Reflowering Bigleaf and Mountain Hydrangeas

The discovery and development of reflowering bigleaf and mountain hydrangeas revolutionized the market and demand for hydrangeas. Reflowering hydrangeas produce an initial flush of flowers in late spring or early summer followed by sporadic flowering or later flushes of flowers in the same growing season. Though some older cultivars are considered "free-flowering," it is the newer, "everblooming" cultivars that have been lauded and subsequently promoted with elaborate marketing campaigns.

Everblooming bigleaf and mountain hydrangea cultivars provide more garden impact and strengthen the ongoing appreciation of hydrangea. This revitalized enthusiasm is bringing greater attention to the historic varieties of bigleaf and mountain hydrangeas as well as inviting efforts to improve hydrangea through new selection and breeding programs.

Origin and Development of Bigleaf and Mountain Hydrangeas

Bigleaf hydrangea, Hydrangea macrophylla, is native to China and coastal areas of Japan. Mountain hydrangea, H. serrata, is native to China and upland areas of Japan and Korea. Mountain hydrangea is generally smaller in size and flowers earlier. Some experts consider it more cold hardy than bigleaf hydrangea. The two species are very similar, can easily hybridize, and have been considered different varieties of the same species in the past. Both types may produce mophead or lacecap flowers, depending on the selection.

Bigleaf and mountain hydrangeas have long been valued as garden plants in Japan. Hundreds of old cultivars are recorded there, and Japanese breeders continue to develop new forms. When introduced to Europe and the Americas in 1789, these hydrangeas acquired the additional common name "French hydrangea." The shrubs became hugely popular as garden and potted flowering plants in the late 1800s and early 1900s before falling out of favor in the mid-1900s. Today, gardeners and landscapers alike have renewed appreciation of bigleaf and mountain hydrangeas because they flower in shade, often produce blue flowers, and are considered "heirloom" plants that remind us of the garden heritage of our forebearers.

The first everblooming bigleaf hydrangea to be recognized and commercially promoted was discovered by noted plantsman, Dr. Michael Dirr, in the trial garden of a wholesale nursery in Minnesota. One of the nursery employees noticed that a neighbor's bigleaf hydrangea flowered late in the year, and he obtained permission to propagate and test the plant. Dr. Dirr recognized that the plant had value as a garden plant along with great commercial potential. This plant was given the cultivar name of 'Bailmer' and is the progenitor of the Endless Summer® series of hydrangeas. Other similar reflowering hydrangeas have also been released under various other trademarked names.

Bigleaf hydrangea (Hydrangea macrophylla) is not documented in any undisturbed natural areas in Florida. It has been evaluated using the UF/IFAS Assessment of the Status of Non-Native Plants in Floridas Natural Areas (https://assessment.ifas.ufl.edu/), is not considered a problem species, and therefore may be recommended for use in Florida. Mountain hydrangea, H. serrata, has not yet been assessed and may be recommended.

Growing Bigleaf and Mountain Hydrangeas

Culture

Bigleaf and mountain hydrangeas require shade (either continuous shade or morning sun and afternoon shade), moist (but not wet) soils, and cool winters. Within these environmental criteria, bigleaf and mountain hydrangeas are adapted to north and central Florida, although good site selection and plant care increases in importance the farther south it is planted. After plants are well established, bigleaf and mountain hydrangeas are moderately drought tolerant and moderately salt tolerant.

Description and Growth Habit

Bigleaf and mountain hydrangeas grow as a mounded or rounded shrub composed of stout, upright stems that usually are unbranched. Leaves are large, often 4 to 8 inches long and 2 to 5 inches wide, and leathery. Leaf color ranges from medium to dark green and from dull to glossy, depending on cultivar.

Bigleaf and mountain hydrangeas are deciduous in most areas of Florida, dropping leaves in autumn as days shorten, especially after cool weather. However, hydrangea may not drop all leaves in frost-free areas or during mild autumns and winters. In these cases, remaining leaves often fall with onset of new growth in spring.

Bigleaf and mountain hydrangeas are fully cold hardy throughout Florida, and plants rarely die back. Consequently, bigleaf and mountain hydrangeas often reach a mature size of 6 feet tall and wide, and the can grow as large as 12 feet tall and wide. When planting bigleaf and mountain hydrangeas in the garden, place them in a location providing enough space for the plant to mature. Alternatively, use a cultivar known to mature at a smaller size or plan for occasional pruning to reduce plant size.

Pruning

Most plants will need little or no pruning unless dieback occurs or plants outgrow their preferred size. Prune plants when needed in mid to late summer, removing no more than a third to a half of each main stem. Pruning in mid to late summer allows for current year's flowering as well as sufficient time for some regrowth and for flower buds to form on the new growth.

