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Missouri Environment & Garden



AUTHOR

David Trinklein
University of Missouri
Division of Plant Sciences
(573) 882-9631
trinkleind@missouri.edu

Hydrangea: The chameleon of plants

David Trinklein
University of Missouri
(573) 882-9631
trinkleind@missouri.edu

Published: March 7, 2018

hydrangea illustration

From the Easter season through Mother's Day, hydrangea is one of the most sought-after flowering plants from retail florists and garden centers alike. It's huge, globe-like clusters of blooms impart a regal elegance that is uncommon among potted plants. Unlike other flowering plants, however, the flower color of most hydrangeas can be changed from year-to-year, with a little help from their caregiver. Many gardeners, therefore, consider hydrangea to be the chameleon of the plant world.

Hydrangea is a genus in the plant family Hydrangeaceae. The genus contains nearly 75 species of shrubs or small trees, most of which arenative to the region of Asia now occupied by China, Korea and Japan. The word, hydrangea, is derived from the Greek hydro meaning "water" and angeion meaning "vessel". Although the name was given to the plant because of the shape of its seed pods, it is fitting in another way. Hydrangea plants have a very high water requirement and should never be allowed to dry out.

Florists' hydrangea (Hydrangea macrophylla var. macrophylla) has been an important greenhouse crop for decades. It produces flowers in flowerheads that (botanically) are classified as either corymbs or panicles. Individual flowers of florists' hydrangea contain large, showy sepals which surround a center core of smaller, less conspicuous flowers. The color of hydrangea flowers can be controlled by altering the plant's soil environment. Flowers are pink if the plant is growing at a soil pH that is nearly neutral. Blue flowers can be produced by acidifying the soil with aluminum sulfate. It actually is the element aluminum which turns the flowers blue. However, aluminum is more readily available for plant uptake at low pH values, hence the need to keep pH low. The color of white hydrangea flowers cannot be changed.

When grown as a potted plant, as mentioned above, it is essential to keep hydrangea moist at all times. Given adequate amounts of water, potted hydrangeas may be kept attractive indoors for several weeks. If the plants are allowed to dry out, the flowers will collapse quickly, even before the leaves show any sign of wilting. Once flowers become badly wilted, they will never recover.

After potted hydrangeas have finished flowering, one of several choices must be made. The plant may be discarded, kept for reblooming or planted outdoors in the garden. Your choice might depend upon your geographical location as well as your interest in gardening. If you have little or no gardening interest, then enjoy the plant until the flowers wither and then discard it. If you like a challenge, you might want to attempt to rebloom the plant.

To rebloom a hydrangea indoors, cut its shoots back after the plant has finished flowering so that two pairs of leaves are left on each shoot. If necessary, repot using a soilless growing medium containing a high percentage of peat moss. After the danger of frost has past, move the plant outside and sink the pot into the soil where it gets full morning sun but light afternoon shade. Water the plant regularly and fertilize with a complete liquid fertilizer about every two weeks. For extra-large flower heads, allow only about three stems to develop. When removing extra shoots, take out those that grow toward the center of the plant. Lift the pot occasionally during the summer to keep root growth from moving outside the pot. To keep shoots from becoming too long, pinch them back during the summer. The last pinch should be made no later than July.

Keep the plant outdoors as long as possible in the fall, but bring it indoors before a hard freeze. Allow the plant to retain its leaves until about November 1st. Then, pick off all the leaves by hand or put the plant in total darkness until all leaves drop naturally. The leafless plant must be vernalized (exposed to cool temperatures) in order to induce subsequent flowering. This involves keeping it at temperatures from 35 to 40 degrees F for about six weeks. During this time, the plant can be kept dry since it has no leaves.

After the cooling period, move the plant to a sunny, cool room, with night temperatures of 55 to 60 degrees F. Water it well and fertilize about every two weeks. It should flower in about four months.

Alternatively, if you live in the southern third of Missouri (hardiness zone six) potted hydrangeas can be planted in the garden for years of enjoyment. Wait until the danger of frost has past and make sure to water the plant regularly until it is established in the garden.

In the garden, hydrangea is prized both for its foliage and its flowers. Flower buds form on the plants in the fall; therefore, these buds must survive the winter if they are to flower the following summer. Keep in mind that the flower buds are not as cold hardy as the remainder of the plant. During severe winters protection from the cold might become necessary.

Large baskets or boxes may be inverted over the plants to help protect the tender buds. Alternatively, a wire cylinder filled with a loose mulch can be place over the plants. Evergreen boughs work well. Loose mulch is important since a dense mulch that retains moisture can promote disease infestation of the flower buds. In milder winters, protection is not necessary.

Other types of hydrangeas are more suitable for outdoor, garden conditions. One of these is 'Hills-of-Snow' hydrangea (Hydrangea arborescens). Unlike florists' hydrangea, this plant is able to flower on new growth. Therefore, plants can be pruned more severely and still flower well. Although available only in white, it bears large flower clusters that are very attractive.

'Pee Gee' hydrangea (Hydrangea paniculata) is a woody, tree-like hydrangea that produces white flowers in large clusters that are somewhat pyramidal in shape. In mild climates, it may eventually reach a height of 25. Somewhat tender, 'Pee Gee' may suffer from winter damage at our latitude which tends to keep plants smaller.

Oak-leaf hydrangea (Hydrangea quercifolia) is very attractive and well adapted to our climate. It has large, showy leaves which turn red in the fall. It grows and flowers well in shade and is a good choice for difficult, low-light areas. It produces white flowers in early summer which gradually change color to pinkish-purple. The latter color is maintained until the flowers turn brown in the fall. Oak-leaf hydrangea is a shrub with relative few problems and deserves more attention than it currently receives.

Whatever the species, it must be noted that hydrangea tissue contains cyanogenic glycosides and is considered moderately toxic. Therefore, hydrangea should be handled with proper care and kept away from children and pets.

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REVISED: February 21, 2017