(Editor's Note: This article was originally published on 1uly 18, 2009. Your comments are welcome, but please be aware that authors of previously published articles may not be able to promptly respond to new questions or comments.)
The dogwoods are an important group of flowering and/or decorative landscape plants for gardeners in cool to cold climates. They belong to the genus Cornus, which contains about 45 species, mostly native to the northern temperate regions of the world. Their flowers consist of a cluster of quite small blooms which are white, yellow to green in colour. In some species, the flowers are surrounded by a four to seven large bracts (modified leaves) which can make for an impressive floral display. Others are grown more for their decorative bark which can be particularly attractive during the winter months. Several have striking fall colour. Still others produce a modest display of attractive fruit, which in one species at least, is quite edible. Plant form is quite variable from low groundcovers, through various-sized shrubs to small trees. Few woody plants offer as much diversity as the dogwoods.
Perhaps the most common group of dogwoods are the so-called ‘red-twig dogwoods' or ‘red-osier dogwoods'. These small to large shrubs are members of the species C. stolonifera, C. sericea, C. sanguinea and C. alba. This group of dogwoods have twigs which range from dark red, through scarlet, orange, yellow and bright green, depending on the cultivars. Leaf colour may be bright yellow in some selections or variegated with various amounts of white, yellow or pink streaking. All produce flat-topped clusters of white flowers in late spring-early summer followed by white to bluish fruit in late summer. Several have notable fall colour. For more details on this group of dogwoods, refer to my previous article entitled ‘Red-twig Dogwoods - a Shrub for All Seasons'.
The emphasis of this article will be the tree-like, flowering dogwoods. These are perhaps the most showy of all the dogwoods but alas, are not as hardy as the red-osier dogwood group. Among the genus Cornus, the flowering dogwoods have the largest bracts and hence the most attractive floral display. There are three species within this group: C. florida of eastern North America, C. nuttallii of western North America and C. kousa from China. All grow as small-sized trees 7-10 m tall. The white, pink or reddish bracts number four to seven and may be rounded or pointed depending on the species and/or cultivar. In some cultivars, the bracts overlap lending the bloom a saucer-like appearance. In other cultivars, the bracts are narrow and spaced to give a more star-like look. The bracts remain in good condition for several weeks; some change from white to pink as they age. This group also exhibits spectacular fall colours in purple, red to orange shades. Red, vaguely strawberry-like fruit are produced in autumn.
Comparion between the three species: Cornus florida (top left), C. nuttallii (top right) and C. kousa (bottom)
In the wild, flowering dogwoods grow as understory trees so are adapted to dappled shade. They are wonderful focal points in open woodland gardens and combine beautifully with rhododendrons, witch-hazels and mountain laurels since they too prefer soil on the moist, organic, acidic side. Flowering dogwoods will not tolerate drought.
These dogwoods do suffer from a number of foliar, fungal diseases. Anthracnose and powdery mildew rank highest among these fungal problems. There is no simple cure. However, recently there have been a number of new cultivars released that show resistance to some of these fungal diseases. If you live in a region where fungus is a problem on these dogwoods, check with your local nursery to see if they sell any of the disease-resistant types. The ‘Cherokee' and ‘Celestial' series of hybrids in particular appear to be mostly fungus-resistant. I will describe these in more detail shortly. In regards to insects, the only serious pest is the dogwood borer. The adult is a clear-wing moth. The larvae burrow into the trunks and larger branches of the trees causing severe die-back. Again there is no simple cure. Oftentimes, trees which are attacked are already under stress from drought or fungal diseases.
Hardiness varies among the flowering dogwoods. The eastern flowering dogwood, Cornus florida, is rated hardy to zone 5 but they require long, hot summers to do well. They do not do so well in most coastal areas of northeastern New England, Atlantic Canada and the Pacific Northwest (they are impossible in my zone 6!). They typically bloom just as the leaves unfurl. There are many named cultivars with ‘flowers' that vary from white through deep red, as well as cultivars with variegated foliage that can provide interest all summer. Some of the notable selections include ‘Bonnie' (white), ‘Apple Blossom' (light pink), ‘White Cloud' (white), ‘Cherokee Chief' (deep rose), ‘Cherokee Brave' (two-tone pink and white), ‘Eternal' (double white) and ‘Royal Red' (deep red). If variegated leaves are your thing, look for ‘Purple Glory' (purple foliage), ‘Rainbow' (yellow variegated in summer, multi-coloured in autumn), ‘Tricolor' (cream, pink and red), ‘Welchii' (yellow and red variegation), ‘Cherokee Daybreak' (cream and pink) or ‘Cherokee Sunset' (bright yellow variegation). There is even a weeping form of C. florida called ‘Pendula'. More selections seem to be released each year so keep your eyes open!
