; that is, the scion (the top growth) was out-growing the rootstock. While this phenomenon did not affect the health of the young ‘Wisselii’, it was not particularly attractive. There was also the possibility that, as the plant grew and matured, it might become too heavy for the slow-growing rootstock, which could result in a break at the graft site in a heavy snow or in a strong wind. He then tried using the faster growing boulevard cypress (Chamaecyparis pisifera ‘Boulevard’) as a rootstock instead of moss sawara cypress; boulevard cypress produces a “beefier” trunk more rapidly. He has been watching all of these grafted plants grow for two and a half years now, and they are vigorous and healthy, both in containers and in the field. His observations have lead him to conclude that C. pisifera ‘Boulevard’ and C. pisifera f. squarrosa rootstocks are suitable for dwarf forms of Lawson cypress, such as ‘Minima Glauca’, ‘Nestoides’, or ‘Lutea Nana’, but he remains skeptical about the long-term physical compatibility of the faster growing selections on either of these rootstocks. Hallgren is convinced, however, that grafting will resolve the conundrum gardeners have faced with this wonderful group of garden plants.

Foliage and cones of Chamaecyparis pisifera ‘Plumosa Vera’

Foliage and cones of Chamaecyparis pisifera ‘Plumosa Vera’

Earlier Experiences

The work of two other nurserymen suggests that Hallgren has discovered an effective means of propagating viable Lawson cypress for garden use in the twenty-first century. Tony Van den Akker and Maurice Ravensberg  (both now deceased) emigrated from Holland to the United States after World War II to pursue their careers in horticulture. Both came from families that had been in the nursery business for more than a generation. After working for a few years for other growers in the Seattle area, both started their own businesses. Van den Akker’s Nursery soon became known for its conifers and its exceptional deciduous azalea hybrids (known as the Van den Akker hybrids). Ravensberg Landscaping created many award-winning residential and commercial landscapes in the Seattle area. Ravensberg continued to propagate plants for use in the landscapes he created.

Both of these nurserymen were knowledgeable grafters, having learned the art as part of their education in Europe. Both grew Lawson cypress and both used a selection of Chamaecyparis pisifera as the understock, though different from the one Gordon used. Their reasons for using C. pisifera were also different. They chose C. pisifera ‘Plumosa Vera’ (syn. ‘Plumosa Viridis’), which is similar in many respects to other plume sawara cypress; it will grow into a large tree with dense prickly juvenile foliage and scale-like softer adult foliage. Their concern was producing a plant that would be well suited for landscape uses; Phytophthera lateralis was not an issue when they began their grafting work. They carried the knowledge of C. pisifera as root-stock with them from the nursery center of Boskoop, where the Dutch had long studied and experimented with the process of grafting conifers. Collective evaluation recommended C. pisifera ‘Plumosa Vera’ as a suitable root-stock for C. lawsoniana. Ravensberg and Van den Akker valued the vigorous, fibrous root system of plume sawara cypress and the ease with which it handled transplanting. They were also aware that the growth rate of the rootstock was compatible with that of Lawson cypresses; they had no trouble with overgrowth using this particular cultivar as a rootstock.

The plantings of the Lawson cypress these men grafted in the 1950s, 1960s, and 1970s offer testament to the wisdom of Hallgren’s strategy for protecting this conifer by grafting the susceptible plants onto a rootstock resistant to Phytophthora lateralis. His choice of Chamaecyparis pisifera as a rootstock is one that is effective for the long run, provided care is given to match the growth rates of the rootstock and the scion. Several cultivars of Lawson cypress that Van den Akker and Ravensberg grafted still grow in commercial and residential landscapes in the Seattle area; some are now more than forty years old.

Continuing Research

Hallgren continues to search for other rootstocks that might protect the delightfully ornamental selections of Lawson cypress from phytophthora and grow a well-shaped, easily cared for cultivar. The Oregon Department of Agriculture has been experimenting with seedlings of Chamaecyparis lawsoniana in an attempt to find one resistant to Phytophthora lateralis. They sowed thousands of seeds and then inoculated the seedlings with the fungus. They have found a seedling variant that is, thus far, totally resistant. Hallgren hopes to obtain some plants of this resistant Lawson cypress next year, to serve as the understock for grafting some of the cultivars he favors. Grafting Lawson cypress onto itself carries little risk of any of the problems that can at times be associated with the grafting process. Thus, there may now be at least two rootstocks for nurserymen to consider when grafting to protect the most desirable cultivars of Lawson cypress. The search for the “best” rootstock may not be over, but, as long as interested, committed, and creative nurserymen, dedicated to growing the best plants for ornamental horticulture, are in the businessmen like Van den Akker, Ravensberg, and now Hallgren-gardeners need not fear that Lawson cypress will disappear from the landscape.

In a year or two, gardeners in the maritime Pacific Northwest will be able to appreciate the results of the work of these committed nurserymen. Great cultivars like ‘Blue Surprise’ ‘Minima Glauca’, ‘Penberry Blue’, ‘Wisselii’, ‘Tamariscifolia’, ‘Van Pelts’, and ‘Stewartii’ should be available in our local nurseries on protective rootstock. Before you buy, however, ask if the Lawson cypress you have chosen was grafted and on what rootstock.

Hallgren may also realize his wish to return Lawson cypress to the landscape of two great parks in Everett, Washington: Forest Park and Legion Park. In the 1980s, Lawson cypress cultivars such as ‘Lutea’, ‘Erecta’, and ‘Alumii’ were removed as they became diseased and were replaced with giant sequoia (Sequoiadendron giganteum). It would be wonderful to have these parks once again graced by the appealing form of Lawson cypress-elegant pyramidal trees densely clothed to the ground with gracefully cascading fans of foliage. Our Canadian neighbors in Vancouver and Victoria may also be able to reverse the decline of Lawson cypress in their landscape, choosing their cultivars with a discerning eye for this new century.