Mention "Hydrangea" and we might envision cloud shaped flowers of either pink or blue, and large, soft-green leaves together composing images of lazy summers in the shade. Wow! That's a lot of responsibility for one shrub. Fortunately, Bigleaf Hydrangea (those with blue or pink flower clusters) is a resilient and hardy plant for southern landscapes. There are actually several sturdy shrubs in the genus Hydrangea.
Hydrangeas have long been a loyal member of many Southern gardens. Most people are familiar with the large, showy pink or blue flowers of the Bigleaf, or French hydrangea (Hydrangea macrophylla), but look at the entire group (genus) to find a variety of shrub size, texture, flower shape and color. Although widely planted in the Southeast, hydrangeas have their origins in Asia and the Americas. The two Alabama native species are Smooth Hydrangea (Hydrangea arborescens) and Oakleaf Hydrangea (Hydrangea quercifolia) discovered by John Bartram in 1791.
The Bigleaf hydrangea is perhaps the best known member of this genus. It graces the landscapes of many homes and originally came to us from France by way of Japan. Known by such common names as House hydrangea, Snowball hydrangea, French hydrangea, Common hydrangea, Garden hydrangea, Hortensia, and Bigleaf hydrangea, the scenario can be quite confusing. Refer to this plant's scientific name, Hydrangea macrophylla, for reliability. Scientific names are always the dependable tag (see plant descriptions below).
White, pink, or blue - the color is up to you. Well, not really. Gardeners only have a color choice with Bigleaf and Serrated Hydrangeas, and several factors may affect the outcome. Bloom color and intensity depend on the specific variety, weather, plant health, and soil. Of these, soil pH is the strongest influence the gardener can adjust.
Blue or pink blooms depend on soil pH and corresponding available aluminum. At lower pH levels (acid soils), aluminum is more readily available to the plant and flowers become blue. At higher pH ranges (more alkaline soils), aluminum is less available and flowers are pink. Want pink flowers? Add lime to raise soil pH. Want blue flowers? Add sulfur to lower soil pH.
Changing soil pH is a gradual process and requires a soil test for accuracy. Broadcast 1/2 cup of wettable sulfur per 10 square feet and water-in to make flowers blue. To make the flowers pink, broadcast one cup of dolomitic lime per 10 square feet and water it into the soil. Raising pH takes longer than lowering. It may be next year before a noticeable change to pink flowers.
A faster option for changing flower color is liquid soil drenches. Want to enhance your blue flowers or to change pink ones to blue? Dissolve one tablespoon of alum (aluminum sulfate) in a gallon of water and drench the soil around the plant in March, April and May. Desire pink flowers? Dissolve one tablespoon of hydrated lime in a gallon of water and drench the soil around the plant in March, April and May. Avoid splashing either of these solutions on leaves, bark, or skin as it can cause burning.
HARDINESS AND COLD DAMAGE
Bigleaf Hydrangea is the more sensitive species. Hydrangea macrophylla suffers from early or late freezes since the flower buds have a weak dormancy. Surprise warm weather in winter or early spring causes the buds to emerge from dormancy, grow and become more susceptible to freeze damage. This results in fewer or no flowers as this year's flower bud was formed last fall.
Remedy? Site and variety* selection are key. If this type weather is common in your landscape, plant Bigleaf Hydrangea on northern and eastern slopes under tall pines. This reduces the temperature fluctuations causing the problem. Choosing resilient varieties that either bloom later or produce new buds for the current season are another option (see plant descriptions below).
[*Variety refers to either a naturally occurring variation of the species or a cultivated variation. Cultivated variety names are shown in single quotes, Hydrangea macrophylla 'Lilacina', and the term can be abbreviated as cultivar or cv.]
Hydrangeas are relatively easy to grow in well-drained soils containing plenty of organic matter. They, like most Southeners, prefer warm, morning sun and cooler, afternoon shade. Avoid planting in hot, dry, exposed sites. Buying any Hydrangea during bloom does help to verify variety and flower form, but the best time to plant them is actually fall. Early spring is the next best planting time. With extra care, they can be planted at other times.
Dig the planting hole two feet wider than the plant's root ball and only as deep as the root ball. In heavy soils, consider preparing a planting bed instead. Amend with compost to create a mound for planting. Planting high on a mound allows better drainage for roots.
While many Hydrangeas benefit from shade, planting any shrub under a large, established tree is a tough situation. Large trees require and consume enormous amounts of water during the growing season. Newly planted shrubs often cannot compete successfully in this location. The gardener is then hard-pressed to supplement the needs of the new shrub.
