|Hydrangea subsp. var.||Hydrangea, Hortensia|
Hydrangea is a genus of about 70-75 species of flowering plants. By far the greatest species diversity is in eastern Asia, notably China and Japan (See: Japanese Ajisai flower). Most are shrubs 1-3 m tall, but some are small trees, and others lianas reaching up to 30 m by climbing up trees. They can be either deciduous or evergreen, though the widely cultivated temperate species are all deciduous.
Hydrangea flowers are produced from early spring to late autumn; they grow in flowerheads (corymbs or panicles) at the ends of the stems. In many species, the flowerheads contain two types of flowers, small fertile flowers in the middle of the flowerhead, and large, sterile bract-like flowers in a ring around the edge of each flowerhead. Other species have all the flowers fertile and of the same size.
In most species the flowers are white, but in some species (notably H. macrophylla), can be blue, red, pink, or purple. In these species the exact colour often depends on the pH of the soil; acidic soils produce blue flowers, neutral soils produce very pale cream petals, and alkaline soils results in pink or purple. Hydrangeas are one of very few plants that accumulate aluminium. Aluminium is released from acidic soils, and in some species, forms complexes in the hydrangea flower giving them their blue colour.
Hydrangeas are moderately toxic if eaten, with all parts of the plant containing cyanogenic glycosides. However, poisoning is rare, as the plant does not look like an enticing food source.
Species in the related genus Schizophragma, also in Hydrangeaceae, are also often known as hydrangeas. Schizophragma hydrangeoides and Hydrangea petiolaris are both commonly known as climbing hydrangeas.
Hydrangeas are popular ornamental plants, grown for their large flowerheads, with Hydrangea macrophylla being by far the most widely grown with over 600 named cultivars, many selected to have only large sterile flowers in the flowerheads. Some are best pruned on an annual basis when the new leaf buds begin to appear. If not pruned regularly, the bush will become very 'leggy', growing upwards until the weight of the stems is greater than their strength, at which point the stems will sag down to the ground and possibly break. Other species only flower on 'old wood'. Thus new wood resulting from pruning will not produce flowers until the following season.
Pests and diseases
- BACTERIAL DISEASES - Bacterial blight (Pseudomonas solanacearum), Bacterial leaf spot (Pseudomonas cichorii)
- FUNGAL DISEASES - Bud and flower blight, (Botrytis cinerea), Interveinal chlorosisn (Lack of iron uptake due to alkaline soils), Fungal leaf spots (Cercospora hydrangeae), (Corynespora cassicola), (Phyllosticta hydrangeae), (Septoria hydrangeae), Powdery mildew (Erysiphe polygoni), Root rot (Pythium spp.), Rust (Pucciniastrum hydrangeae), Southern blight (Sclerotium rolfsii), (teleomorph: Athelia rolfsii (Curzi) Tu & Kimbrough)
- VIRAL DISEASES - Phyllody (Hydrangea ringspot virus), Ringspots (Hydrangea ringspot virus), (Tobacco ringspot virus), (Tomato spotted wilt virus), (Tomato ringspot virus)
Partial list of species:
- Hydrangea anomala (Climbing Hydrangea). Himalaya, southwest China.
- Hydrangea arborescens (Smooth Hydrangea). Eastern North America.
- Hydrangea aspera. China, Himalaya.
- Hydrangea bretschneideri. China.
- Hydrangea candida. China.
- Hydrangea caudatifolia. China.
- Hydrangea chinensis. China.
- Hydrangea chungii. China.
- Hydrangea cinerea (Ashy Hydrangea). Eastern United States.
- Hydrangea coacta. China.
- Hydrangea coenobialis. China.
- Hydrangea davidii. China.
- Hydrangea dumicola. China.
- Hydrangea gracilis. China.
- Hydrangea heteromalla. Himalaya, west and north China.
- Hydrangea hirta. Japan.
- Hydrangea hypoglauca. China.
- Hydrangea integrifolia. China.
- Hydrangea involucrata. Japan, Taiwan.
- Hydrangea kawakamii. Taiwan.
- Hydrangea kwangsiensis. China.
- Hydrangea kwangtungensis. China.
- Hydrangea lingii. China.
- Hydrangea linkweiensis. China.
- Hydrangea longifolia. China.
- Hydrangea longipes. Western China.
- Hydrangea macrocarpa. China.
- Hydrangea macrophylla (Bigleaf Hydrangea). Korea, Southern Japan.
- Hydrangea mangshanensis. China.
- Hydrangea paniculata (Panicled Hydrangea). Eastern China, Korea, Japan, Sakhalin.
- Hydrangea petiolaris (Climbing Hydrangea). Japan, Korea, Sakhalin.
- Hydrangea quercifolia (Oakleaf Hydrangea). Southeast United States.
- Hydrangea radiata (Silverleaf Hydrangea). Southeast United States.
- Hydrangea robusta. China, Himalaya.
- Hydrangea sargentiana. Western China.
- Hydrangea scandens. Southern Japan south to the Philippines.
- Hydrangea serrata. Japan, Korea.
- Hydrangea serratifolia. Chile, western Argentina.
