PACIFIC DOGWOOD (Western Flowering Dogwood)
Written by Moralea Milne and Jim McPherson
The Pacific dogwood is found in temperate coastal British Columbia and south to California with a disjunct population in Idaho
HABITAT AND LIFE HISTORY
Pacific dogwoods are found at low elevations in association with Douglas-fir, grand fir and western hemlock in southwestern BC, preferring sites with deep, coarse, well-drained, moderately dry to moist, nitrogen rich, acidic soils with high water holding capacities. Although they are moisture loving trees, they are also drought tolerant. They are adapted to shade but can grow well in full sun although the bark is susceptible to sun scalding. They are considered to have low frost tolerance and high flood tolerance.
Many birds feast on the fruit, bears and beavers eat the fruit and foliage and deer browse the twigs. Trees with many new sprouts (after a disturbance) are grazed heavily by mule deer and elk but black-tailed deer do not appear to favour it. Some shrews and voles eat the seeds and it is pollinated by many insects.
The Pacific dogwood is a small to medium sized deciduous tree (to 20 m) with a deep taproot.
Trunk: Trunk diameter is rarely more than 60 cm. The bark is thin, smooth and grey to blackish-brown, developing fine ridges with age.
BC’s Big Tree Registry records a specimen near Burgoyne Bay, Saltspring Island with a circumference of 3.05 m, a height of 26.75 m and spread of 16.45 m.
Leaves: Leaves are opposite, oval, pointed at the tip, and 4-10 cm long. The have prominent, curving, parallel leaf veins to the leaf edge. They are dark green above and grayish brown on the underside, turning a lovely red in the fall.
Flowers: What we think of as the spectacular flowers are actually 4-6 white, 2-7 cm modified leaf bracts surrounding a cluster of inconspicuous green flowers, flowering in April and May.
Some trees will occasionally repeat bloom in the autumn, producing trees with red leaves and fruit and large white “flowers”, this might be an adaptation to late summer water deficit.
Fruit: The fruit ripens into colourful red berries that adorn the trees in the fall.
Pacific dogwoods reproduce both vegetatively and by seed. Seed production varies from year to year and it is thought that seeds are dispersed by birds and animals. They readily re-sprout from the crown after a disturbance and layering has been observed in newly sprouting plants.
Seeds are cleaned by maceration and pulp and debris floated off (the fruit flesh is considered to contain a germination inhibitor). Seeds are dried on mesh trays. Can be stored at 1-2º. Seed should be cold stratified for 3-4 months and sown early in the spring or sown outdoors in the fall soon after harvest. Nicking the seed and allowing a 3 day period of warm (15º C) stratification before cold stratification may help germination. Some seeds may take up to 18 months to germinate although some sources say seeds will germinate in a few weeks.
Some websites claim that Pacific dogwood does not root well from cuttings or layering, others that it can be successful.
Take cuttings of new growth from the root crown in June/July or of half-ripe side shoots in July/August, cuttings of mature wood of current year’s growth, taken with a heel, if possible, in autumn in a cold frame and layering of new growth can be tried in June/July, takes 9 months.
Since the mid-1970’s, the Pacific dogwood has suffered from dieback and mortality. Dogwood anthracnose (dogwood leaf blotch) from the introduced fungus Discula destructiva, has been established as the culprit. It is thought that the fungus might have arrived with imported Japanese dogwoods (Cornus kousa).
There is a second Discula species found on Pacific dogwoods that is probably a native, less virulent pathogen. The disease readily attacks young seedlings and trees in the forest understory but all trees are susceptible. Most infection occurs during cool, wet weather. Stressors like drought and winter injury seem to increase the likelihood of infections. Some trees do appear to have a natural immunity. Infections begin with brown leaf spots which can coalesce into large splotches and shot holes. The infection can travel to the twigs, causing cankers and girdling, and eventually progress to the main truck, killing the tree.
Some sources recommend the following treatments: clean up fallen leaves, prune and destroy all infected parts, prune to open the canopy, spray with lime sulfur, copper and/or fungicides in spring at budbreak (usually 3 treatments at 2 week intervals).
Other sources warn against any unnecessary pruning. Do not over-fertilize, which will encourage succulent growth. Mulch trees to conserve moisture. Don’t use overhead watering devices. Consult with a professional before treatment.
Pacific dogwood has been British Columbia’s floral emblem since 1956 and was at one time protected by provincial law. That law was repealed in 2002.
First Nations used the fine-grained, heavy wood in the production of bows, arrows, implement handles and clothing hooks and the Cowichan people have used it for making knitting needles. The young shoots can be used to make baskets and the boiled bark was used to make a brown dye. Medicinally it was prepared to alleviate stomach troubles, as a blood purifier and a lung strengthener.
The Pacific dogwood makes a beautiful specimen tree but dogwood leaf blotch has restricted its use.
FURTHER INFORMATION AND REFERENCES
Clark, L.J., 1976. Wildflowers of the Pacific Northwest. Evergreen Press, Vancouver, BC.
Douglas, G. et al, 1999. Illustrated Flora of British Columbia, Volume 4 (Dicotyledons). Crown Publications, Victoria, BC.
Klinka, K., et al, 1989. Indicator Plants of Coastal BC. UBC Press, Vancouver, BC.
Parish, R and Thomson, F. The Tree Book, Learning to Recognize Trees of B. C. http://www.for.gov.bc.ca/hfd/library/documents/treebook/index.htm
Pojar, J. and A. Mackinnon, 1994. Plants of Coastal BC. Lone Pine Publishing, Vancouver, BC.