To look and perform their best, Bigleaf and Smooth Hydrangeas need more water than do their relatives. Becoming tattered in appearance during periods of drought, they still survive only to need minor pruning to restore their form. The large, soft leaves lose water quickly and the hot sun wilts them. Well watered plants may still appear wilted if too much sun (or heat) is the added problem. Afternoon shade is a must for both these hydrangeas. One inch of water per week either by rainfall or irrigation is recommended, and of course, mulching helps conserve water loss and cool the root system.
Different Hydrangeas have different fertility needs. Bigleaf hydrangea responds to several light fertilizer applications during the growing season. A general-purpose fertilizer, such as 12-4-8, 16-4-8 or 10-10-10** applied in March, May and July is suggested (one pound per 100 square feet, divided applications). Panicle and Oakleaf Hydrangeas benefit from an April and June application of the same rate divided twice, while Smooth Hydrangea needs just one application in late winter. It is not necessary to remove the mulch when fertilizing, but water soon after application to help dissolve fertilizer into the soil.
If blue flowers are desired for Bigleaf (only), use fertilizers low in phosphorus (the middle number 10-10-10). Phosphorus ties up aluminum making it unavailable for root uptake. This leads to pink flowers.
[**Periodic soil tests are recommended to ensure phosphorus levels remain at acceptable levels. Too much soil phosphorus causes plant injury.]
Understanding flower development for each species is required. Some hydrangeas bloom on "old wood" while others bloom on "new wood." Bearing flowers on woody stems produced the previous year describes blooms on old wood. Flower buds forming on this year's current growth appear on new wood. Shrubs blooming on old wood must be pruned shortly after current flower so as not to remove developing flower buds. Next year's flower buds begin forming in August-October. If a shrub blooms on new wood, prune late winter or in spring stimulating new growth for additional blooms.
General maintenance on Bigleaf and Oakleaf hydrangeas is recommended annually. Remove all dead wood and cut about 1/3 of the older stems to the ground. This improves plant vigor, overall shape and bloom volume. Remember to prune these two species early; soon after flowering. Bigleaf and Oakleaf bloom on old wood. There are a few Bigleaf hydrangeas that bloom on both old and new wood. These should be chosen if you are in an area prone to late frosts.
Smooth Hydrangea can be cut at six to twelve inches from the ground, or at half its height, every year in late winter or early spring. Height to prune is strictly personal preference. Pruning after initial flowering can even stimulate a second flower period. Panicle Hydrangea is most effective in tree form. Remove lower suckers and up to 1/2 of older stems for greater flowering. Both Smooth and Panicle Hydrangeas bloom on new wood.
Bigleaf and Panicle Hydrangeas are easy to propagate by layering. In early to mid summer, dig a trench near the plant and bend a flexible branch down into it. Scratch the bark area having soil contact for better rooting. Cover the wounded branch section with soil and either pin it or weight it down such as with a brick. Six inches to one foot of branch tip should stick out of the ground uncovered. (Note: Rooting hormone may be dusted on the wound to increase rooting rate). Smooth and Oakleaf Hydrangeas may also be propagated this way, but their colonizing habit (spread by underground stems) makes them easy to propagate by division during the dormant season.
DISEASE AND INSECT PESTS
Hydrangeas are susceptible to leaf spots, blights, wilts and powdery mildew. Insect pests on hydrangea include aphids, leaf tiers, rose chafers, oyster scale and red spider mites. These pests are uncommon and if seen may be a sign of another problem causing plant stress. Powdery mildew is common on some varieties, but is rarely life threatening. As always, gardeners should make choices based on varietal resistance, and desired traits. A little research goes a long way in making the best selection for your garden.
Hydrangea species and cultivars
Common Name: Bigleaf Hydrangea, French Hydrangea, Garden Hydrangea, etc Botanic Name: Hydrangea macrophylla Size: varies by cultivar; from 2 to 6 feet high and as wide (unmanaged plants may reach 10 feet height) Hardiness: varies by cultivar; ranges from zones 6 to 9 Leaf texture/interest: Coarse, large leaves with serrate (jagged) edges; color rages from medium to dark green and can be leathery or glossy in some cultivars Flowers: This most common hydrangea has two classifications by flower type:
1. Hortensias (or mopheads) display masses of sterile, ball shaped flower clusters providing a bold appearance in mid to late summer. The infrequent, fertile flowers are hidden inside the cluster. Blue or pink mopheads can stand on their own in a bed planting, but may dominate the show. Use this flamboyant character to your advantage.
2. Lace Caps look something like a lace doily having a center of non-showy, tiny, fertile flowers ringed by a pinwheel of showy, sterile flowers. Lace caps, blue or pink, are more subtle in effect and may be easier to place in a landscape. They work well in wooded, natural areas near plants like dogwoods and azaleas.
