Michael Dirr at the National Arboretum

In 2007, Dirr started a company, Plant Introductions, Inc., which develops breeding programs to address consumer needs and improve the performance of ornamental garden plants. Dirr explained that, at the company’s onset, he did not have the marketing expertise to make the project financially viable so he collaborated with Bailey Nurseries in Minnesota, a wholesale grower, to bring his products to market. PII is now a subsidiary of Bailey and serves as their in-house breeding division. Dirr was exploring the possibility of a further collaboration with the Arboretum.

Dirr pointed out that the shrubs people purchase the most frequently are boxwood, viburnum, and hydrangeas, so much of his work is focused on those. He discussed his efforts to develop new cultivars of distylium, “…the most beautiful plant you never heard of.” Distylium are underused in American gardens but are an extremely attractive alternative to plants such a cherry laurel, junipers, hollies, and boxwood, which are subject to ruinous diseases, such as boxwood blight. Members of the witch hazel family, Hamamelis, dystylium are compact, evergreen, disease- and insect-resistant shrubs for full sun to partial shade. They do well in zones 6b to 9 and are tolerant of heat and drought as well as wet soil. They have small, reddish maroon flowers in late winter, adding interest to add late winter garden. Among Dirr’s favorites are Vintage Jade, Blue Cascade, Cinnamon Girl, and Emerald Heights. You can learn more about this plant and see pictures on the website of Plant Introductions, Inc.

Dirr also discussed his extensive work with hydrangeas. Macrophylla is still the most familiar type but paniculata is gaining many fans, Dirr said. Having been involved with the development of the extremely popular Endless Summer series — nurseries sold 18 million plants last year — Dirr has continued to breed them for increased bloom time, larger flowers, and better cold resistance. Two spectacular varieties of Endless Summer are “Twist and Shout” and “Bloomstruck”, both of which bloom on new wood as well as old, producing flowers all summer long on sturdy stems which can support the extraordinary large and numerous blossoms. They are also more drought resistant so the leaves do not wilt as readily in the heat. Perhaps Dirr’s most interesting project, however, is his effort to develop a purple leaf hydrangea which he is almost ready to bring to market.

Richard Olsen, the new director of the Arboretum, spoke briefly to the audience, as well. He pointed out that the landscape industry generates more than 120 billion dollars in business each year, which is 1% of the U.S. economy. The government supports this important industry by funding the research and development of ornamental plants at the Arboretum through its Agricultural Research Service, which also funds the development of improved agricultural varieties at sites across the country. Asked about the role of native plants in light of extensive hybridizing efforts, both Dirr and Olsen agreed that they continue to play and important role in the industry. Native plants provide all the genetic material for hybridized plants that breeders develop from them and, most important, display the characteristics that breeders want to retain in new cultivars, chiefly, disease resistance and hardiness.