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A genus of well over 100 species, mainly in the temperate regions of the northern hemisphere, but extending into Malaysia and S. America. They are shrubs or small trees, deciduous or evergreen. Winter-buds naked or few-scaled. Leaves opposite, simple, toothed, sometimes lobed; stipules sometimes present, small, adnate to the petiole, but more commonly lacking. Inflorescence compound, consisting of cymes arranged in the form of a flat or dome-shaped umbel, or more rarely of a panicle; it is usually terminal on short growths of the season, sometimes almost sessile on the previous season's wood. Ovary inferior. Calyx small, shortly five-toothed. Corolla white or cream-coloured, sometimes flushed with pink or almost wholly pink, rotate, campanulate or more rarely cylindric, five-lobed. Stamens five, inserted on the tube. Style sessile on the free upper part of the ovary (i.e., the portion above the insertion of the corolla and calyx). Fruit a red, blue or black one-seeded drupe.
A curious feature of several species of Viburnum is the presence of two distinct types of flower in one inflorescence – the one sterile and showy, consisting of a corolla without stamens or pistil, the other much smaller but perfect and fertile. The function of the large sterile ray-flowers is that of advertisement, to attract insects to the inflorescence. This really represents an interesting and unusual division of labour, for most insect-fertilised do their own advertising by means of the petals attached to the individual flower. In three species – V. opulus, V. macrocephalum and V. plicatum – mutations have occurred in which the inflorescence is entirely made up of sterile ray-flowers, which represents a striking increase in flower beauty. These phenomena are also exhibited by several species of Hydrangea (according to one controversial theory Viburnum actually derives from the Hydrangaceae).
The viburnums that have spread most widely in gardens are grown for their flowers. But many species have beautiful fruits, and some would be as common as the cotoneasters and pyracanthas were it not that most of these show the phenomenon of self-incompatibility. Like so many of the orchard apples, pears, plums and cherries they do not as a rule set fruit unless pollinated by another seedling or by a plant belonging to a different clone. There may be exceptions (V. opulus and its allies are said to be self-fertile), but cross-pollination appears to be necessary precisely in those groups that have the most ornamental fruits – the blue-fruited V. davidii and its allies, and the red-fruited members of the section Odontotinus such as V. betulifolium and V. dilatatum. Ideally, two or more compatible clones should be available, as is already the case here with V. davidii. A possible alternative would be to plant together different but related species that flower at the same time. But either way, insect visitors are still necessary to do the work of cross-pollination.
Viburnums are as a rule of easy cultivation, but there are some exceptions and some are not very hardy. They love moist conditions and a deep, rich loamy soil.
Most viburnums can be increased by cuttings taken in late summer and placed in gentle bottom heat, and this is the usual way of propagating the evergreen species. Seed is often slow to germinate and may not come true if taken from plants in a collection. Layering is another means, and is often used for V. plicatum, V. lantanoides and V. furcatum.
The following is based on Rehder's standard classification of Viburnum in Sargent, Trees and Shrubs, Vol. II (1908). Characters based on the stone of the fruit are omitted, as the fruits of Viburnum are too rarely seen on some species to make them of much value. Only species given a full treatment are listed; those given shorter descriptions belong to the same group as the one under which they are mentioned.
sect. Viburnum (Lantana). – Indumentum of stellate hairs. Winter-buds naked. Leaves usually deciduous but evergreen in some species, toothed (sometimes slightly so). Inflorescence corymbose, without ray-flowers except in V. macrocephalum (wild form). Fruits black or bluish black, but at first red in most species. An Old World group.
V. buddleifolium, V. burejaeticum, V. carlesii, V. cotinifolium, V. lantana, V. macrocephalum, V. rhytidophyllum, V. utile, V. veitchii.
Of the several garden-raised hybrids in this group, V. × carlcephalum and V. × burkwoodii are of particular interest as confirming the relationship of V. carlesii to the outwardly dissimilar V. macrocephalum and V. utile respectively.
The section pseudotinus is in most respects similar to the sect. Viburnum, but differs in technical characters of the fruit-stone. See V. lantanoides.
sect. Pseudopulus. – Winter-buds with one pair of scales. Indumentum stellate. Leaves with straight veins, running to teeth. Flowers in cymes, with ray-flowers, terminating short lateral branches.
sect. Lentago. – Winter-buds with one pair of outer scales. Leaves deciduous, finely serrate or entire; lateral veins anastomosing before reaching the margin. Flowers in cymes, all fertile. Fruits blue-black. All species American.
V. cassinoides V. lentago, V. nudum, V. prunifolium, V. rufidulum.
sect. Odontotinus. – Winter-buds with two pairs of outer scales. Hairs when present fascicled. Leaves usually deciduous, toothed (sometimes obscurely lobed), with straight lateral veins running out to teeth. Flowers in cymes. Corolla rotate. Old and New Worlds. The fruits are blue-black in the American species V. dentatum and V. molle and in V. acerifolium of southwest Asia, red in the following species, all natives of E. Asia:
V. betulifolium, V. dilatatum, V. erosum, V. foetidum (semi-evergreen, leaves often lobed), V. hupehensis, V. japonicum (evergreen, leaves obscurely toothed), V. phlebotrichum, V. setigerum, V. wilsonii, V. wrightii.
sect. Thyrsosma (Solenotinus). – Leaves deciduous or persistent. Flowers in panicles (often somewhat condensed in the winter-flowering species). Corolla rotate to cylindric. Fruits blue-black or purple. Himalaya and E. Asia.
V. farreri, V. grandiflorum, V. henryi, V. odoratissimum, V. sieboldii, V. suspensum.
sect. Tinus. – Leaves persistent, entire or V. davidii, V. harryanum, V. propinquum, V. rigidum, V. tinus. sect. Opulus. – Buds with one pair of outer scales, connate at the base. Leaves deciduous, three- or five-veined from the base and usually lobed. Inflorescence with ray-flowers in some species. Fruits red or scarlet. Old and New Worlds. V. opulus, V. kansuense. Similar to this section in general appearance is V. cylindricum of the section Megalotinus; it has a cylindric corolla with short lobes. In An Enumeration of the Flowering Plants of Nepal, this genus is placed in the Sambucaceae (see Sambucus in this supplement).
From the Supplement (Vol.V)
V. davidii, V. harryanum, V. propinquum, V. rigidum, V. tinus.
sect. Opulus. – Buds with one pair of outer scales, connate at the base. Leaves deciduous, three- or five-veined from the base and usually lobed. Inflorescence with ray-flowers in some species. Fruits red or scarlet. Old and New Worlds.
V. opulus, V. kansuense.
Similar to this section in general appearance is V. cylindricum of the section Megalotinus; it has a cylindric corolla with short lobes.
In An Enumeration of the Flowering Plants of Nepal, this genus is placed in the Sambucaceae (see Sambucus in this supplement).