Use in the Garden

Bigleaf and mountain hydrangeas are valued as garden plants for their ability to flower in shade and for their blue flowers. Sooner or later, most gardens develop areas shaded by trees, house walls or other structures. Shade limits the plants that can be grown in these areas. Bigleaf and mountain hydrangeas are well adapted to shaded areas, and few other plants can produce as many colorful flowers under shade. In addition, bigleaf and mountain hydrangeas produce relatively rare and thus highly valued blue flowers on most Florida soils.

Large leaves and flower clusters lend a coarse-textured appearance to bigleaf and mountain hydrangeas. Within the environmental constraints of shade and moist soil, bigleaf and mountain hydrangeas can be an attractive component of a mixed-shrub bed or border, where their coarse-textured leaves are an effective foil for finer-textured plants and where their flower clusters can create bursts of color. A mixed bed also will "hide" the leafless stems in winter, or at least make them less noticeable. Bigleaf and mountain hydrangeas are well suited for use in large massed-beds where their peak flowering in early summer will create impressive swaths of color. Reflowering types of bigleaf and mountain hydrangeas are especially useful for their showy display of flowers in early summer followed by sporadic flowering throughout the rest of the year.

Conversely, most people may not want to use bigleaf and mountain hydrangeas in a foundation planting because the plants lose their leaves in winter and would thus expose the structure behind it. Similarly, they may not be a desirable specimen plant because the winter appearance of the leafless, unbranched stem structure is too open to offer a prominent profile. Furthermore, the upright, leafless stems create a more vertical effect rather than the mounded appearance presented by the fully leafed plant.

Use in Containers

Bigleaf and mountain hydrangeas have long been used as small flowering pot plants that are discarded after flowering. Many cultivars have been developed exclusively for this use. Container-grown bigleaf and mountain hydrangeas are still popular flowering plants at holidays such as Mother's Day and Easter. If planted in the garden, some will grow and flower the same as those purchased for garden use. However, many of these cultivars are not adapted to garden use in Florida and often are particularly susceptible to diseases.

Bigleaf and mountain hydrangeas bred for garden use also perform well in a large container provided the container is placed in shade and the potting mix is moisture-retentive yet well-drained. Remember the plant will be leafless in winter. Smaller, compact cultivars are better-adapted and easier to manage in containers.

Flowering

Bigleaf and mountain hydrangeas flower for 5 to 9 weeks in late spring or early summer. The mophead or lacecap flowers develop from buds formed in the tips of stems the previous year (on old wood). Most older cultivars bloom just once. Some bigleaf and mountain hydrangeas also form flower buds the previous year in buds up and down the stems (not just at the tips). These flower buds may develop and flower weeks or months after the terminal flower buds bloom in early summer. Bigleaf and mountain hydrangeas with this pattern of flowering are called "free-flowering."

"Everblooming" hydrangeas produce flowers from buds formed the previous year and also from buds formed on new growth. In this latter case, flower buds form in the tips of new growth, and these buds will continue to develop and ultimately flower later in the same year. Technically called "remontant," bigleaf and mountain hydrangeas with this characteristic have the ability to flower almost continuously throughout the growing season. Some of these hydrangeas flowered 30 weeks or more at the UF/IFAS North Florida Research and Education Center. Everblooming types of bigleaf and mountain hydrangeas are especially popular and dominate the nursery trade.

Flower Types

"Mophead" or "hortensia" type hydrangeas form ball-shaped flower clusters 3 to 8 inches across (Figure 1). Most of these individual flowers are sterile and do not produce seed. "Lacecap" hydrangeas have flat clusters 3 to 8 inches across composed of tiny, spidery flowers surrounded by a ring of prominent flowers (Figure 2). Each spidery flower is ¼ to ½ inch across and fertile, with the capacity to produce seed. The showy, prominent flowers on the outer periphery of the flower clusters are sterile and are similar to those found on mophead hydrangea.

Characteristics of Individual Flowers

Showy sterile flowers are characterized by four wide, showy petal-like sepals (hereafter called "petals"), each of which may be 1 to 3 inches across. Some flowers have double or triple the number of petals, creating a more full flower and/or a lacy appearance. Petals also may be serrated, enhancing a lacy appearance.

As flowers age or are pollinated, petal color may change to green, blue or burgundy. Individual flowers also may invert themselves so that the undersides ("backs") of the petals are displayed. Aging flowers may be quite ornamental, and may extend the season of flower color for many weeks (Figure 3).

