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The Return of the ELM: The Status of Elms in the Industry in 2000

Keith Warren, J. Frank Schmidt and Son Co.

A paper from the Proceedings of the 11^th Metropolitan Tree Improvement Alliance (METRIA) Conference held in Gresham, Oregon, August 23-24, 2000, and cosponsored by the Landscape Plant Development Center.

Abstract

Elm trees were a major nursery product in the early 20th century. Dutch Elm Disease (DED) virtually halted production. Recently, new cultivars and hybrids have been introduced with high levels of tolerance to DED, and elms are again important nursery products. However, nursery production and customer expectations have changed since the early days of elm plantings, and DED is not the only problem facing landscape elms. Present propagation consists of vegetative production by cuttings or micropropagation, or by bud grafting on DED resistant rootstock. J. Frank Schmidt & Son nursery is now producing 19 elm cultivars, representing selections and hybrids of seven parent species. The features of each cultivar are discussed in this paper.

It is clear that today there is a revival in interest in the elms. The American elm was the dominant street tree in the U.S. in the early part of 20th century, only to be decimated by Dutch elm disease. Following this, nursery production and urban planting of elm trees virtually ceased. Today, a large number of trees have been found or bred that are highly tolerant of the disease, and the genus Ulmus is making a comeback in the green industry. Twelve years ago, we listed our first two elm cultivars in our catalog. Today, we have 19 cultivars in some stage of production, and we are evaluating at least a dozen more. This comeback has been largely fueled by the work of three determined plant breeders (Denny Townsend, Gene Smalley, and George Ware) and has been rapidly pushed to market by the needs of urban foresters.

J. Frank Schmidt and Son Co. is playing a leading role in the comeback of elms, and is making available the best of the new cultivar introductions.  As the breeding and improvement of elms is presently an area of great activity, it is anticipated that the "top choices" among cultivars will be in a state of frequent change for a number of years. Here, I will summarize the status of cultivars in production or intended for production at Schmidt Nursery, and make some predictions for their future.

The elms of present horticultural importance in the United States belong to four main groups:

• Chinese elm (U. parvifolia)
• Asian elms other than U. parvifolia
• European and European-Asian Hybrids
• American elm (U. americana)

While the object of most elm breeding has been to find a replacement for the American elm, the true Chinese elm, Ulmus parvifolia, has generated interest in its own right.  This unique and highly variable species ranges from evergreen to deciduous in its native range.  Often called "lacebark elm," its bark characteristics vary from a beautiful exfoliating mosaic of tan, orange, green and gray to a dark bark that is of no special interest. Tree size, leaf size, and growth rate are all about half that of the American elm, so it is used differently in the landscape. It fills the role of a small to medium sized tree with ornamental characteristics, and is as often used as a single specimen as it is as a street tree. It is quite heat resistant, and was first appreciated in the south, where evergreen and semi-evergreen selections with ornamental bark were developed and widely planted. The first northern selections of note were 'Dynasty' and 'King's Choice'. These are fully deciduous with good hardiness, but lack ornamental bark. 'King's Choice' has experienced some problems with branch breakage due to its fast growth. 'Allee' and 'Athena' appear to combine hardiness with good ornamental bark characteristics, and seem to be the most desirable releases to date.

Following the Dutch elm epidemic in Europe, breeding toward disease resistance began in Holland using European species.  Moderate levels of disease resistance were found and cultivars were developed, but none had the habit or environmental tolerance of the American elm, so they never became popular here. U. S. tree breeders Denny Townsend and Gene Smalley used the best of the resistant Dutch germplasm in their own programs and also hybridized these with tough Asian species that exhibit high levels of resistance to Dutch Elm Disease (DED). The result were the first hybrid elms to gain significant street tree use in the U.S. as American elm replacements. 'Regal', 'Pioneer', and 'Homestead' are the most important of these.

George Ware has now released five cultivars of mostly Asian heritage, drawing largely on Ulmus wilsoniana and Ulmus japonica. These tough, hardy parent species come close to duplicating the American elm in form. A hybrid of these parent species, 'Accolade' looks like it will be outstanding, giving us the first real "look-alike" of the American elm, and it has excellent horticultural and nursery characteristics. 'Accolade' should have a very bright future in the nursery trade, and I anticipate strong demand. Promising cultivar releases of non-hybrid U. wilsoniana and U. japonica have been made by others and are now becoming available as well. These parent species impart the added advantage of resistance to the feeding of the elm leaf beetle.

