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A genus of around seventy species of shrubs in the temperate regions of the northern hemisphere. Leaves deciduous, without stipules, simple, toothed or more rarely entire, sometimes lobed. The flowers are very uniform in size, being rarely more than 3⁄8 in. wide, but about 1⁄2 in. across in the double-flowered forms of S. prunifolia and S. cantoniensis. They are either white or in some shade of pink or crimson (all the early flowering sorts have white flowers), and in most species they are hermaphrodite. The inflorescence is a condensed raceme or rounded corymb produced from buds on the previous season’s wood, or it terminates the growths of the season, then taking the form of a panicle or a sometimes complex corymb (see further under classification). Receptacle cup-shaped, campanulate or top-shaped, bearing on its rim five short sepals, five petals and numerous stamens, which in some species are longer than the petals. The receptacle is usually edged with a many-lobed, nectar-secreting disk. Gynoecium composed of five free or almost free carpels, inserted at the base of the receptacle, each bearing a style at its apex (or just below the apex on the outer side). The carpels usually enlarge as they mature, each becoming a dry follicle which opens along the inner suture, releasing a few minute seeds.
Spiraea, as once understood, was a much larger genus than now. In previous editions, following the older classification, the following were included in it (the most obvious differential characters are given in brackets): Sorbaria (leaves pinnate); Chamaebatiaria (leaves doubly pinnate and fern-like); Petrophytum (creeping or tufted shrubs; flowers in long-pedunculate racemes or heads); Luetkea(dwarf shrub with much divided leaves); Holodiscus (flowers very small in plume-like panicles; carpels indehiscent in fruit); Sibiraea (near to Spiraea but carpels more united; easily recognised by its stout branches and the combination of slender panicles of white flowers and narrow entire leaves).
The old Spiraea also contained herbaceous plants, e.g., Aruncus dioicus (Walt.) Fern. (Spiraea aruncus L.); Filipendula ulmaria (L.) Maxim. (Spiraea ulmaria L.), the native Meadow Sweet; Filipendula purpurea Maxim. (Spiraea palmata Hort., not Thunb.); Filipendula rubra (Hill) Robins. (Spiraea lobata Jacq.; S. venusta Hort.).
As ornaments in the garden, the best of the spiraeas fill an important place. They flower with great freedom, are often very graceful, and except that some of the earlier flowering kinds are liable to injury by late frost, they are perfectly at home under cultivation. All like a good loamy soil, abundant moisture, and full sunlight. ‘Arguta’, ‘Grefsheim’ and S × vanhouttei make excellent low hedges.
Propagation. – Some of the spiraeas spread by means of sucker growths from the base, and such are easily increased by dividing the plants into small pieces. The rest can nearly all be propagated easily by means of cuttings made of moderately firm wood placed in light sandy soil in gentle bottom heat in July and August. If this be not available, cuttings made of harder wood in September may be placed under bell-glasses out-of-doors in a sheltered spot.
The spiraeas produce fertile seed in abundance, but they cross-breed with such facility that seed can only be depended on to come true when the plants are fairly isolated from other species. Some of the very best spiraeas are hybrids, as may be gathered from the following descriptive notes, but they have become so numerous that they make the genus, as represented in gardens, excessively difficult to study and classify. Zabel of Münden devoted a volume of one hundred and twenty-eight pages exclusively to their elucidation, but many are so similar to each other that their differentiation on proper is no longer possible within convenient limits.
Pruning. – Few shrubs repay careful pruning better than the spiraeas, and in this matter they may be divided into two groups, viz. (1) those that flower early and from the buds of shoots made the previous year, such as S. ‘Arguta’, S. ‘Grefsheim’, S. thunbergii, S. × vanhouttei, S. veitchii; and (2) those that flower later in the year at the ends of the shoots of the current season, such as S. japonica, S. douglasii, S. salicifolia, etc. This matter is fully discussed in the introductory chapter on pruning, and from what is there stated it will be evident that the first group must only be pruned by thinning out the older and weaker wood; any shortening back of the shoots must mean a reduction in the next crop of blossom. The second group, on the other hand, is benefited by the shoots being shortened back. This should, of course, be done in later winter or early spring, and at the same time superfluous old shoots should be cut clean out. Unless pruning of either kind is done, many of the spiraeas get into a weedy, thin condition, and their blossoms will not bear comparison either in quantity or quality with that of properly pruned plants.
