Hydrangeas

Hydrangeas are workhorses in the garden. Most species of hydrangeas can be grown in either full sun or partial shade. They are resistant to most insects and diseases. Hydrangeas should be planted where watering will not be a problem because they need a lot of water and must be irrigated at least weekly to replace moisture loss. Leaves begin to wilt when the plant needs water. Avoid dry windy sites, as their large, soft leaves lose water quickly, especially on hot, windy days, causing the foliage to wilt. For winter interest, flowers remain on the plant in a semi-dried condition long after the leaves have fallen.

Fertilize hydrangeas in early spring with a complete granular fertilizer applied  directly to the soil over the roots. They can be grown in a wide range of soil but prefer a rich, moist, acidic soil. The acidity allows aluminum to be taken up by the plant, thus making the flowers blue. Check the soil pH, but if the flowers start to lose their blue color or start to turn pink it is time to amend the soil (see Color, below). There are also products available that fertilize and acidify.

Species and Cultivars

While there are approximately 23 species of Hydrangea, only five are widely cultivated in the U.S.: H. macrophylla (commonly known as bigleaf, French, garden, or  florist’s hydrangea), H. paniculata (panicle hydrangea), H. quercifolia (oakleaf hydrangea), H. arborescens (smooth hydrangea), and H. anomala subsp. petiolaris (climbing hydrangea).

H. macrophylla: The most popular species is Hydrangea macrophylla, which is commonly known as bigleaf, French, garden or florist’s hydrangea, is a Japanese native that is hardy to Zone 6 and produces large inflorescences of white, pink, or blue flowers in early summer. As with most other Hydrangea species, the inflorescence is composed of a combination of large, showy, and small inconspicuous flowers.

Mophead, or hortensia cultivars, (H. macrophylla var. macrophylla) have many large, round, showy flowers while lacecap cultivars (H. macrophylla var. normalis) have done-shaped flowers with an inner ring of small, fertile flowers surrounded by an outer ring of large, showy flowers. New growth can be on old wood only for some cultavars and on both old and new wood for others. Both mophead and lacecap hydrangeas can be used as specimen plants, in mixed borders, or in mass plantings.

Mophead is a good seashore shrub; flowering is more profuse in an open, sunny location. This, however, increases its vulnerability to bud killing. Winter protection of the plant should be initiated in December to preserve buds for next year's flowering. Tie the shoots together and wrap with burlap. If left unprotected, delay any spring pruning until the buds swell in order to determine which wood needs to be removed, and then cut back to below the point of injury.

The first ever repeat-blooming macrophylla is Endless SummerTM which was discovered by Bailey Nurseries of St. Paul, Minnesota. It is hardy to Zone 4 and will bloom on both old and new wood, providing blooming color all season. Its pink, blue, or white (Blushing BrideTM) mophead type flowers are big and showy. By cutting back to half the emerging growth, the amount of blooms is significantly increased. Deadheading will promote blooms from April through October in warmer regions, and June through frost in colder areas. There are now many similar varieties available.

H. paniculata: This species can be reliably grown in Zones 4 to 7. Panicle hydrangea tolerates more sun than do other species. It produces massive displays of large creamy-white flowers borne in 6- to 18-inch long panicles in mid-to-late summer. Plants, particularly those of the cultivar 'Grandiflora' ('Pee Gee'), are sometimes pruned into a tree form and grown as a specimen plant. Panicle hydrangea is also suitable for use in a mixed border or as a deciduous hedge.

H. quercifolia: The oakleaf hydrangea is one of two Hydrangea species that is native to the U.S. It is found growing primarily in moist woodlands in the Southeast. Large (4 to 12 inches in length) panicles of creamy white flowers are produced in early summer. As flowers age, they often turn a medium- to deep-rose color. Oakleaf hydrangea is the only member of the genus to develop significant fall foliage color. Leaves turn a deep mahogany-red and stay on the plant until late fall. Exfoliating bark adds to winter interest. Oakleaf hydrangea is best suited used in a mixed border or as a mass planting.

H. arborescens: Smooth hydrangea is the other U.S. native. It is found in the eastern U.S. from New York to Florida and west to Iowa and Louisiana. The species is hardy for Zones 4 to 9. Flowering occurs in early to midsummer. The most common cultivar, 'Annabelle', produces rounded inflorescences that may reach up to a foot in diameter. Plants found in the wild typically have a lacecap type inflorescence. At the peak of flowering, smooth hydrangea flowers are a pure white but as they age they develop a pale green color. Smooth hydrangea is extremely striking in mass plantings.