With the release of Ulmus americana 'Valley Forge' by the National Arboretum, American elms are once again in the news and in demand. While a number of U. americana cultivars with a moderate level of disease tolerance have been introduced over the years, these earlier cultivars have not developed the trust of the green industry and have been little used. Denny Townsend has now done the research to quantify the level of disease tolerance of elm clones, and introduced the best U. americana selection as 'Valley Forge', and the runner-up as 'New Harmony'. He also found that the older cultivar 'Princeton' has a good level of disease tolerance, as well as outstanding horticultural characteristics. In addition, the National Park Service has found two apparent American elm hybrids of unknown origin with triploid chromosome levels. These two, 'Jefferson' and 'Washington' show promise as well, but are not widely tested outside the Washington, D.C. area. Unfortunately, these American elm selections are difficult to propagate, so they have been slow to become available. They must be propagated from softwood cuttings; if they were grafted onto American elm seedlings, they would be susceptible to DED infection through the roots.

No tree is trouble free, and elms are no exception. The death sentence from Dutch Elm Disease has been eliminated for these modern cultivars. However, they are susceptible to other troubles. Fungal cankers, leaf spot, insect problems, and storm breakage are still potential problems. Elm cultivars of European and Asian parentage are resistant to elm yellows, but DED tolerant American elms are still susceptible to this disease. However, every tree species faces risks, and in comparison to other genera, elms can be expected to be tough and reliable urban performers.

Elm Production

We use a number of propagation techniques for the production of our elm cultivars: softwood cutting, tissue culture, and budding onto seedling rootstock. There are advantages and disadvantages to each of these techniques, depending on cultivar. In some cases, one technique will be used for rapid build up of a new release, then we may change to a more economical technique at a later date.

Summer softwood cuttings are widely used on elm cultivars. Cuttings are taken in late June and July, about 6" long, treated with an IBA hormone quick dip, then stuck into unheated bark beds in poly tunnels under intermittent mist. Rooting occurs in 3 to 4 weeks, and most cuttings put out a flush of new growth by the end of the summer. In late winter, these rooted cuttings are carefully dug from the beds and put into cold storage. In the spring, when the soil dries, these cuttings are planted outside in transplant beds. Here, they are grown for one season to put on size and acclimatize to outside conditions. In the winter, they are dug, graded, then held in cold storage preceding spring field planting.

Tissue culture allows for rapid build up of new releases. We do not have our own lab, but instead buy plantlets from several commercial suppliers. Tissue culture is a viable option for elms, but has produced some unique challenges. Plants in tissue culture often sustain minor and temporary morphological changes. I have observed this across a broad range of species. For lack of a better term, I call this "juvenile reversion," because the plants tend to take on many of the characteristics of young seedlings. In Ulmus parvifolia, this phase change is particularly striking and noticeable. Tissue culture grown plants are bushier, with more fine branching and more numerous but smaller leaves. As they grow older, they eventually change to the adult morphology and have all expected characteristics.

Tissue cultured plants are normally brought in very early in the spring. They are rooted or established in fog tents inside our shaded greenhouse. All stress must be avoided for the first two weeks. Gradually, they are weaned away from fog to a normal greenhouse environment. Then, during cool cloudy weather, we move them outside to transplant beds under immediate sprinkler irrigation. Here, they are grown for the summer to gain size.

Budded (or bud grafted) trees are produced by first raising a seedling tree in an outdoor seedbed, then transplanting it into a nursery row. There, during the month of August, a single bud is cut from the desired cultivar and grafted onto the seedling rootstock using the "chip bud" technique. This bud is tightly wrapped with plastic for four weeks until it forms a union with rootstock. The bud remains dormant for the winter, then the seedling trunk is cut away above this bud, and the entire energy of the root system is forced into the growth of the cultivar bud.

Tissue culture and cutting produced trees are handled the same way once they are planted out into the nursery row. They are allowed to sprawl and simply establish their root system for the first year. The following season, all growth is cut away to within 2 inches of the ground. All of the plant's growth is then forced into a single shoot, which grows rapidly into a tree.

Regardless of propagation technique, all trees are handled the same from this point. Trees are staked with steel rod and tied regularly with a vinyl tie to develop are very straight trunk. Lower branches are pruned away and tip growth is taped as necessary to ensure a straight central leader. As the well established root system has a three year age advantage over the top, growth is extremely rapid, and six to ten feet of growth is typical for various elm cultivars.

Trees may be sold at this size, or grown on to a larger size. If kept in the field, stakes are removed, and the tree is heavily dormant pruned to develop its initial branch structure. Masking tape is used to reestablish a strong central leader after pruning. Vigorous but limber cultivars such as 'Prospector' are staked for the second year with bamboo.

Dormant, bare root trees are dug during the winter months, heeled in to sawdust holding beds, then shipped all over the country in the early spring by refrigerated semi truck.

Elm Cultivars

The following are the 19 cultivars, in production or on the way from J. Frank Schmidt and Son Co. Many of these are brand new and are in a stock increase phase, and some may not be available for several years. As with many new introductions, ultimate size and zone information is my best estimate based on existing knowledge.

Lacebark Elm Cultivars - Ulmus parvifolia

European and Eurasian Hybrid Cultivars

Asian Cultivars and Hybrids

American Elm Cultivars - Ulmus americana

Parentage of Elm Cultivars of Importance to the American Nursery Trade

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