US National Arboretum

 


Hydrangea Questions and Answers

  • How do I change the color of my hydrangea?
  • How and when should I prune my hydrangeas?
  • Are hydrangeas bothered by any diseases or insects?
  • Why are the new leaves on my bigleaf hydrangea yellow?
  • How can I dry hydrangeas?

  • What types of hydrangeas are available?

    image of Hydrangea macrophylla var. macrophylla - mophead While there are approximately 23 species of Hydrangea, only five are widely cultivated in the U.S. The most popular species is Hydrangea macrophylla, which is commonly known as bigleaf, French, garden or florist’s hydrangea. This Japanese native is rated as hardy to USDA cold-hardiness zone 6. It 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. In mophead, or Hortensia, (H. macrophylla var. macrophylla) cultivars, many showy flowers are arranged on the outside of the rounded inflorescence (see image at right). On the interior of the inflorescence, a few small flowers are present; these are the flowers that produce seed.

    image of Hydrangea macrophylla var. normalis - lacecap



    Lacecap cultivars (H. macrophylla var. normalis) have an inner ring of small, fertile flowers surrounded by an outer ring of large, showy flowers (see image at left). Growing 4 to 6 feet in height and width, both mophead and lacecap hydrangeas can be used as specimen plants, in mixed borders or in mass plantings.

     

    image of Hydrangea macrophylla subsp. serrata


    The mountain hydrangea (H. macrophylla subsp. serrata) is a subspecies of bigleaf hydrangea that has slightly smaller leaves and flowers (see image at right). It is often listed in nursery catalogs as H. serrata. Cultivars may have either mophead or lacecap type flower arrangements. Delicate in appearance, mountain hydrangea is particularly well suited for placement at the edge of a wooded area.


    image of Hydrangea paniculata - panicle white flower



    Panicle hydrangea (H. paniculata) is the most cold hardy member of the genus. It can be reliably grown in USDA cold-hardiness zones 4 to 7. Native to Asia, it grows 10 to 15 feet tall. Large creamy-white flowers, which are borne in 6- to 18-inch long panicles, are produced in mid-summer (see image at left).


    As flowers mature, they may turn pink (see image below).

    image of Hydrangea paniculata - panicle pink flower





    Plants, particularly those of the cultivar 'Grandiflora' ('Pee Gee'), are sometime 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.



    image of Hydrangea quercifolia - oakleaf plant


    The oakleaf hydrangea (H. quercifolia) is one of two Hydrangea species that is native to the U.S. (see image at left) It is found growing primarily in moist woodlands in the southeastern U.S. Plants generally grow 6 to 8 feet in height, although a few cultivars with smaller and larger plant habit are available.


    image of Hydrangea quercifolia - oakleaf white flower



    Large (4 to 12 inches in length) panicles of creamy white flowers are produced in early summer (see image at right).




    image of Hydrangea quercifolia - oakleaf pink flower



    image of Hydrangea quercifolia - oakleaf pink flower

    As flowers age, they often turn a medium- to deep-rose color (see images at left and right).



    Oakleaf hydrangea is the only member of the genus to develop significant fall foliage color.

    image of Hydrangea quercifolia - oakleaf red foliage





    Leaves turn a deep mahogany-red (see image at left) 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.



    image of Hydrangea arborescens 'Annabelle' - smooth


    Smooth hydrangea (H. arborescens) 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. In cultivation, plants usually reach about 5 feet in height, with a similar or greater spread. The species is rated as hardy from USDA Cold Hardiness Zones 4 to 9. Flowering occurs in early to mid-summer. The most common cultivar, 'Annabelle', produces rounded inflorescences that may reach up to a foot in diameter (see image at right).

    Plants found in the wild typically have a lacecap type inflorescence consisting of a combination of a few large and many small flowers. At the peak of flowering, smooth hydrangea flowers are a pure white. As they age, they develop a pale green color. Smooth hydrangea is extremely striking in mass plantings.

    image of Hydrangea anomala subsp. petiolaris - climbing
    While not as well-known as the previous four species, climbing hydrangea (H. anomala subsp. petiolaris) is becoming increasingly popular (see image at left). Hardy from zones 4 to 7, climbing hydrangea is a true clinging vine. 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. The scientific name of this plant is listed by some sources as H. petiolaris.

    Where can I purchase hydrangeas?

    Most local garden center and mass merchandisers carry hydrangeas, but may only offer a few cultivars. Several specialty nurseries, many of which offer on-line sales, carry a wider array of hydrangea cultivars. Gardeners looking for the newest cultivars and widest selection should consider searching the internet for mail order sources of hydrangeas if they are unable to find them locally.