The group including S. douglasii, S. tomentosa, S. salicifolia, and their hybrids form dense thickets, and spread rapidly by means of underground suckers. These should be pruned as in group (2) (being late flowering), and it is also advisable at intervals of a few years to dig them up, divide them into smaller pieces, and after enriching the ground, replant them more thinly. This, of course, applies to ordinary cultivated shrubberies and borders, but they also make admirable masses for the wilder portions of the demesne, where they can safely be left to take care of themselves. In such places the reddish or rich brown stems of many spiraeas make a cheerful feature in winter.
It has been customary to group the species of Spiraea into three sections, according to the nature of the inflorescence, but the classification is to a large extent artificial, as Schneider pointed out early this century. The species treated here, and their hybrids, can be informally arranged as follows:
salicifolia Group. – This is the well marked section Spiraea (Spiraria), in which the inflorescence is a panicle terminating a long shoot of the season’s growth. Here belong the Old World S. salicifolia (the type-species of Spiraea), and the American S. alba, S. douglasii, S. latifolia and S. tomentosa. There are numerous garden hybrids in this group, of which S. × billiardii is treated here.
japonica Group (sect. Calospira, in part). – Inflorescence a flat or slightly convex, much branched corymb, usually borne at the end of a long shoot, as in the first group, more rarely on a short lateral (S. bella); the peduncles arc usually furnished with slightly reduced leaves. This group occurs in both the the New and the Old World. The Asiatic species treated here are S. japonica, S. bella, its close ally S. amoena, and S. betulifolia. The last named also occurs in N. America where the other representative (in the western part of the continent) is S. densiflora. The geographically isolated European species S. decumbens and S. hacquetii should perhaps also be placed here. Among the hybrids that have arisen in this group are ‘Margaritae’ and those mentioned under it.
There are some fine hybrids between the Salicifolia and Japonica groups, for which see S. × sanssouciana.
canescens Group (sect. Calospira, in part). – Inflorescence corymbose as in the preceding group, but of simpler form, not leafy, and borne at the end of a short leafy lateral. S. canescens, S. henryi, S. wilsonii, S. veitchii, S. longigemmis and S. rosthornii may also belong here.
S. × brachybotrys and S. × fontenaysii are hybrids between S. canescens and members of the Salicifolia group.
The remaining species, all natives of Europe and Asia, are usually grouped in the section Chamaedryon, a rather artificial aggregate. The inflorescence is usually simple, i.e., the peduncles each bear a single flower and arc arranged in the form of a condensed raceme or umbel. But in several species the inflorescence is partly compound, the lower peduncles being branched and bearing several flowers. There is also great variation in foliage and in the form of the leaf-buds. The flower-clusters are borne in spring or early summer on the branches of the previous season on short leafy laterals, or are sessile on them. Grouped according to probable affinity the species treated here are:
S. prunifolia, S. thunbergii
S. crenata, S. hypericifolia
S. cana, S. media
S. chamaedryfolia, S. flexuosa
S. blumei, S. chinensis, S. trilobata, S. yunnanensis
S. alpina, S. myrtilloides
S. calcicola, S. calcicola
S. arcuata, S. gemmata, S. mollifolia, S. nipponica, S. trichocarpa
Hybrids within this group are: S. ‘Arguta’, S. × cinerea, S. × multiflora, S. × schinabeckii, S. × vanhouttei. See also S. × nudiflora, p. 476.
H. Zabel, Die strauchigen Spiräen der deutschen Garten (1893). The bulk of Zabel’s collection at the Forstakademie, Hannover-Münden, was propagated by Messrs Hesse of Weener, Hannover, and distributed by them from about 1894 onwards, each plant being sent out with the name and number under which it appears in Zabel’s work. His collection consisted mainly of hybrids raised by open pollination, some by himself, others in nurseries or gardens. A generation or so later such hybrids would have received ‘fancy names’, but Zabel followed the then usual practice of giving botanical status to these garden productions. In the present revision this status is recognised only if the name is the valid designation for a simple interspecific cross.