H. anomala subsp. Petiolaris: While not as well-known as the previous four species, climbing hydrangea has become increasingly popular. It is a true clinging vine and it is hardy from zones 4 to 7. While initially slow growing, the plant can eventually cover tall (up to 80 feet) structures. White, lacecap type inflorescences are produced in early- to mid-summer. Plants can be slow to flower, but patience is rewarded by a spectacular floral display on established plants. The stems of climbing hydrangea leave a residue that is very difficult to remove. An alternative to planting climbing hydrangea on the side of a building is to allow it to climb up a tall tree or to cascade over a horizontal surface like a rock pile. Climbing hydrangea grows well in shade, but can also tolerate a sunny location. 

Pruning 

There are many different types of hydrangeas; many require different methods of pruning. First, one must know the identity of the hydrangea in the landscape, so the proper pruning procedure can be implemented. Established bigleaf, panicle, oakleaf, and smooth hydrangea plants can often benefit from regular pruning. Removing about one-third of the oldest stems each year will result in a fuller, healthier plant. This type of pruning is easiest to do in early spring since the absence of leaves makes it easier to see and reach inside plants. Gardeners may also want to prune to control height or to remove old flower heads. The best time for this type of pruning depends on the species. Bigleaf and oakleaf hydrangea, which flower on previous year's growth,should be pruned shortly after flowering is complete. Panicle and smooth hydrangea flower on current year's growth and can be pruned anytime from late summer until early spring. If pruning these two species in the spring, try to prune before leaves appear.

H. macrophylla: Pruning can be accomplished in late summer or early spring. Late summer is more desirable since most of these plants flower only from the end buds of upright or lateral shoots produced during late summer and fall of the previous season. Prune as soon as the flowers have faded and strong shoots are developing from the lower parts of the stems and crown. Remove at the base some of the weaker shoots that are both old and new. Always try to keep several stems of old productive wood, with a sufficient number of stout new stems that will flower the following season. Pruning this species too late in the fall (after September) is harmful. New growth, both vegetative and reproductive, will not develop proper maturity. Early spring pruning (March), although acceptable, could result  in sacrificing of bloom for that growing season if care isn’t taken. Stems that have been damaged by cold should be pruned as soon as it is determined that they are dead.

H. paniculata 'Grandiflora' (Pee Gee Hydrangea): Pruning involves the removal of dead flowers, if unattractive, and an annual corrective pruning of vigorous shoots. Thin and/or cut back the previous season's growth in late winter or early spring, since flower clusters occur on newly developing branches. Without regular pruning, this hydrangea can rapidly become quite overgrown and out of scale in the landscape. It can, however, be developed into a single or multi-stemmed tree form.

H. quercifolia: This plant, if grown in an exposed site, it is subject to some winter dieback. Prune back in early spring to remove dead wood. Cut back to below the point of injury and remove old wood to the base. 

H. arborescens: Prune to the ground line each winter or early spring because it flowers abundantly on new growth and is frequently killed back during winter. If a larger shrub is desired (3+ feet) and/or it is not killed back over the winter, prune less severely. Remove certain branches to the ground; cut others back at varying heights from 1 to 3 feet.

H. anomala subsp. Petiolaris: Climbing hydrangea requires little or no pruning. If certain shoots have grown out of bounds, reduce their length in summer. While there is some concern about climbing vines that may be inundating a tree and causing irreparable damage there has never been a proven case of damage occurring from climbing hydrangea.

Color

The color of the pink and blue varieties is determined by the degree of soil acidity. Pink varieties develop  color best at a soil pH of 6.5 to 7.0 while the best blue color occurs in acid soils at pH 5.0 to 5.5. The anthocyanin pigment in hydrangea flowers will be pink or blue, depending upon the presence of either iron (pink) or aluminum  (blue). Insufficient aluminum is absorbed by the plant at a pH of 6.5 to 7.0 so iron is used. As the pH drops to 5.0 to 5.5, aluminum becomes more soluble, is absorbed by the plant, and becomes part of the anthocyanin molecule resulting in blue coloration.

To maintain a pink color enough limestone should be incorporated into the soil at planting to keep the pH high. Depressing pH with aluminum sulfate will make soluble aluminum available. Since soluble phosphates precipitate aluminum, a fertilizer containing low phosphorous or none at all should be used (13-0-20 and 15-0-15 are examples).