    Where in the yard should I plant my hydrangea?

    image of Hydrangea landscape Hydrangeas grow best in moist, well-drained soil. Most hydrangeas benefit from some shade, especially in hot climates. Bigleaf, oakleaf and smooth hydrangeas will usually perform well on the north side of a house or planted at the edge of a woodland (see image at right). As discussed earlier, growing hydrangeas in deep shade is not necessary and can greatly reduce flowering.

    The amount of sun that hydrangeas can tolerate depends on species, climate and availability of water. Panicle hydrangea tolerates more sun than do other species. Plants grown in hot climates require more shade than do those grown in the colder limits of adaptation. In southern climates, providing frequent and adequate watering will allow hydrangeas to tolerant more sun than if they were subjected to moisture stress.

    Why doesn’t my hydrangea bloom?

    There are three possibilities for lack of flowering among the hydrangea species. The first two – too much shade and improper pruning – apply to all hydrangeas, while the other – weather-related damage to flower buds – applies primarily to the bigleaf hydrangea.

    While most Hydrangea species benefit from some shade, too much shade can reduce flowering. This is particularly true of panicle hydrangea, which is the one Hydrangea species that grows well in full sun. If you have a hydrangea that used to bloom well but now flowers only sparsely, evaluate whether the growth of nearby trees has reduced the amount of light that reaches the hydrangea. If so, you may want to consider moving the hydrangea to a sunnier location.

    Improper pruning can also reduce flowering in Hydrangea. Since bigleaf and oakleaf hydrangeas flower on previous year’s growth, potential flowers buds would be removed if the plants were pruned in fall, winter or spring. Panicle and smooth hydrangea flower on this year’s growth, so pruning them in early summer would reduce or eliminate flowering for that year.

    The most common reason for lack of flowering in the bigleaf hydrangea is unfavorable weather. Most H. macrophylla cultivars flower primarily on previous year’s growth. Weather conditions that damage aboveground parts of the plant can reduce flowering. Damaging weather conditions include early fall freezes that occur before the plant is completely dormant, extremely low winter temperatures, and late spring freezes that occur after the plant has broken dormancy. In USDA Cold Hardiness zone 6 and warmer, which is the recommended growing area for H. macrophylla, the most common of these unfavorable weather events is late spring freezes that damage tender new growth. This is particularly true in the southeastern U.S., where "see-saw" temperatures are very common in the spring.

    image of Hydrangea 'All Summer Beauty' Bigleaf hydrangea responds quickly to warm temperatures in late winter and early spring by breaking dormancy and producing new leaves. Unfortunately, these spells of warm weather are often followed by periods in which temperatures reach well below freezing. The severity of the damage caused by these freezes depends on how many of the buds had broken dormancy. If a substantial portion of the buds on a stem were actively growing, the whole branch may die. For some cultivars, the loss of the aboveground part of the plant will completely eliminate flowering the following summer. The plant will produce new buds from the base of the stems, but stems produced from these buds will not flower in these cultivars.

    Other cultivars,
    such as 'All Summer Beauty' (see image at right),
    image of Hydrangea 'Nikko Blue'



    'Nikko Blue' (see image at left),

    and 'Madame Emile Mouillere' (see image below),
    image of Hydrangea 'Madame Emile Mouillere'








    are a bit more flexible, and will flower from the buds that develop from the base of the stem. However, if another cycle of warm weather followed by freezing temperatures damages shoots developing from these buds, these cultivars may flower only lightly, if at all.

    Is there anything I can do to make my bigleaf hydrangea flower more reliably?

    image of Hydrangea 'Endless Summer'
    It may be possible to protect plants from weather-related flower bud damage by covering them during late spring freezes with blankets, sheets, etc. There is also evidence that some bigleaf hydrangea cultivars have the ability to flower on current year’s growth, which means that, even if the plant is killed back to the ground, it should still flower during the subsequent summer. One such cultivar is 'Endless Summer' (Bailmer©), which was released to the retail market in 2003 (see image at left).

    image of Hydrangea 'Penny Mac'






    Re-flowering, or remontant, ability has also been reported in the cultivars 'Decatur Blue', 'Oak Hill', 'David Ramsey' and 'Penny Mac' (see image at right). These five cultivars, which are similar in appearance, produce blue flowers in acidic soil and pink flowers in neutral soil.



    image of Hydrangea 'Mariesii Variegata'



    While the primary reason for growing bigleaf hydrangea is for the flowers, a few cultivars have ornamental foliage that makes the plant attractive even when not in flower. Cultivars with variegated or gold foliage, such as 'Mariesii Variegata' (see image at left),


    image of Hydrangea 'Lemon Zest'





                  or 'Lemon Zest' (see image at right), can
                  brighten up a dark spot in the garden.