Some named cultivars of Cornus florida include 'Cherokee Brave', 'Cherokee Chief' and 'Eternal'
Some variegated forms of Cornus florida include 'Summer Fun', 'Cherokee Sunset' and 'Rainbow'
The Pacific or western flowering dogwood, Cornus nuttallii, is relatively tender, only rated for zone 7. Like the eastern flowering dogwood, the western counterpart also blooms just after the leaves appear. There are less selections than for C. florida. Some worth considering include ‘North Star' (white, star-like flowers), ‘Colrigo Giant' (huge 15 cm diameter white flowers), ‘Ascona' (pendulous branches) and ‘Goldspot' (leaves spotted and mottled yellow).
Details of Cornus nuttallii
The Chinese flowering dogwood, C. kousa, is the hardiest species (zone 5). This species does not have the high summer heat requirements of C. florida thus is suitable for cool summer regions. Certainly in my area of Atlantic Canada it is the only reliable flowering-dogwood. They are last of the three species to bloom, well after the trees have already produced their foliage. There are many selections on the market and new ones are appearing regularly. Some cultivars of note include ‘Milky Way' (white), ‘Satome' (deep pink), ‘Summer Stars' (white), ‘Beni Fuji' (pink), ‘Heart Throb' (red) and ‘Prophet' (white). Like the other species, variegated forms exist including ‘Wolf Eyes' (white edged), ‘Sunsplash' (yellow edged), ‘Summer Fun' (cream and pink edges), ‘Trinity Star' (cream, yellow and pink) and ‘Gold Star' (central yellow blotch). This species also has a weeping form called ‘Lustgarten Weeping'. The Chinese flowering-dogwood has the advantage of wonderfully patterned bark and superior insect and disease resistance compared to C. florida and C. nuttallii.
Details of Cornus kousa fruit and fall foliage
Some named forms of Cornus kousa include: 'Satome', 'Lustgarten Weeping' and 'Prophet'
Some of the variegated forms of Cornus kousa include 'Gold Star', 'Sun Splash' and 'Wolf Eyes'
There are also hybrids between these three species. ‘Pink Blush' (pink), ‘Ormonde' (white) and ‘Eddies White Wonder' are hybrids between C. florida and C. nuttallii. They are slightly hardier than C. nuttallii being rated for zone 6. Hybrids between C. florida and C. kousa are often very floriferous. They are almost as hardy as C. kousa but like the latter species, do not require hot summers to do well. Some of the hybrids include ‘Ruth Ellen', ‘Stardust', ‘Constellation', ‘Celestial', ‘Aurora' and ‘Stellar Pink'. They were all developed at Rutgers University in New Jersey and have been released as the ‘Celestial' series. ‘Starlight' and ‘Venus'(both zone 7) are among the few hybrids between C. kousa and C. nuttallii. They were also developed by Rutgers University.
Examples of hybrid flowering dogwood include 'Eddie's White Wonder' and 'Stellar Pink'
New selections and hybrids of flowering dogwood, especially disease-resistant forms, are appearing on a regular basis. There are no doubt cultivars out there that I have not mentioned. If you are fortunate to live in an area where you can grow these spectacular dogwoods, I highly recommend you try them as few flowering trees have as long a blooming season along with the added bonus of spectacular fall foliage.
I have many people to thank in regards to pictures in this article: Brent_in_Nova ('Gold Star'), DaylilySLP ('Prophet'), EricInSF (C. florida), fdetroch ('Stellar Pink'), Gustichock ('Lustgarten Weeping'), lauriwilson ('Cherokee Sunset'), paigeplants ('Rainbow'), passiflora_pink (fall foliage C. kousa), picea ('Sun Splash'), plantfreak ('Eternal' and 'Cherokee Brave'), PudgyMudpies ('Cherokee Chief'), RoyB ('Eddie's White Wonder'), Scorpioangel (C. nuttallii), TomH3787 ('Summer Fun') and ViburnumValley ('Wolf Eyes')....whew! Thanks everyone!