Hydrangea

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Hydrangea

Hydrangea
Light mauve hydrangea.jpg
Genus: Hydrangea
Family: Hydrangeaceae
Type: Trees, shrubs, and vines
Pest issues: White-tailed deer are a serious issue
Pollination: Insect

Hydrangea (common names also Hydrangea, pronounced haidréindʒiə, and Hortensia) is a genus of about 70-75 species of flowering plants native to southern and eastern Asia (from Japan to China, the Himalaya and Indonesia) and North and South America. By far the greatest species diversity is in eastern Asia, notably China and Japan.

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.

Description[edit]

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.

Growing conditions[edit]

Species[edit]

A hydrangea flower.
Hydrangea flowers, dead in winter.

Uses[edit]

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 the following season.

Hydrangeas are moderately toxic if eaten, with all parts of the plant containing cyanogenic glycosides. However, poisoning in humans is rare, as the plant does not look like an enticing food source.

Maintenance[edit]

Deadwood tends to accumulate on most species, and needs to be pruned out. Pruning is best performed just after flowering in order to ensure new bud sets for the following year. The plants are frequently deadheaded as well after the flowers fade, but in some cases the flowers do provide some winter interest for the garden.

Propagation[edit]

Primarily propagated by cuttings, though some species freely self-seed.

Harvesting[edit]

Flowers can be harvested for use either as fresh or dried flowers. To dry, hang the stems upside-down until fully dry.

Pests and diseases[edit]

Bacterial diseases
Bacterial blight Pseudomonas solanacearum
Bacterial leaf spot Pseudomonas cichorii
Fungal diseases
Bud and flower blight Botrytis cinerea
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

Athelia rolfsii [teleomorph]

Viral diseases
Phyllody Hydrangea ringspot virus
Ringspots Hydrangea ringspot virus
Tobacco ringspot virus Tomato spotted wilt virus

Aphids

Scales

Whiteflies

Mealybugs

  • Citrus Mealybug

Bugs

Beetles

Caterpillars

Mites

Slugs and Snails

Gallery[edit]

References[edit]

  • Flora of China: Hydrangea
  • Flora of Nepal: Hydrangea species list
  • USDA Plant Profile: Hydrangea
  • Hydrangea arborescens images at bioimages.vanderbilt.edu
  • Common Names of Diseases, The American Phytopathological Society
  • Britton, Nathaniel Lord; Addison Brown (1913). An Illustrated Flora of the Northern United States and Canada, Volume 1 (second edition ed.). Dover Publications, inc.. pp. 230-231. 
  • Ann Fowler Rhoads and Timothy A. Block (2000). The Plants of Pennsylvania: An Illustrated Manual. Anna Anisko, illustrator. Morris Arboretum, University of Pennsylvania Press. pp. 455-456. 
  • P. D. Strausbaugh and Earl L. Core (1977). Flora of West Virginia (Second ed.). Seneca Books, Grantsville, W. Virginia. pp. 456-457. 
  • Christopher Brickell and Judith D. Zuk (1997). The American Horticultural Society A-Z Encyclopedia of Garden Plants. DK Publishing. pp. 538-540. 
  • Staff of the L. H. Bailey Hortorium (1976). Hortus Third: A Concise Dictionary of Plants Cultivated in the United States and Canada. Cornell University Press. pp. 578-579. 
  • Pirone, Pascal P. (1978). Diseases & Pests of Ornamental Plants (Fifth Edition ed.). John Wiley & Sons, New York. pp. 309-310. 
  • Cranshaw, Whitney (2004). Garden Insects of North America: The Ultimate Guide to Backyard Bugs. Princeton University Press. pp. 600. 
  • Pippa Greenwood, Andrew Halstead, A.R. Chase, Daniel Gilrein (2000). American Horticultural Society Pests & Diseases: The Complete Guide to Preventing, Identifying, and Treating Plant Problems (First Edition ed.). Dorling Kindersley (DK) Publishing, inc.. pp. 196.