- Hydrangea stenophylla. China.
- Hydrangea strigosa. China.
- Hydrangea stylosa. China.
- Hydrangea sungpanensis. China.
- Hydrangea xanthoneura. China.
- Hydrangea zhewanensis. China.
Hydrangea sp. painted by the botanical artist Redouté
Standard Cyclopedia of Horticulture
|Standard Cyclopedia of Horticulture|
Hydrangea (Greek, hydor, water, and aggeion, vessel; alluding to the cup-shaped fruit). Saxifragaceae. Ornamental woody plants, grown chiefly for their showy white, pink or blue flowers.
Deciduous shrubs: lvs. opposite, without stipules, petioled, serrate, sometimes entire, rarely lobed: fls. perfect, in terminal panicles or corymbs, often with sterile marginal fls.; calyx-lobes and petals 4-5; stamens usually 10; ovary inferior or half-inferior; styles 2-5, short: caps. 2-5-celled, dehiscent at the base of the styles, with many minute seeds.—About 35 species in N. and S. Amer., Himalayas and Cent, and E. Asia, of which more than 20 occur in China; for a key to the Chinese species, see Rehder, Synopsis of the Chinese Hydrangeas (in Sargent, Plante Wilsonianse 1:34-41).
The hydrangeas are highly ornamental mostly low shrubs, rarely vines climbing by rootlets, with medium- sized or rather large leaves and small white, bluish or pinkish flowers in corymbs or panicles, bearing usually marginal sterile flowers, with enlarged showy sepals. or in some varieties all the flowers are sterile and enlarged. H. paniculata is the hardiest of all, but H. arborescens, H. radiata, H. xanthoneura and H. bretschneideri are also almost hardy North, while H. quercifolia and H. petiolaris are hardy as far north as Massachusetts, and H. involucrata, H. opidoides, H. sargenliana, H. heteromalla and H. davidii, are still more tender, and cannot be grown outdoors North.
They grow best in a rich, porous and somewhat moist soil and thrive well in partly shaded positions, but flower more freely in full sun if they only have sufficient moisture. All hydrangeas are well adapted for borders of shrubberies, and H. paniculata and H. opidoides, especially the varieties with sterile flowers, are very showy as single specimens on the lawn. In warmer climates the latter is sometimes used for ornamental hedges (see G.C. III. 24:337, 456); but it is not hardy in the North. These and also most of the other species should be pruned in fall or early spring, and the branches of the previous year cut back to one to three pairs of buds, according to the growth of the branches and the desired size of the panicles; if only slightly pruned, the panicles will be many but small. Sometimes they are cut back every year almost to the ground and produce then enormous panicles, which, however, usually need artificial support and lack the gracefulness of less severely pruned plants. H. paniculata var. grandiflora can be grown into a small standard tree; for this purpose vigorous young plants should be selected and planted in rich soil, and cut down to the base. The strongest shoot of each plant will attain by fall the height of 4 to 6 feet, if freely manured and watered during the summer; in autumn, all the weaker branches arc cut off, and in colder climates the plants should be lifted and stored in a frost-proof pit or cellar, since the wood is usually not sufficiently ripened to withstand severe frost. In the following year the top of the stem is allowed to branch. The weaker basal shoots may be pegged down to make new plants. Strong- growing varieties of H. opuloides may be treated in the same way if standard plants are desired.
The method of winter protection of hardy hydrangeas adopted around Newport, Rhode Island (and possibly other places) may be worthy of mention. In the case of individual specimens, after the leaves have dropped in the fall, the branches are tied together and the plant covered with a box having open ends. The box is then filled with earth. When the plants are growing together in a bed or border, they may be treated in a similar way by placing boards along the side of the bed, to assist in retaining the earth that is used as a covering material. If, after the branches are tied, they are bent over somewhat, a saving of labor is effected by reason of a smaller quantity of earth being sufficient to cover them. (Montague Free.)
H. opuloides, which cannot withstand much more than 10° of frost, is in the North much grown as a pot- plant, especially the more showy varieties with large heads of sterile flowers, and is extensively used for outdoor decoration during the summer. Late in fall, when the leaves have fallen after frost, the plants are moved to a frost-proof cellar and kept rather dry until spring, when they are repotted in new soil and the growth of last year cut back to one or two pairs of buds. As a suitable soil may be recommended a mixture of loam, leaf-mold and sand, with ground bone, dried cow-manure or some other kind of manure added. During the summer a liberal supply of water should be given, also occasionally applications of liquid manure, until the flowers have developed. They may also be planted in the open ground during the summer, lifted late in fall with a large ball of earth, stored over winter in a coldframe or pit and planted out again in spring; this will not injure in any way the profusion of ' flowers. In certain kinds of soil the pink hortensias show a tendency to turn blue, and perhaps this can_ be caused by adding iron filings or alum to the soil. H. opuloides is also a valuable plant for forcing, and is much grown for Easter, especially the var. otaksa, on account of its dwarfer habit. Handsome pot-plants can be grown in one year from cuttings. In February or March cuttings are inserted in the propagating-house with slight bottom heat, and planted in small pots as soon as they are rooted. During the summer they may be easily grown in pots and plunged outdoors in coal-ashes or in any kind of porous soil, transplanted several times, and freely watered and occasionally manured; or they may be planted out in rich soil, exposed to the full sun, where water should be liberally given and now and then an application of liquid manure. Last of September they should be repotted in 8-inch pots, kept shady some days until established, and afterward exposed to the sun. After the first frosts they may be brought into a cool greenhouse. If intended to have them in flower for Easter, they should be transferred not later than the fore part of January into a warmer house, with a temperature gradually rising from 50° to 60°; the plants should be freely watered, and about once a week an application of liquid manure given until the flower-buds are developed. The flowers should be almost fully developed some time before they are desired, that they may be hardened off in a cooler house, since overforced plants are likely to collapse if exposed to sudden changes of temperature. After flowering, the plants are pruned and repotted or planted out and treated as above described for cuttings, or they may be thrown away and another set of plants raised from cuttings.