Pests and Disease

While hydrangeas in landscape settings are relatively pest free, under certain growing conditions some diseases and insects can become problems. For bigleaf hydrangea, the major disease problem is powdery mildew. It is most common on plants growing in shade and under high humidity conditions. Infested leaves are covered with a light gray powdery-looking substance. Powdery mildew rarely kills plants, but is unattractive. Powdery mildew may occur on other hydrangea species, but is most severe on bigleaf hydrangea. 

Several fungal leaf spot organisms attack Hydrangea. Leaves develop brown to gray lesions surrounded by purple halos. These leaf spots are most common in late summer and early fall, and seem to be more common among plants grown in sunny locations. Again, plants are rarely killed, but severe infestation can be very unattractive. All the cultivated species of Hydrangea are susceptible to one or more of these leaf spots.

Oakleaf hydrangeas are susceptible to root rots. Infested plants will appear wilted but will not recover when watered and will eventually die. Planting hydrangeas on poorly drained soils will increase incidence of root rots and should be avoided.

Smooth hydrangea is susceptible to rust, which will appear on the back side of leaves as small, orange spots. The disease is usually seen near the end of the growing season and rarely kills plants.

Japanese beetles will feed on oakleaf hydrangea, but are rarely a problem on the other species. The problem is rarely severe enough to merit use of an insecticide.

Aphids can be a problem on the new growth of all hydrangeas, but can be easily controlled by washing, using an insecticidal soap, or an insecticide spray. As leaves become tougher during the growing season, aphid problems usually diminish.

Mites can cause distorted growth, mainly seen in new shoots problems, on hydrangeas. They are too small to see with the naked eye, but webbing between leaves will also be noticed with spider mites. Mite problems are usually worse during hot, dry weather. Adequate watering of plants during hot weather is the best preventative against mite problems. Mites are difficult for the homeowner to control using insecticides.

Bigleaf hydrangea is susceptible to iron chlorosis. Because iron becomes less available as pH increases, iron chlorosis is most likely to be found on plants growing on high pH soil. In contrast to nitrogen deficiency, which is expressed as yellowing of old leaves, iron chlorosis is found on new leaves. Iron chlorosis can be corrected by the addition to iron to the soil. The best way to do this is to use a chelated iron product. Lowering soil pH through application of aluminum sulfate or mulching with acid organic materials like pine bark is a long-term solution for iron chlorosis.

Drying Flowers

One of the easiest ways to preserve hydrangea flowers is to allow them to almost completely dry on the plant. Do not collect them until the flowers have developed a papery feel. On a dry day with low humidity, cut the stems the length you need for making floral arrangements. Strip off all leaves and then find a dry place indoors where the flowers can finish drying. Some people recommend using a warm, dark location, such as an attic, while others prefer a cool, dry location. Flowers can be hung upside down while being dried, or can be placed in a vase with or without water. Whichever method is used, individual inflorescences should be separated as they dry so that none of the flowers get squashed. The flower heads of some cultivars dry better than others.

Holiday Plants

Full blooming potted hydrangeas should be kept in moist soil at all times as this plant has a high water requirement and  tends to dry rapidly in the home. It should also receive direct light. After the flowers fade, they may be removed and the plant treated as a house plant. When danger of frost is past, it can be planted in a sheltered location in the garden. Shelter is necessary because the hardiness of this plant is questionable in northern gardens. This plant forms flower buds in the fall which freeze out or are destroyed in colder climates. Many times the plant will survive and produce green leaves, but no flowers. Planting in a sheltered location plus covering the plant with burlap will offer some protection.

Adapted from: Cornell Cooperative Extension, Suffolk County Long Island New York; Connecticut Cooperative Extension; Virginia Cooperative Extension; The United States National Arboretum

Useful Websites

The American Hydrangea Society

http://www.americanhydrangeasociety.org/

Pruning Hydrangeas (Cornell Cooperative Extension)

http://counties.cce.cornell.edu/suffolk/grownet/tree-shrub-maintenance/h...

Pruning Hydrangeas (NC State University)

http://www.ces.ncsu.edu/fletcher/staff/rbir/hypruning.html

The United States National Arboretum

http://www.usna.usda.gov/Gardens/faqs/hydrangeafaq2.html#How-when-prune