    How do I change the color of my hydrangea?

    image of Hydrangea macrophylla purple flower
    Flower color in H. macrophylla is dependent on cultivar and aluminum availability. Aluminum is necessary to produce the blue pigment for which bigleaf hydrangea is noted. Most garden soils have adequate aluminum, but the aluminum will not be available to the plant if the soil pH is high. For most bigleaf hydrangea cultivars, blue flowers will be produced in acidic soil (pH 5.5 and lower), whereas neutral to alkaline soils (pH 6.5 and higher) will usually produce pink flowers. Between pH 5.5 and pH 6.5, the flowers will be purple (see image at left) or a mixture of blue and pink flowers will be found on the same plant.

    image of Hydrangea macrophylla mixed color flower





              It isn’t unusual to find inflorescences that are
              part pink and part blue (see image at right).




    image of Hydrangea macrophylla 'Alpengluhen' pink






    There are a few cultivars that never produce blue flowers. In low pH soils, flowers of these "non-bluing" cultivars turn a dull reddish-purple. In higher pH soils, the flowers of these cultivars are a vivid deep pink color. Some non-bluing cultivars are 'Alpengluhen' (see image at left),

    image of Hydrangea macrophylla 'Pia' pink




                                               'Pia' (see image at right),




    image of Hydrangea macrophylla 'Kardinal'




    and 'Kardinal' (see image at left).

    To change the color of a hydrangea planted in the garden, it is necessary to change the soil pH. To raise the pH, add lime. One way of lowering the pH of the soil is to add aluminum sulfate. Additions of acidic organic matter (peat, pine bark) can also help lower soil pH. Exact amounts of lime or aluminum sulfate necessary to get the desired flower color will vary depending on current soil pH and soil type.

    How and when should I prune my hydrangeas?

    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 winter, 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 differs between 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. Plants of H. arborescens 'Annabelle' have been known to produce a second flush of flowers if pruned lightly after the first flowering.

    Stems of bigleaf hydrangea that have been damaged by cold should be pruned as soon as it is determined that they are dead. Watch for new growth at the base of the plant. If your plant has basal shoots that are 6 to 8 inches in length, but the upper parts of the stems are still bare, then the bare stems need to be removed. For bigleaf hydrangea plants that are subject to frequent weather-related dieback, other than removing the dead stems, you probably won’t ever need to do any other pruning – Mother Nature has been doing the work for you.

    Are hydrangeas bothered by any diseases or insects?

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

    image of Hydrangea leaf-spot disease
    There are several fungal leaf spot organisms that attack Hydrangea. Leaves develop brown to gray lesions surrounded by purple halos (see image at left). 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. The most common is Armillaria root rot. 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.

    image of Hydrangea arborescens rust disease
    Smooth hydrangea is susceptible to rust, which will appear on the back side of leaves as small, orange spots (see image at right). Rubbing the back of the leaves will release an orange dust which contains spores of the fungus. 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.  Japanese beetles can be controlled by spraying or dusting with the insecticide Sevin, but the problem is rarely severe enough on hydrangeas to merit the 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. The presence of ants crawling on plant leaves is often an indicator of an aphid problem. The ants feed on the sticky honeydew (excrement) left by the aphids. If you see ants on the leaves of your hydrangeas, turn the youngest leaves on the plant over and look for small green insects. As leaves become tougher during the growing season, aphid problems usually diminish.

    image of Hydrangea mite damage


    Mites can cause problems on hydrangeas. Mites are too small to see with the naked eye, but mite infestation can cause distorted growth, mainly seen in new shoots (see image at left). 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.

    Why are the new leaves on my bigleaf hydrangea yellow?

    image of Hydrangea iron chlorosis 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 (see image at right).

    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. You should be able to find one of these products at a garden center or the garden section of a hardware store or mass merchandiser. Follow package directions carefully. You should begin seeing results in a couple of weeks.

    As long as the soil pH remains high, you will probably need to re-apply the chelated iron product yearly or whenever symptoms reappear. 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.

    How can I dry hydrangeas?

    image of Hydrangea 'General Vicomtesse-de-Vibraye' dried flower head As hydrangea flowers age on the plant, they often turn attractive colors. Bigleaf hydrangea flowers may develop muted tones of blue, purple, rose, violet and green. Panicle and oakleaf flowers develop pink and rose colors, while smooth hydrangea flowers turn pale green. One of the best cultivars is 'General Vicomtesse de Vibraye' (see image at left).

    One of the easiest ways to preserve these 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. 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 you choose, be sure to keep individual inflorescences separated as they dry so that none of the flowers get squashed. The flower heads of some cultivars dry better than others.

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    Last Updated  September 28, 2005 2:41 PM
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