H. petiolaris is a handsome climbing plant for covering walls and trunks of trees, and grows well in the shade, but flowers freely only in the full sun.
The hydrangeas are readily propagated by cuttings of half-ripened or nearly ripe wood under glass in summer; also by hardwood cuttings, layers, Buckers or division of older plants. H. quercifolia is best propagated by suckers or by layers of growing wood put down in summer. Rarely increased by seeds, which are very small, and should be sown in fall in pans or boxes and only slightly covered with soil. acuminate, 7 (1). albo-variegata, 7. attisoima, 16. anomala, 16. arborescens, 8. aspera, 13. azisai, 7 (1). belzonii. 7 (1). bretschneideri, 4, 5. buergeri, 7 (1). cinerea, 9. cordata, 8. cyanea, 7 (1). cyanoclada 7 (2). davidii, 6. fimbriata, 7 (3). floribunda. 2. glabrescens, 4. prolifera, 7 (3). grandiflora, 2, 8. heteromalla, 3. hortensia, 7 (2). hortensis, 2, 7, 14. involucrata, 14. japonica, 7 (1). lindleyana, 7 (1). macrophylla, 13. macroaepala, 7 (1). mandshurica, 7 (2). mariesi, 7 (1). monstrosa, 7 (2). nigra. 7 (2). nivalis, 7. nivea, 10. opuloides, 7. otaksa, 7 (2). paniculata, 2. pekinensis, 4. petiolaris, 15. plena, 7 (2). praecox, 2. prolifera, 7, (3). pubescens, 3. quercifolia, 1. radiata, 10. ramulis, 7 (2). rosalba, 7 (1). rosea, 7, (2). roseo-marginata, 7. rosthoranii, 12. rubro-plena, 7 (3). sargentiana, 11. scandens , 15. serrata, 4, 7 (1). setchuenensis, 5. stellata, 7 (3). sterilis, 8, 9. strigosa, 13. tardiva, 2. thunbergii, 7 (1). tricolor, 7. urticifolia, 8. variegate, 7. veitchii, 7 (1). vestita, 3,4. volubilis, 15. xanthoneura, 5.
H. aspera. Don. Shrub, to 20 ft., similar to H. strigona: lvs. oblong-lanceolate, fimbriate-denticulate, densely villous beneath: sepals usually toothed: styles usually 3. Himalayas. Tender.—H. canescens, Koch (H. arborescens X H. radiata). Very similar to H. cinerea, but the hairs smooth or nearly smooth under the microscope, in H. cinerea tuberculate. Garden origin.—H. hirta, Sieb. & Zucc. Shrub, to 4 ft.: lvs. broad-elliptic, coarsely incised-serrate: cymes without sterile fls. Japan. S. Z. 62. Not very decorative. H. longipes, Franch. Allied to H. rosthornii. Lvs. thinner, smaller, more coarsely serrate, sparingly strigoso or glabrescent below; petioles 2-7 in. long. Cent, and W. China.—H. robusta. Hook. f. & Thorns. (H. cyanema, Nutt.). Closely related to H. rosthornii. Spreading shrub, to 15 ft., with large ovate lvs., pubescent on both sides: sterile fls. with toothed sepals. Himalayas. B. M. 5038. Handsome in bloom, but tender.—H. roseo-paniculata, Foucard. Supposed to be a hybrid of H. paniculata and H. opuloides. Fls. rose-carmine. R, H. 1912, p. 324.—H. villosa, Rehd. Allied to H. strigosa. Branchlets, petioles and cymes clothed with spreading villous hairs: lvs. strigose above, with a rough woolly tomentum below, 4-7 in. long. W. China. Var. strigosior, Rehd. Branchlets and petioles with shorter and fewer or without spreading hairs: lvs. smaller.—H. virens, Sieb. Slender shrub, to 6 ft.: lvs. elliptic or lanceolate, coarsely serrate, 1-2 ½ in.: cymes rather few-fld., sterile fls. with 3 or 4 large, unequal sepals, white. Japan. S. Z. 60. A desirable shrub, with graceful and delicate fls. and with the lvs. often handsomely variegated along the veins, but tender.
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