Common Hydrangeas and Less Common Friends
The blue flowers of bigleaf hydrangea (Hydrangea macrophylla) are the epitome of Cape Cod in the summer. Hydrangea species are some of the most popular landscape plants, which is evident by the number of new cultivars introduced every year. Hydrangea macrophylla is likely the best-known Hydrangea species worldwide. Hydrangea paniculata, or panicle hydrangea, has increasingly grown in popularity in American gardens due to better winter hardiness, greater adaptability, and better drought tolerance compared to other Hydrangea species. The number of distinct species of Hydrangea is debated by taxonomist and plantsmen, with sources ranging from 23 to 80 or more! The increasing number of hybrids and cultivars furthers the confusion, with hundreds of cultivars of Hydrangea macrophylla alone. There are six common species of hydrangea grown in American gardens:
- Hydrangea macrophylla (bigleaf, French, or mophead hydrangea)
- Hydrangea paniculata (panicle hydrangea)
- Hydrangea arborescens (smooth hydrangea)
- Hydrangea quercifolia (oakleaf hydrangea)
- Hydrangea anomala ssp. petiolaris (climbing hydrangea)
- Hydrangea serrata (mountain hydrangea)
Of the common species, H. quercifolia and H. arborescens are native to Eastern North America. Less common hydrangea species that can sometimes be found in the trade include Hydrangea angustipetala, Hydrangea aspera, Hydrangea hetermalla (wooly hydrangea), and Hydrangea involucrata. Dr. Michael Dirr also mentions some additional species in his book Hydrangeas for American Gardens.
Soil. Hydrangeas need consistently moist, well-drained, soil rich in organic matter. Panicle hydrangeas are tolerant of a wider range of soil conditions as long as they are well-drained.
Light. Bigleaf, panicle, oakleaf, smooth and mountain hydrangea need full sun to part shade, but perform best when provided morning sun and afternoon shade or dappled light. It is important for moisture to be adequate when plants are sited in full sun. Panicle hydrangea is the most tolerant of full sun conditions. Climbing hydrangea can do well in full sun to shade.
Water. Adequate water is important for healthy hydrangeas. Bigleaf hydrangea in particular is very sensitive to drought-stress and may sometimes wilt even when water is adequate due to its high transpiration demand. In general, about 1” of water per week is needed. During especially hot, dry weather, additional water may be needed.
Fertilization. Hydrangea have moderate nutrient requirements. Slow-release fertilizers are great for providing nutrients slowly over time. Liquid fertilizer can provide more rapid greening. Keep in mind that there is such a thing as too much of a good thing; too much nitrogen can result in longer than normal stems with fewer flowers.
Flower Color. The flower color of bigleaf and mountain hydrangeas is the result of aluminum availability in the soil. Lower pH (or more acidic soils) have more available aluminum, which results in blue flowers. Higher pH results in lower aluminum availability and pink flowers. Soil pH of 5.5 or less results in blue flowers and above 6.5 leads to pink flowers. In between 5.5-6.5 are shades of purple. Aluminum sulfate can be added to the soil to make flowers bluer; apply according to package recommendations. The intensity of flower color is also cultivar dependent.
Pruning. Care should be taken when pruning hydrangeas since different species and cultivars of a species set buds at different times of the year. Bigleaf, oakleaf, and mountain hydrangea normally bloom on old wood and should be pruned after flowering. Newer cultivars of bigleaf and mountain hydrangea bloom on both new and old wood. It is best with these cultivars to plant them where they can grow to size without pruning (or be realistic that pruning will reduce the number of blooms). Panicle and smooth hydrangea bloom on new wood and should be pruned in late winter to early spring. Winter dieback can be common with bigleaf and smooth hydrangeas. Before pruning, it is best to wait until growth resumes in spring and then prune out any stems that do not have new growth.
Winter hardiness of some hydrangea species can be problematic in Massachusetts. Panicle hydrangea and climbing hydrangea are the hardiest of the common species. Bigleaf and mountain hydrangea are hardy to zone 6(5) and are best sited in more protected areas in most areas of the state.
|Hydrangea anomala ssp. petiolaris
||Vine: 30-40’ long
||Lacecap; creamy white to greenish to yellow; fragrant
|| Shrub: 3-5’ x 3-5’
||Pale yellow; not ornamental
||Shrub: 3-6’ x 3-6’
||Rounded or lacecap; color can be pH dependent; pink, blue, purple, white, green
||Old (some cultivars bloom on new and old)
||Pale yellow; some cultivars can have good color
||Shrub: 8-15’ x 6-10’
||Panicle; white; flower can mature to lime, pink, or red
||Shrub: 6-8’ x 6-8’
||Panicle; white; flower can mature to pink or red
||Red to red-purple
||Shrub: 2-4’ x 2-4’
|| Lacecap; color depends on soil pH, pink, blue, purple
||Old (some cultivars bloom on new and old)
||Leaves can develop red tints in summer that deepen in fall
*Cultivar can alter size, flower color
Other Members of the Hydrangeaceae Family That Look Similar to Hydrangea Species
- Schizophrgma hydrangeoides (Japanese hydrangea vine) – similar in look to climbing hydrangea (hydrangeoides means “like the genus Hydrangea”).
- Deinanthe bifida (false hydrangea) – herbaceous perennial, leaves resemble those of hydrangea and flowers somewhat resemble hydrangea flowers.
- Platycrater arguta, cobweb flower – overall resembles mountain hydrangea but with narrower leaves. Zones 7-9.
Hydrangea macrophylla (bigleaf, French, or mophead hydrangea)
There are 100’s of cultivars, with new ones introduced every year! Cultivars can offer different flower attributes (lacecap vs. mophead, double flowers, larger flowers, longer flowering, repeat bloomer, etc.), dwarf sizes, mildew resistance, variations in foliage (dark green, red veins, fall color, variegated leaves).
Hydrangea paniculata (panicle hydrangea)
Dozens of cultivars and varieties are available. Newer cultivars offer smaller sizes (‘Little Lamb’, ‘Jane’ Little Lime™, ‘PIIHP-I’ Baby Lace™), variations in flower colors (lime-white, pink, white aging to pink), size of flowers, etc.
Hydrangea arborescens (smooth hydrangea)
The species is seldom planted as cultivars offer a better floral show. Cultivars include larger flowers, rounded flowers, pink flowers, double flowers, lacecap flowers, and stronger stems.
Hydrangea quercifolia (oakleaf hydrangea)
Not used as much as it could (or should) be. Offers multiple seasons of interest with interesting oak-shaped leaves, summer flowers, good fall color, and exfoliating bark. Cultivars offer larger flowers, flowers that age to variatons of pink and red, smaller size, and double flowers.
Hydrangea anomala ssp. petiolaris (climbing hydrangea)
True clinging vine, can also grow as a woody groundcover. Not many cultivars, but there are a couple that offer variegated leaves or multi-colored emerging leaves.
Hydrangea serrata (mountain hydrangea)
Similar features to bigleaf hydrangea, but smaller (although not always true of cultivars). Cultivars include lacecap and mophead flowers, double flowers, and improved cold hardiness.
Dirr, M. 2004. Hydrangea for American Gardens. Timber Press.
Dirr, M. 2011. Dirr’s Encyclopedia of Trees and Shrubs. Timber Press.
Mandy Bayer, Extension Assistant Professor of Sustianable Landscape Horticultur