HYDRANGEAS: Classic Garden Shrubs
Mary Perdue-Jones, Provisional Master Gardener 2004
Celebrated for their beauty, durability, and diversity, hydrangeas are enjoying a renaissance in today's gardens. Their name is derived from the Greek "hydra" meaning "water" and "angeon" meaning "vessel" referring to the plant's preference for moisture and to the shape of the seed capsule. Known for their bold leaves and long-lasting clusters of flowers in shades of white, pink, red or blue, hydrangeas are classic garden shrubs that add plenty of impact to the garden.
History:
Hydrangeas have been cultivated in western gardens since the American plant H. arborescens was brought to England from Pennsylvania in 1736. In the Orient, hydrangeas had doubtless been growing in the gardens of China and Japan long before any records were kept; however, the first to reach Europe was one brought from China by one of Joseph Banks’ protégés. That plant, first named H. hortensis, was later renamed ‘Sir Joseph Banks’.
During the 17th and 18th centuries, hydrangeas increased in popularity. As Japan was closed to foreigners from 1639 to 1856, access to Japanese plants was restricted. With the establishment of the Dutch East India Company’s trading post on the island of Deshima, in Nagasaki Bay, a base for a few intrepid plant hunters was provided. Subsequently, seeds and plants of Japanese species were introduced and gradually reached Europe.
Classification:
The genus comprises flowering shrubs, small trees, and climbing plants and is divided into two sections. The first, Hydrangea, is comprised of eleven deciduous species. The second, Cornidia, contains twelve evergreen self-supporting climbers.
Reclassification and renaming has been a feature of their history, especially among the most popular macrophylla species. Many names that have emerged over the years and this proliferation may be partly due to large variability within the species, particularly the changes in inflorescence. Leaf size and shape vary as well, not only from one shrub to another - but within the same plant. This capacity for change presents problems for even the most dedicated taxonomist.
Native Species:
Today, gardeners are showing a renewed interest in hydrangeas, particularly in the native American species arborescens and quercifolia. H. arborescens is a native of the Eastern United States and shows considerable variations throughout the regions where it occurs. A small, open and erect deciduous shrub, H. arborescens is hardier than many other hydrangeas (zone 4/5) and grows to approximately 3 feet tall. The inflorescence is a fairly flat corymb, 2-6 inches across and is comprised of numerous small, whitish, fertile flowers and may or may not include a few large sterile flowers on long pedicels. The leaves are dark green above and paler beneath, with ovate to acuminate tips. Flowering time is from July to September.
H. arborescens is divided into three subspecies: H. arborescens ssp. arborescens; H. arborescens ssp. discolor; and H. arborescens ssp. radiata. A quick way to distinguish between the three is by looking at the reverse side of the leaf: arborescens – the leaves are green beneath; discolor – the leaves are gray beneath; and radiata – the back of the leaves are white. This species and its cultivars flower on the new season’s wood, tolerates light shade and some degree of drought. Two favorite cultivars are ‘Annabelle’ and ‘Grandiflora’.
Found in the southeastern part of the United States, H. quercifolia is one of only two in the genus to have pinnate leaves. An open deciduous shrub, this plant grows 3 to 7 feet tall. The inflorescence is an open panicle 6-10 inches long and up to 5 inches in diameter at the base. The numerous white sterile flowers with long pedicels and dense clusters of white fertile flowers bloom from July to September. The leaves are 3-10 inches long and 2-7 inches wide. Usually 5-lobed, the leaves resemble an oak leaf, hence the term oak leaf hydrangea. The color of the leaves, light green early in the year, darkens to shades of gold and crimson in the autumn, making this plant a splendid addition to the garden. Readily available, H. quercifolia grows vigorously and produces large inflorescences on erect stems. A few favorite cultivars are ‘Harmony’, ‘Snowflake’, and ‘Snow Queen’.
Culture:
Hydrangeas are relatively easy to grow. They are hardy in most gardens in USDA zones 6 through 10, though those that bloom only from tip growth may never bloom where heavy freezing is common. Hydrangeas have a long life span, so it advisable to have some idea of the size to which your plant will grow before choosing a site.