Flower Colors: Blue, Purple, Pink, Red, and White

Spidery fertile and showy sterile flowers may be white, dark pink, pink, purple, or blue on both mophead and lacecap flower types. Showy sterile flower petals may have picotee patterns such that each petal is edged in a thin margin of white. Most bigleaf and mountain hydrangeas produce blue, purple or pink flowers, depending on cultivar and soil conditions. Many people are aware that soil acidity or alkalinity affects and changes bigleaf and mountain hydrangeas' flower color, but most do not know why.

Amounts of flower pigment and aluminum determine blue or pink flower color

The amount of a flower pigment, delphinidin 3-monoglucoside, and the amount of aluminum within flower cells determines the flower color of bigleaf and mountain hydrangeas. This flower pigment combines with aluminum to form the blue color; without aluminum, pigment color is pink. The intensity and shade of flower color depends on relative amounts of pigment and aluminum. Resulting flower color can vary from intense blue to purple to pink to almost red.

Soil pH affects aluminum and flower color

The amount of aluminum in the flower is determined by the amount of aluminum in the soil, soil pH, phosphorus in the soil, and the plant's ability to absorb aluminum through plant roots. pH is a measure of acidity or alkalinity. In acidic soils (pH 5.5 or lower), aluminum assumes a form that is easily absorbed by plant roots, and thus flowers are predominately blue. In soils where the pH is 6.5 or higher, aluminum is unavailable and flower color is pink. Purple flowers are produced when soil pH is between 5.5 and 6.5 or when a pink or "red" cultivar is grown in acid soil. Sometimes a single plant will have both blue and pink flowers because of varying soil conditions around the plant.

Most soils in north and central Florida are slightly acid (pH between 5.0 and 6.5). Thus, most bigleaf and mountain hydrangea flowers are blue or purple. Exceptions occur when bigleaf and mountain hydrangeas are planted in soils derived from limestone, marl or sea shells; when soils are contaminated by alkaline mortar or concrete debris (left after construction); when beds are situated next to stucco walls, mortared walls, concrete foundations and sidewalks (where alkaline runoff affects soil); or when irrigation water is alkaline and contains significant amounts of dissolved limestone. These situations often will result in higher soil pH and purple or pink flowers.

Soil phosphorus content affects aluminum levels in flowers

Another soil element and plant nutrient, phosphorus, can confound development of blue flower color. Phosphorus interacts with and effectively ties up available aluminum in the soil. Thus if soil phosphorus levels are high, pink flowers will result, even if soil is acidic and aluminum is present. This is important because many Florida soils naturally contain high levels of phosphorus or are former agricultural lands that received excessive phosphorus fertilization. A soil test will measure soil pH and phosphorus content to provide information on potential flower color of bigleaf and mountain hydrangeas.

White- and "red"-flowered bigleaf and mountain hydrangeas are different

Plant genetics also can affect flower color. Some cultivars produce flowers that are predominately white. These flowers have very little of the pigment delphinidin 3-monoglucoside. Because small amounts of this pigment usually are present, white flowers often are tinted pink or blue, depending on availability of aluminum.

Some cultivars produce flowers that are predominately dark pink, "red" or purple. The roots of these cultivars are poor absorbers of aluminum. Because they can't absorb much aluminum, the flower pigment is predominately in the pink form.

Cultivars with "red" flowers generally are reddest in cooler climates, such as the Pacific Northwest. These cultivars rarely produce red flowers in Florida, and at best the flowers are dark pink.

Changing Flower Color to Blue, Purple, or Pink

Lime applied to soil raises pH and reduces the availability of aluminum, resulting in pink or purple flowers. For long-term changes to soil pH, a soil test can determine current pH and provide recommendations for annual liming to achieve and maintain the pH of 6.5 needed for pink flowers.

For short-term changes to soil pH, most recommendations suggest drenching soil around well-watered bigleaf and mountain hydrangeas with hydrated lime (available at garden supply stores) mixed at a rate of 1 tablespoon per gallon of water. Apply drenches monthly in February, March and April.

Change blue flower color to pink or purple with lime

Lime applied to soil raises pH and reduces the availability of aluminum, resulting in pink or purple flowers. For long-term changes to soil pH, a soil test can determine current pH and provide recommendations for annual liming to achieve and maintain the pH of 6.5 needed for pink flowers.

For short-term changes to soil pH, most recommendations suggest drenching soil around well-watered bigleaf and mountain hydrangeas with hydrated lime (available at garden supply stores) mixed at a rate of 1 tablespoon per gallon of water. Apply drenches monthly in February, March and April.

Change pink flower color to blue or purple by adding aluminum

Addition of aluminum sulfate to soil will increase the amount of aluminum available to roots, almost guaranteeing blue or purple flower color. Also known as alum, aluminum sulfate is widely available at garden supply stores.

