Dogwoods, as a group, belong to the taxonomic family Cornaceae. Historically, this family has included Cornus and other genera of familiar ornamentals such as Alangium, Aucuba, Davidia, Helwingia, and Nyssa, among numerous genera less familiar to gardeners. Current taxonomic treatments of the family include as few as one genus (Cornus) to as many as 17 genera. The number of Cornus species generally ranges from 45 to 65 depending on who holds the stage. Readers interested in sorting through this academic challenge are invited to start with the references at the back of the book.
Most garden reference books love to list dogwoods (at least Cornus florida and C. kousa) as the penultimate four-season plants in the landscape: spring flowers, summer fruit and foliage, autumn fruit and leaf color, and winter bark and form. In fact the supermarket checkout-stand gardening magazines have made such comments downright cliché. Yet if one is trying to capture the essence of the dogwoods, it is hard to avoid joining such unimaginative company. Ounce for ounce, dogwoods add more to the full season of the garden than just about any other group of garden plants.
Dogwoods range from medium-sized trees to multistemmed suckering (rhizomatous) woody shrubs to herbaceous, mat-forming plants. They are primarily deciduous, although some evergreen species do exist. Most dogwoods bear opposite leaves. They provide the "D" in the MAD Horse rule — that introductory rule taught to all plant materials students trying to remember the majority of plants with opposite leaves: "M" is for maple, "A" is for ash (including the genus Fraxinus, along with all the other members of its family, Oleaceae), and "D" is for dogwood. The "horse" refers to all members of the genus Aesculus, including the horsechestnut. (There's another useful purpose for that pesky plant taxonomy.) Those who take the rule a step further know it as the MAD Horse Cap rule, "Cap" standing for the family Caprifoliaceae, which includes such genera as Lonicera and Viburnum.
Dogwood leaves are typically ovate, simple, and entire along the margin. The lower leaf surface is often covered with copious quantities of single-celled T-shaped trichomes (liquid-filled, hairlike structures) loaded with calcium carbonate. All dogwood leaves show a rather unique arcuate veination, the major veins arranged much like the lines of longitude on the globe. In addition, the major leaf veins contain a latex-like substance that forms strands when pulled apart. Thus, dogwood leaves make a wonderful magic trick for amusing small children. "Separate" the leaf into two halves and watch the second half rise when you lift the first. Ok, not a very good bar trick, but it does work on very young kids.
Most dogwood species sport medium green summer foliage, and with the exception of many variegated forms, few are standouts in this regard. But it is in autumn that the leaves can really shine. Reds, yellows, oranges, and burgundy shades can develop alone or in concert to form spectacular autumnal dress. To be truthful, on the full family scale, there are as many poor fall-coloring species as those that would inspire poets. Dogwoods seem to have benefited from wonderful public relations in this realm.
Of course, the flowers are what make the dogwood. Just the mere mention of the name dogwood conjures up images of sprightly spring days of crystal blue skies, fresh morning breezes, and the clear proud glow of a flowering dogwood in bloom. While this does a bit of injustice to the entire clan (by far, most dogwoods don't have flowers that most casual observers would call dogwood flowers), for many this is the image of spring. In the eastern and central United States, the Pacific Northwest, throughout much of temperate Asia and parts of Europe, there is probably no better-recognized, -loved, or -planted small flowering tree than Cornus florida, the flowering dogwood. There are dogwood festivals, subdivisions, shopping centers, and more. In Kentucky, we even celebrate annually dogwood winter, that late spring cold snap at dogwood bloom time that reminds us all that the tomato plants we just set out can still experience a rude reminder of Mother Nature's occasional mean-spirited jokes. No, there are few who would argue the flowering dogwood's position as top dog among the spring-flowering trees.
Yet, as many learn in grade school, or at least in an introductory plant materials class, the flowers of Cornus florida aren't white at all. Of course, the true flowers are yellow to yellow-green and not more than 0.25 inch (6mm) across. The show is apparent in what are called involucral bracts that subtend the boss of 20 to 30 true flowers. These are the outer protective structures evident in the winter bud that sits at the end of most dormant branches. Still, a sure way to win a quick beer at a spring neighborhood barbeque is to bet the host on the color of his or her dogwood's flowers. It's a winner every time.
All Cornus species bear their true flowers in compound inflorescences described as cymose, paniculate, corymbose, umbellate, or capitate. Translated, that means a more-or-less rounded, flat, or spherical mass of from as few as 8 to as many as 50 flowers, mostly born at the ends of branches of the previous season's growth. And while there is fair variation in the show provided by the bloom, the individual flowers from all species are quite consistent. Most are four-petaled flowers of creamy white to yellow-green, perfect (dioecious — male and female flowers born on separate plants — in the African species), and almost all are self-sterile. In some species like C. florida and C. Kousa the show of the true flowers is easily overwhelmed by the show of the expanded bracts. Others produce more pin-cushion-like inflorescence of creamy white with no showy bracts. This is typical in C. alba, C. alternifolia, and all their kissin' cousins. Still others have followed a different path and accentuated the true flowers with a bright splash of sulfur yellow: C. mas and friends. Across the family, flowering time can vary from the February–March gold of C. mas to the midsummer show of C. macrophylla, nearly six months of flowering from a single small group of plants in the garden.