Hydrangeas are natural woodland plants and, in general, demand only two things: moisture and shade. Too much of either is not advisable, however. Planting hydrangeas in too heavy shade is one of the most common mistakes gardeners make. Most hydrangeas are happy with a good dose of early morning direct sun or dappled all day sunlight. Hours of midday sun can bleach the flowers and scorch the leaves. No hydrangea will tolerate drought; however, don’t assume that a wilted plant needs to be watered. Hydrangeas, like some people, will wilt in the summer heat. As long as the soil is moist, a wilted hydrangea will perk right up after the sun goes down without extra watering. Remember that overly wet soils can compact, lose aeration, and ultimately damage the plant the extra water was intended for.
Most hydrangeas flower on the previous year’s wood. Flower buds are carried through the winter and will begin to develop in spring. They are at their most tender in April and May and can be completely destroyed by searing cold winds. More flower buds will develop later in the year but will not produce until the following year so some protection is advisable.
Soils should be well amended so as to accommodate the very fine root systems, and mulch is recommended to preserve moisture. Pay attention to the soil pH! Hydrangeas tolerate a range of pH from 5.0 to 7.5, but too high a pH can be detrimental to the plants. Signs of chlorosis can indicate a soil too alkaline for the proper uptake of nutrients.
All blooming plants need a good schedule of fertilization and hydrangeas are no exception. A balanced fertilizer such as 8-8-8 or 10-10-10 sprinkled around the plant at the drip line in March, May, and July will do well. If you want to avoid repeat applications, apply a time-release formula in early March.
Color Change:
One fact that is widely known about Hydrangea macrophylla and its cultivars is that they can change color. It is widely assumed that all hydrangeas are alike and if a different color is desired, we should merely apply different chemicals to affect flower color. Color is ultimately determined more by variety than by what we do to the soils they grow in. Two factors affect the color in Hydrangea macrophylla cultivars: soil acidity and the presence of aluminum in the soil. Flowers can be made to “blue” only when aluminum is present in the soil. Most soils contain aluminum, although if it is absent we can add it in the form of aluminum sulphate. If the soil is alkaline, however, plants are unable to absorb the existing aluminum and the flowers will not blue. It is important when addressing the pH of your soil to remember that the ideal range of pH for the maximum absorption of nutrients is in the range of pH 6.0 to pH 7.0.
It is more difficult to acidify soils. The addition of lime to the soil will boost alkalinity, but increased acidity is achieved only by more laborious methods. The addition of acid organic matter such as peat moss will gradually affect soil acidity, and the use of acid plant foods, such as those formulated for azaleas and rhododendrons will help as well. A soil test will save much trial and error. Achieving a pH of 7.0 to 7.5 will assure good pinks and reds, with 5.5 to 6.5 being ideal for good blues, as long as a supply of aluminum exists as well.
Remember to be patient. If you know your soil is acid and have supplied some supplemental aluminum, the flowers will turn blue, but it may take the better part of a year. Be sure to supply aluminum on an annual basis; for the best blues feeding twice a year with aluminum sulfate dissolved in water and applied evenly over the roots is best. Aluminum can be reused by the plants but only if the flower heads are left until the following spring, when the element will be reabsorbed into the stems. When flowers are removed as fresh cut or dried specimens the aluminum will be removed as well.
Pruning:
How and when to prune hydrangeas is an issue for many gardeners, even those with experience. No pruning is better than wrong pruning when it comes to these shrubs. Provided they are fed and watered, hydrangeas will grow and flower if they are never pruned. Correct pruning can help the plant and rectify mishaps such as storm damage. Annual cosmetic pruning should never take place until after the last spring frost. Below are pruning guidelines for five popular types of hydrangea:
Bigleaf Hydrangea (Hydrangea macrophylla)
Flowers bloom on last year’s wood. In early spring remove older, woodier branches at ground level for more light and air circulation. After the last frost date, clip or pull off any frost damaged leaves. Do major pruning and reshaping after the blooms begin to fade. On a mature bush (5-6 years old) remove 1/3 of the oldest woody stems each year by cutting to the ground. Encourage new foliage growth by deadheading.
Oakleaf Hydrangea (Hydrangea quercifolia)
This is a large shrub that should be left to grow in its natural form. Flowers bloom on previous season's wood. Prune after flowering only to remove dead wood, old flower heads and 1/3 of the oldest wood to increase light and air circulation. If necessary, severe pruning (back to only two buds at the base of each stem) may be done in late February to early March to rejuvenate plants.
