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Growing Hydrangeas

Hydrangeas are among the most popular flowering shrubs for Massachusetts landscapes. Indeed, the "blue" hydrangea (Hydrangea macrophylla or bigleaf hydrangea) is one of the signature plants in the Cape Cod garden. Whether it is in the shrub border, in mixed plantings with perennials, or a specimen flowering shrub, hydrangeas provide color and beauty through the summer and into the fall.

Several different species of hydrangea are grown in our landscapes. In general, they all share similar cultural requirements. Hydrangeas prefer a soil that is consistently moist, well drained and well supplemented with a good quantity of organic material such as compost or well-aged manure. They are not very drought tolerant and will not thrive in very dry, sandy soils. In general, hydrangeas need approximately one inch of water a week, applied in a deep soaking. During very hot and dry weather, they may require up to 2" of water per week to keep from wilting. Drip irrigation or soaker hoses work well in a shrub border of hydrangeas, as they keep the soil moist but also keep the foliage dry, reducing the potential for leaf spots. Hydrangeas may be grown in either full sun or light dappled shade. They do well if shaded from the hot afternoon sun, particularly the bigleaf hydrangea, which may wilt in full sun even if the soil is moist.

Hydrangeas prefer a moderate nutrient level in the soil; fertilize with approximately 4 oz. of a balanced fertilizer in early spring and again after flowering. Before planting, check the root quality of container grown plants and loosen the roots if they appear pot bound or to be circling the container. Amend the planting area with organic material and set the plant so that it is at the same depth as in the container. The top of the root ball should be level with the surrounding soil. Water in the new plant and apply 2" to 3" of good quality mulch, keeping the mulch spread over the roots but off the crown of the plant.

Smooth hydrangea (H. arborescens) is native to the Eastern United States, where it grows as an under-story shrub in woodlands. This species prefers part shade and ample moisture. It grows as a rounded shrub approximately 3'-5' high by 3'-5' wide. It blooms in late June to early July, producing white flowers in clusters 4" to 6" in diameter. 'Annabelle' is a popular cultivar that has extremely large flower heads, up to 12" across. 'White Dome' has an open, lace cap flower head that holds up well in heavy rain. Smooth hydrangea blooms on new wood; prune in late winter or early spring before new growth appears. Smooth hydrangea is hardy to zone 4, but may die-back to the ground in an especially cold winter. It will still bloom the following year as it forms its flower buds on new wood.

Bigleaf hydrangea (Hydrangea macrophylla) is one of the stars in the landscape during the month of July. Growing between 3' and 6' high with an equal spread, it has a rounded habit composed of many stiff, unbranched erect stems. It is late to leaf out in spring and, during cold winters, may die back to the ground. Bigleaf hydrangea produces two types of flower heads: the Hortensia group has flowers clusters made up of many sterile flowers in large globes. The Lacecap group has a center of small, fertile flowers surrounded by a ring of showy sterile flowers, all in a flat-topped cluster.

There is often variation in the flower color of Bigleaf hydrangea, with color ranging from shades of blue through white and deep pink to red. For a Bigleaf hydrangea to turn pink or blue, it has to possess certain pigments. If there is aluminum present in the soil, the flower color will be a shade of blue. If aluminum is absent, the flower color will be pink. Soil pH determines the availability of aluminum: in soils with a low pH, aluminum is more available; in higher pH soils, it is less available. If soil ph is 5.5 and below, the flowers will be blue; at soil pH 6.5 and above, flowers will be pink, rose or red, depending on cultivar. The intensity of color depends on how much pigment is in a given cultivar. 'Nikko Blue' will always be either pastel blue or pastel pink while 'Dooley' will be rich blue or pink.

There is much confusion surrounding the pruning of Bigleaf hydrangea. Typically, flower buds form on mature wood in summer and bloom the following year in late June or early July. The primary flower bud is on the tip of the stem, with additional buds forming lower along the stem in leaf axils. If the primary bud flowers, these additional buds remain dormant. If the primary bud is winter killed, some of these buds may bloom. However, winter temperatures below 10 ºF, as well as early fall and late spring freezes, may kill all flower buds, resulting in little if any bloom the following summer.