Once the flowering subsides, it is time for the fruit to take center stage. Botanically, the fruit is described as a drupe or drupacaeous berry (any more question as to why science writing as a genre has taken so long to make it onto the best-seller lists?) and can range from white, to yellow, cherry red, and dark bluish black. From a show standpoint, leading the charge once again is Cornus florida. Individual bright red, oval fruits up to 0.4 inch (9 mm) long form in groups of up to about 15 or so. They are closely rivaled in showiness by that of C. canadensis and its relatives. But all make a splendid show of bright, glossy red fruits nestled into a delightful foil of bright green. They are formed brightly, held proudly, and often taken quickly by foraging birds. Still, some can remain on the plant to extend the show into late autumn. Yellow forms of the normally red-fruited dogwoods result from the absence of two anthocyanins (peonidin and petunidin), pigments typically found in fruit exocarp (skin).
In Cornus kousa, the individual football-like fruits, which resemble those of C. florida, have fused into a syncarp, a compound structure most closely resembling a large round raspberry. It is almost as if one took the C. florida fruit mass down to the local filling station and inflated it with one of those over-powered air hoses.
Most dogwood species produce fruit of a more somber nature. Creamy white to blue-black is the norm for most of the shrubby species and some of the small trees as well. They are generally produced in flat, rounded arrangements and at best are considered moderately showy. While plant geeks around the planet like to wax eloquent about such modest features, the bottom line is that black is black, and few non-plant enthusiasts consider it showy. How many garden center customers come in on a sunny Saturday morning looking for something with black fruit?
While dogwoods win many pageants in the show category, they are fairly limited in the culinary world. Cornus mas is the true one-and-only standout in this category. The bright cherry red fruits, while incredulously tart in early summer, ripen to a range of pleasantly tart to downright sweet. About the best way to know one is harvesting appropriately ripe fruit is to sit beneath a plant with open hands. When a fruit drops into your hands, it is just about ready. Of course, the birds would have been there the day before so you may be standing out there for quite some time. Each fruit does have a large stone and limited pulp, but a bucketful cooked down and strained can produce fabulous jellies, jams, and preserves. Several cultivars from eastern Europe have been selected for large, pulpy fruit and are grown commercially on a limited basis.
Most of the other red-fruited species bear edible but not overly palatable fruit. Historically, Cornus canadensis has been used as a minor food crop by some northern North American tribes, but it is not likely to show up in trendy restaurants any time soon. There seem to be no historical references to any food uses of the white- to black-fruited species. In fact, like many plants that civilization has chosen not to domesticate, these have an effect on the human anatomy that might earn them the name "Zumilax."
Finally, we come to dogwood bark. Many a bored undergraduate eye has rolled at the one comedic entry of the genus. How can you tell a dogwood? By its bark, of course! My (Paul's) 10-year-old has gotten great mileage out of that one. But unlike most bad jokes, this one actually can point us in a reasonable direction.
Bark texture and coloration not only provide an outstanding identification feature, but also open another whole realm of ornamental usefulness for the genus. It is here that some of the shrubby species, so often relegated to the back row of the choir in terms of flower and fruit show, finally get their due. Where would the winter landscape be without the red, yellow, orange, and coral bark of Cornus alba, C. stolonifera, and C. sanguinea? They provide the seldom-found combination in garden plants of industrial strength with outstanding ornamental appeal. Placed against an evergreen backdrop of Abies balsamea (for the northerners), Picea orientalis, Cephalotaxus, Buxus, even the venerable Taxus, dogwoods provide a brilliant contrast for the winter-weary gardener. Better still, a light dusting of the winter white stuff over the brilliant-colored stems casts all the makings of calendar and postcard.
Continuing to turn the horticultural tables on the typical front-runners, our old favorites, Cornus florida and C. kousa, now take up a position back in the pack. Their bark, distinctive as it is, doesn't hold a candle to the bark of C. alba and its friends. Bark on a mature C. florida is often described as blocky or produced in a jigsaw puzzle pattern. Good for winter ID but not showy.
A few rungs up the bark ladder from Cornus florida is C. kousa. While not a showstopper from 50 paces, the micro-camouflage patterning of olive, cream, gray, and coppery brown does create a nice, close-up impact in the garden. The fine exfoliation pattern plays beautifully against bold foliage beneath. Think Rodgersia, even large blue hostas.