Peegee Hydrangeas (Hydrangea paniculata)
Flowers develop on new wood. Cut back severely to leave only two buds at the base of each stem in early March or allow plant to develop without pruning. Blooms will still appear on new growth. Severe early pruning produces a smaller plant with larger blooms; not pruning produces a larger plant with many smaller blooms.
Smooth Hydrangea (Hydrangea arborescens)
Flowers bloom on current year's wood. Prune entire plant back to 6-12” from ground in early March only when necessary.
Climbing Hydrangeas (Hydrangea anomola petiolaris)
Little or no pruning is required. In the summer, if needed, remove unwanted shoots that have grown out of bounds.
Pests and Diseases:
Hydrangeas are generally hardy plants that, once established in the garden, have few enemies or ailments. The greatest hazards are cold winds and frost. Deer and rabbits avoid H. macrophylla, but all other species are vulnerable to their ravages. Aphids cause the leave to curl, damage flowers and can spread viral disease. Scale insects form brown scales on the underside of leaves, usually along the veins. Thrips cause discoloration of leaves and flowers. Slugs and snails love young hydrangeas so it is essential to maintain some type of deterrent against these pests.
There are a few diseases that can affect hydrangeas, including rust and leaf spot. Gray mold can occur during propagation and powdery mildew can appear on leaves, stems and buds. Affected parts may become distorted and the entire plant weakened. Preventative measures include proper mulching, regular watering in dry summer spells and pruning out old wood to allow good air circulation. Treatment by a fungicide is recommended when necessary.
Propagation:
Hydrangeas are some of the easiest shrubs to propagate. To ground layer, select a branch close to the ground, remove the leaves for about 5-6 inches at the spot where the branch touches the ground, and scrape a little of the bark off the under-side of the branch in this area. Make sure at least one leaf node will be under the ground. Do not cut the branch off the mother plant. Dig a little trench about 2 inches deep, lower the branch into it, and cover generously with soil. Put a brick or stone on the buried area so that it will stay under the soil. Keep it watered occasionally. When roots form, the branch can be removed from the mother plant, potted, and treated like a mature cutting.
Softwood cuttings should be taken from H. arborescens and H. paniculata before the end of June. All other hydrangea cuttings may be taken at any time from April through October. Take a 5-6" long cutting from a branch of the hydrangea shrub. Remove the lower leaves of the bottom two leaf nodes and cut the largest leaves down to about half their size. Dip cuttings in rooting hormone and insert into damp vermiculite or sterile medium. Water the pot well and allow it to drain. Cover cuttings and pot with plastic. Keep the plastic from touching leaves by using stakes. Place the cuttings in bright light, and do not water again until the top of soil begins to feel dry as over watering will cause cuttings to rot. Expect cuttings to begin forming roots in 2-3 weeks depending on temperature and humidity.
Hardwood cuttings are 8-10 inch lengths of the current years stem growth taken from the plant in late autumn after the leaves have fallen. Make a horizontal cut just below a node and trim the upper end of the cutting at an angle (This allows you to identify which is the top and which is the bottom). Dip the cutting in rooting compound, shake off any surplus and insert vertically into a pot filled with sand. This pot can then be kept in a cold greenhouse until rooting occurs, usually by next summer. A quicker version of this method is to place the potted cuttings on a windowsill or in a heated propagator at approximately 60 degrees and given sufficient water to keep just moist until new growth appears. Once rooted, the cuttings may be individually potted. Keep cool and frost free until ready to plant outdoors.
Excellent as specimen plants or in containers, the hydrangea is currently enjoying a well-deserved resurgence in popularity. As you explore the world of hydrangeas you will discover their distinctiveness and their wide range of beauty. Planted in masses, their impact is stunning. But, even if you settle for only one, the hydrangea deserves a place in your garden.
References:
http://apps.caes.uga.edu/urbanag/factsheets/HydrangeaJun02.htm
www.vintagegardens.com
hydrangeashydrangeas.com
Hydrangeas: A Gardener’s Guide (Timber Press, 2005)
American Horticultural A-Z Encyclopedia of Plants