For the first few years after planting, simply remove old flowers when they turn brown. Cut them off just above the next set of leaves. As the plant matures, maintain good structure by removing the oldest stems (3 yr+) right to the ground in late winter. In addition, remove any dead tips and dead stems at that time as well. If a Bigleaf hydrangea needs pruning to manage height, cut it back just above a set of leaves as soon as the flowers begin to fade. Better yet, plant it where it can grow to its normal height and width.
New developments in the breeding of Bigleaf hydrangea have produce cultivars that can bloom on new wood as well as mature wood. This "remontant" characteristic allows blooming even if the flower buds are winter killed. 'Endless Summer', Penny Mac', and 'Dooley' are among the cultivars that have this re-blooming capacity. Bigleaf hydrangea is hardy to zone 6.

Panicle hydrangea (H. paniculata) has been used in the landscape for many years and is commonly, if erroneously, referred to as the Pee Gee hydrangea. This large shrub or small tree can reach a height between 10' and 20', with an equal spread. It blooms from July into September, producing large, pyramidal panicles 6" to 8" long. The panicles are composed of showy, sterile flowers interspersed with fuzzy fertile flowers. The flowers are white but over time change to a mauve-pink color, which persist into the fall. Of all the hydrangea species, panicle hydrangea is the most adaptable. It is more tolerant of dry spells and full sun and is the hardiest of the species. It blooms on new wood; prune in late winter or early spring to manage height if needed. Panicle hydrangea is hardy to zone 3.

The old-fashioned Pee Gee hydrangea is the cultivar 'Grandiflora' and it often grows with one primary trunk in a tree form. Heavy flower clusters, 12" to 18" long, cause the branches to arch over, creating a fountain-like affect. Newer cultivars entering the market offer a better plant for the garden. 'Kyushu' is a vigorous, upright selection with more fertile than sterile flowers, giving it an open, airy affect. 'Limelight' has lime green flower buds opening to a pale greenish white color and grows 6' to 8' tall and wide. 'Tardiva' is a late flowering cultivar that blooms from late August to October and grow 8' to 12' tall. 'Little Lamb' is a short cultivar that only reaches 4' to 6'. Its flowers are also small and delicate, with the flower heads said to resemble "little lambs dancing over the plant".

Oakleaf hydrangea (H. quercifolia) is native to the southeastern quarter of the United States. Its large, lobed leaves resemble oak leaves and are up to 8" long and 5' wide. Oakleaf hydrangea grows with upright stems, reaching a height of 8'. It slowly forms a clump by producing stems from the roots and develops a shaggy, reddish bark with maturity. In July, large, white conical flower heads appear, consisting of both fertile and sterile flowers. The flower heads can be up to 12" long and 4" wide at the base. The flowers dry to an attractive pink-mauve as they age and remain attractive in to the fall. The fall foliage can be spectacular, changing to shades of scarlet and burgundy. The flower buds are formed on old wood; prune immediately after bloom if necessary.

'Snow Queen' has been on the market for some time and is a great cultivar. It is a more compact grower, reaching 6' at maturity, and tolerates sun well. Its flower heads are 6"to 8" long and are a creamy white color. 'Alice' is one of the newer cultivars and is quite vigorous, growing to 10' to 12' high. It produces very large flower heads, up to 12" to 14" long with individual florets the size of a fifty-cent piece. 'Snowflake' has multiple bracts that give the flowers the appearance of being doubled. The 12" to 15" flower heads are so heavy that they arch over, creating a weeping effect. This cultivar prefers moist soil and partial shade. 'Pee Wee' is a compact form, only reaching 3' tall by 3' wide, making it a good candidate for smaller gardens. The flower heads are also compact, about 5" long. "Pee Wee" has good fall color. Oakleaf hydrangea is hardy to zone 5.

While not a shrub, Climbing hydrangea (H. anomela petiolaris) certainly deserves mentioning, as it is one of the most spectacular climbing vines available. It has the same growing requirements as other hydrangeas and but is slow to develop after planting. Once established, it may climb to 60'. Climbing hydrangea climbs by clinging to surfaces with root-like holdfasts. It is often planted at the base of a tall tree or against walls or other structures. It blooms in June, producing large, flat lace-cap type flowers up to 10" across that remain effective for up to two weeks. The flowering stems often protrude horizontally from the surface it is growing on, creating a three dimensional effect. Fall color is golden yellow. This is truly a magnificent vine. Climbing hydrangea is hardy to zone 4.

Written by: Roberta Clark
Revised: 08/2011