Culture/Pruning: plant in well drained amended soil; requires cool, moist soil in summer; tolerates beach landscapes; requires afternoon shade in Alabama; most cultivars bloom on old wood prune shortly after flower, never after August to be safe; search newer cultivars for dependable, cold hardy flower buds. Diseases and Insects: Hydrangeas are susceptible to leaf spots, blights, wilts and powdery mildew. Insect pests on hydrangea may include aphids, leaf tiers, rose chafers, oyster scale and red spider mites. A Few Cultivars:
- 'Endless Summer' is a new release, reblooms***, intense blue or pink, 3 4' height
- 'Nikko Blue' has rich, medium blue or pink flowers, 6' height, reblooms
- 'Penny-Mac' has clear blue or pink, 3 5 ' height, named for founder American Hydrangea Society, reblooms
- 'Pia' flowers remain pink, compact, 2 3' height
- 'Madame Emile Mouillθre' has white mophead, 6' height, hardy to -11 oF; reblooms
- 'Fuji Waterfall' has double white, fading to blue or pink, glossy dark green foliage, 3 6' height
Several lacecaps feature variegated foliage: 'Quadricolor,' 'Mariesii Variegata,' and 'Variegata'
- 'Lilacina' has blue or pink ray florets, upright habit, lustrous, dark green foliage, mildew resistant, reblooms
- 'Veitchii' has white fading to pastel blue or pink florets, large, dark green leaves, mildew resistant, hardy to -6 oF; reblooms
- 'Beaute Vendomoise' has largest ray florets of lace caps, palest pink or blue, 3 6' height
- 'Blue Wave' has wavy-edged ray florets, rich blue or pink, 6' height
- 'Lanarth White' has central fertile flowers of blue or pink, ray florets bright white, blooms July August, 3' height; reblooms
[***Reblooms blooms on old and new wood in same season, blooming early summer and again in fall]
Common Name: Oakleaf Hydrangea (Alabama's State Wildflower) Botanic Name: Hydrangea quercifolia Size: from 4 to 6 feet high and as wide or wider (unmanaged plants may reach 8 feet height) Hardiness: ranges from zones 5 to 9 Leaf texture/interest: Coarse, large leaves resembling oak; grayish green new growth in spring, dark green in summer, excellent fall color (varies by cultivar); light tan to cinnamon, exfoliating bark on twisted, wandering stems is striking winter trait Flowers: begin soft green, turn creamy white, fade to rose or purplish pink by July; conical in shape; some cultivars have double florets; flower clusters are conical in shape Culture/Pruning: ideally suited to shaded locations; cool, well drained, fertile soil; some cultivars tolerate sun better than others; part shade recommended; prefers drier locations compared to Bigleaf; colonizes by underground stems; blooms on old wood; prune before August Diseases and other Pests: see Bigleaf Hydrangea; pests are extremely rare on this Southeastern native shrub; wilt can be symptom of root rot; very sensitive to wet soils; plant is a special culinary delight to deer A Few Cultivars:
- 'Snowflake' has double-florets in 12 15" conical flower cluster, clusters may droop from weight, prefers afternoon shade, 8' high and wide, originated Aldridge Nursery, Bessemer AL
- 'Snowqueen' has 6 8" flower clusters held upright, turn strong pink, leaves dark green, tolerates sun better than others, deep red to bronze fall color, 8' high and wide, compact
- 'PeeWee' is dwarf, has 4 5" flower cluster, requires shade, red to purple fall color, 3 4' high and wide
- 'Harmony' has 12" rounded flower clusters, shrub 6 8' high and wide, originated in Atalla AL
- 'Alice' has 14" flower clusters of rich rose pink, upright flower, dark green summer foliage, burgundy fall, sun tolerant, 12' height, originated Athens GA
Common Name: Smooth Hydrangea, Seven-bark (Native American name), Wild Hydrangea Botanic Name: Hydrangea arborescens Size: from 3 to 4 feet high and larger width (unmanaged, wild plants seen 10 x 10') Hardiness: ranges from zones 3 to 9 Leaf texture/interest: coarse, rounded, and large leaves; medium to dark green; dense habit, drooping stems in flower Flowers: produced on new growth, can produce two flower seasons prune after June flowers fade for early fall flowering, up to 12" diameter mound, cultivar flower forms preferred over native, wild species Culture/Pruning: can grow in part sun, light or deep shade; perhaps the most reliable of the hydrangea family; colonizes by underground stems; moist, well drained soil; tattered leaves appear in drought; pruning after flower will often initiate rebloom Diseases and Insects: see Bigleaf Hydrangea for possibilities; none common A Few Cultivars:
- 'Hayes Starburst' has narrow, shiny, wavy-edged, dark green leaves; double, white florets; more compact than native species; pruning young plants produces more uniform shape; 3 to 4' height; found Anniston, Alabama.
