Some species more disease-resistant than others
By Rachel Foster
So many dogwoods, so little space. There are about 45 species of cornus, the botanical name for dogwoods. Almost all of them are shrubs or trees, ranging in height from a few inches to 75 feet. Many are native to North America; both the smallest and the tallest are native to Pacific Northwest forests. Bunchberry (Cornus canadensis) keeps its woody parts below ground, while Pacific dogwood (Cornus nuttallii) grows into a soaring tree. Both of them have large white bracts that look like petals. The true flowers are small, and bunched together in a central disc between the bracts. A number of other dogwood species share this feature. Many other dogwoods are well worth growing, but deciduous species with showy bracts are the subject of this column.
Eastern, or flowering, dogwood (Cornus florida) is a slow-growing tree that usually grows to 20-25 feet, forming a rounded, low-branching crown. Gardeners love this tree for its modest size, long spring display and a beguiling twiggy, layered look. Bracts are normally white, but cultivars with pink or reddish bracts are very popular. This should be the perfect tree for small gardens, and sometimes it is. Unfortunately it is prone to anthracnose, a serious fungal disease that demands regular applications of chemicals for effective control. Adequate water, feeding and good air circulation may help protect it against anthracnose.
The same disease afflicts our Pacific dogwood unless it grows in perfect conditions. It is a taller, narrower tree than Eastern dogwood and less densely twiggy and floriferous. But the bracts are larger, and the tree is very beautiful in bloom. The vigorous selection ‘Colrigo Giant’ has 6-8 inch flower heads. Pacific dogwood frequently reblooms in late summer and has red fall color. For some reason, it is more common in and near Portland than in the southern Willamette Valley. We see it at its best at higher elevations, as along the McKenzie highway.
Unlike the Eastern dogwood, Pacific dogwood dislikes conventional garden conditions with summer irrigation, fertilizer and pruning, and the bark is easily damaged. It prefers very good drainage and minimal summer watering and should certainly be tried in native gardens. Since it can be difficult to transplant successfully, many professionals suggest establishing very young plants. If at all possible, choose a location for either dogwood that is not too hot but has excellent air circulation. Both are understory trees in nature, but being crowded in by larger trees exacerbates disease.
Cornus ‘Eddie’s White Wonder,’ a hybrid between Eastern and Pacific dogwoods, is intermediate in overall shape, has large bracts and blooms more or less with C. nuttallii. It is better adapted to garden conditions than Pacific dogwood, and is said to be somewhat resistant to anthracnose. For dependable disease resistance, though, gardeners turn to Chinese dogwood (Cornus kousa). This one blooms in early summer after the leaves have expanded and therefore lacks the particular beauty of trees that flower on leafless branches, but Chinese dogwood is lovely in its own way and has attractive, clean looking foliage. It naturally makes a big, multistemmed shrub but can be trained to a single trunk.
Although not entirely immune to anthracnose, Chinese dogwood is much more resistant than American “flowering” dogwoods. This resistance apparently carries through in hybrids between Cornus kousa and C. florida. ‘Stella Aurora’ (white ‘flowers’) and ‘Stellar Pink’ are the result of a breeding program at Rutgers University. They grow with a single trunk to 20 feet tall and wide. Bloom is heavy and intermediate in time between the parents. Fall color is said to be brilliant, as is that of C. kousa.
Cornus ‘Starlight’ is a new Rutgers cross between C. kousa and C. nuttallii. I look forward to seeing this one.
If you don’t have room for any of these trees, there is always tiny, creeping bunchberry (Cornus canadensis). Give it a cool, spongy soil in light shade, and it makes the world’s most elegant ground cover.
Last year’s tour was such a success they are doing it again. Coming up from noon to 5 pm Sunday, May 9, is the second Mother’s Day Native Plant Garden Tour in Eugene. The tour will feature a variety of Willamette Valley native plants and habitat types in several private gardens. Download garden descriptions with plant lists and a map from the Emerald Chapter website, www.emerald.npsoregon.org