Most recommendations call for drenching soil around well-watered bigleaf and mountain hydrangeas with aluminum sulfate mixed at a rate of 1 tablespoon per gallon of water. Apply drenches monthly in February, March and April, avoiding contact with leaves because this solution can damage foliage.

Ignore any recommendations listed on bags of aluminum sulfate to apply it at higher rates than those recommended above. These rates will lower soil pH but increase risk of accidental application of excess levels of aluminum that are toxic to many other garden plants.

Lowering soil ph Is often difficult and does not guarantee blue flower color

Efforts to lower soil pH with "acid-forming" fertilizer, aluminum sulfate or elemental sulfur usually are not effective because most soils quickly revert to their natural pH. Acid-forming fertilizer does not significantly lower pH of most soils.

For long-term effectiveness, elemental sulfur or aluminum sulfate must be routinely applied or blended into the soil of the entire root zone to ensure all flowers are blue. This can damage plant roots and usually is not practical. Low soil pH will not produce blue flowers in high-phosphorus soils. Phosphorus ties up aluminum and prevents blue flower color development regardless of soil pH.

Hydrangea Pests

Bigleaf and mountain hydrangeas are nearly pest-free when properly placed in the garden and not stressed. Plant bigleaf and mountain hydrangeas in full shade or at least in areas shaded from afternoon sun. Avoid wet soils and don't plant too deeply, or root systems will be stressed and unable to function properly.

Insects and Mites

The following insects and mites are pests of bigleaf and mountain hydrangeas but are not commonly found unless hydrangea is stressed:

• Aphids are found primarily on new growth.

• Oystershell scale typically occurs on upper stems.

• Four-lined plant bug causes round, brown, sunken spots on leaves.

• Leaf tiers are indicated by webbing between leaf tips.

• Rose chafers are light tan beetles that damage leaves, usually when bigleaf and mountain hydrangeas are grown on sandy soils.

• Mites occasionally may infest bigleaf and mountain hydrangeas.

These pests generally do not threaten plant health and often disberse naturally. When present, these pests can be controlled by removing them by hand, pruning affected branches and applying horticultural soaps and oils.

Deer

Deer occasionally damage bigleaf and mountain hydrangeas. Deer will eat bigleaf and mountain hydrangea leaves, but they seem to prefer the native Hydrangea species H. arborescens and H. quercifolia.

Diseases

A number of diseases can affect bigleaf and mountain hydrangeas, but only powdery mildew and leaf spot are typically encountered in the garden.

Powdery mildew appears as small white patches on the undersides of leaves, followed by purple or yellow blotches on the tops of leaves. It is primarily an aesthetic problem and does not seriously threaten plant health. Powdery mildew occasionally occurs in spring and fall when days are warm and humid and nights are cool. Powdery mildew can be prevented by using resistant selections of hydrangea.

A leaf spot caused by Cercospora often occurs in the garden, usually during late summer and fall. It appears as small brown or purple spots on leaves. The leaf spot is primarily an aesthetic problem and does not threaten plant health, although it can defoliate plants when severe. It can be controlled by removing dead and diseased leaves and avoiding overhead irrigation.

Several diseases rarely occur on garden-grown bigleaf and mountain hydrangeas:

• Anthracnose, caused by Colletotrichum fungi, can occur during hot, wet weather on plants with succulent growth. Anthracnose initially appears as circular brown spots on leaves that then form irregular patches; older spots turn tan.

• Botrytis blight occurs occasionally on flowers during cool, humid, wet conditions. It initially appears as small, water-soaked spots on petals that then form irregular reddish-brown patches.

• Mushroom root rot can occur in gardens where oak trees were or are present. Mushroom root rot appears as sudden wilting of a branch or shoot.

These diseases can be prevented or controlled by keeping plants as healthy as possible, avoiding overhead irrigation and removing dead or diseased leaves.

Future Hydrangeas

Watch for new everblooming bigleaf and mountain hydrangeas. The popularity of these everblooming plants has prompted many new breeding efforts. In addition, breeders are working to develop bigleaf and mountain hydrangeas with additional ornamental characteristics, such as burgundy or purple stems or leaves, as well as dwarf habit, compact growth, and greater disease resistance and hardiness.

References

Dirr, Michael A. 2004. Hydrangeas for American gardens, Portland, OR: Timber Press, Inc.

Knox, Gary W. 2007. Flowering of Hydrangea macrophylla Selections at the University of Florida/IFAS North Florida Research and Education Center in Quincy, Quincy, FL: UF/IFAS North Florida Research and Education Center.

University of Florida, Institute of Food and Agricultural Sciences. 2020. "Assessment of Non-native Plants in Florida's Natural Areas" https://assessment.ifas.ufl.edu.