- 'Annabelle' has enormous, globular flower clusters up to 12" diameter, stems droop when in flower, 4' height, has received many landscape awards for its beauty and prolific blooms
- 'Grandiflora' (or 'Hills of Snow') flower clusters are approximately 6 to 8" round, 4' height; stronger, more upright stems than 'Annabelle'
Common Name: Panicle Hydrangea, P. G. or PEE GEE Hydrangea Botanic Name: Hydrangea paniculata Size: variable by cultivar, 6 to 20' height Hardiness: ranges from zones 3 to 8 Leaf texture/interest: narrow, serrate, dark green and slightly hairy; upright growth, but drooping stems from large flowers Flowers: white fading to purplish-pink by late summer; 6 to 18" long, conical flower clusters; flowers on new wood, prune early spring Culture/Pruning: Panicle hydrangea is a sun lover. With adequate moisture it can tolerate all-day sunlight, but like most of its cousins prefers morning sun and afternoon shade. Diseases and Insects: see Bigleaf Hydrangea; none common A Few Cultivars:
- 'Grandiflora,' sometimes named Pee Gee (Hydrangea paniculata 'Grandiflora'); has 10 to 18" conical flower clusters, creamy white fades to pink; older cultivar introduced from Japan 1862
- 'Brussels Lace' is a compact version (5 to 7 ft. high); has an abundance of fertile flowers giving the cluster a lacy appearance; scattered, sterile flowers become pink spotted with age
- 'Tardiva' flowers later than Pee Gee, September/October; has superior form due to upright stem and flower cluster habit.
- 'Webb's' has a more rounded flower cluster composed mostly of sterile flowers; can become 20 feet tall
- 'Chantilly Lace' is a new cultivar from the Center for Applied Nursery Research in Augusta GA, large, white, sterile florets fade to pink
Common Name: Serrated Hydrangea Botanic Name: Hydrangea serrata Size: variable by cultivar to 5' height Hardiness: ranges from zones 5 to 7 Leaf texture/interest: smaller version of the Bigleaf Flowers: Blue, pink Culture/Pruning: This plant is hardier than Hydrangea macrophylla but still produces blue or pink flowers. Hydrangea serratta is a smaller (up to 5 ft. tall) shrub with slender stems, and smaller leaves and flowers. It is drought tolerant, growing well in full sun or partial shade. Serrated Hydrangea blooms on old and new wood. Cultivars:
- 'Beni-gaku' has white, lacecap flowers opening red and turn purple with age; inner, fertile flowers are purple, then blue
- 'Blue Bird' has both sterile and fertile flowers are blue (or pink depending on pH)
- 'Grayswood' has large, white lacecap flowers turn soft pink eventually becoming burgundy.
- 'Preziosa' has flowers opening in shades of pink then changing to shades of crimson. May have mophead and lacecap flower clusters on same shrub. The foliage turns burgundy in the fall.
Common Name: Climbing Hydrangea Botanic Name: Hydrangea anomala petiolaris Size: variable by support given; can reach 60 to 80 feet Hardiness: zone 4 to 7; tolerates zone 8; not as vigorous in extreme south Leaf texture/interest: rounded leaves; dark green and somewhat glossy; exfoliating bark on woody stems; lateral stems reach out from main vine adding horizontal as well as vertical effect Flowers: creamy white to white; early summer; lasting two weeks or more; light fragrance; flattened flower cluster 6 to 10 inches diameter; lacy appearance with fertile center flowers surrounded by sterile, showy flowers Culture/Pruning: This hydrangea is a clinging vine that prefers a moist, organic, well-drained soil. It should be planted with afternoon shade in zone 8. This vine can be quite substantial needing equally sturdy support. Be patient. It may be slow to establish, but once the flowers start you'll appreciate the wait. Disease and Insects: none serious
Michael Dirr, retired Department of Horticulture, University of Geogia
Rick Bir, Extension Horticulture Specialist, North Carolina State University
Ken Tilt, Extension Specialist and Professor, Horticulture Department, Auburn University
Auburn University is conducting cultivar trials of hydrangea at the North Alabama Horticulture Research Center in Cullman. Visit the Center to view the flowers or return to the AU Hydrangea Trials Page. Follow our progress and get more information on this special garden plant.