Every year, the National Garden Bureau, a non-profit organization promoting the pleasures of home gardening, selects one annual, one perennial, one edible plant, one shrub and one bulb to celebrate. It’s a great way to discover a new plant or to learn a bit more about a plant you may already be growing.
Let’s look at the shrub chosen for 2020, the hydrangea.
Year of the Hydrangea
Hydrangeas are one of the most sought-after garden shrubs in temperate climates! Known for their nostalgia as well as their modern design appeal, hydrangeas are a perfect fit in the landscape, a container garden or for use as cut flowers.
The word Hydrangea comes from the Greek words for water, hydor and vessel, aggeion, because of the juglike shape of the seed capsule. You sometimes also hear certain species called Hortensia, a former botanical name honoring Hortense Lepaute, wife of Jean-André Lepaute (1720–1789), renowned French inventor and clockmaker.
There are some 70 species of hydrangeas, both shrubs and climbing plants, most native to Asia, but a few are also found in the Eastern United States.
Some species, though, have been so widely planted elsewhere they appear native. Such is the case of the Azores, Portuguese islands in the Atlantic Ocean about 1,500 km (930 miles) west of Lisbon, where H. macrophylla now grows so abundantly the island of Faial is now known as the Blue Island.
Hydrangeas belong to the botanical family Hydrangeaceae which contains a few other plants of horticultural interest, including mock oranges (Philadelphus spp.) and deutzias (Deutzia spp.).
Hydrangea flowers are borne in heads at the end of the branches. They typically contain two types of flowers: a central cluster of small fertile non-showy flowers surrounded by a ring of large showy and more colorful yet sterile flowers designed to attract insect pollinators to the fertile ones right nearby. Cultivated varieties have generally been bred to have more of the larger showy sterile flowers and fewer fertile ones. Indeed, some hydrangeas form rounded balls of sterile flowers resembling pom-poms and are called mophead blooms or just mopheads. Those that have flat flowerheads with a circle of sterile flowers surrounding a cluster of fertile ones are often called lacecap hydrangeas or lacecaps.
Hydrangeas are often confused with viburnums, a genus of non-related shrubs with similar lacecap and mophead flowers.
Hydrangeas make great cut flowers, but dehydrate very rapidly. Plunge them immediately into tepid water when you harvest them. If the flowers become wilted, first insert the stem briefly into boiling water, then plunge the entire flowerhead into tepid water. The latter step will help restore hydration to the petals.
Most species also make great dried flowers.
Selecting a Hydrangea for Your Garden
When choosing a hydrangea, keep the following in mind: your hardiness zone, available space (they can get quite big!), soil makeup, moisture levels and amount of sunlight received per day.
Below are the five hydrangea species most often grown in home gardens.
Bigleaf Hydrangea or Hortensia (Hydrangea macrophylla)
The most common type in many areas, bigleaf hydrangea is perhaps the classic hydrangea. It gets the name bigleaf (the meaning of macrophylla) from its serrate, obovate, dark green leaves that often reach 8 inches (20 cm) in diameter. It’s especially renowned for its mophead varieties whose blooms are also huge: up to 10 inches (25 cm) in some greenhouse varieties. And it’s certainly the most colorful of the hydrangeas, with flowers in shades of blue, pink, purple, red, white and more.
These are the hydrangeas that famously change colors depending on the acidity of the soil. More on that under Pink or Blue Flowers? below.
Bigleaf hydrangeas are not only garden plants, but are often sold as potted plants in the spring (Easter, Mother’s Day, etc.). Such potted plants lose enormous quantities of water through their large leaves and flowerheads and may need watering two or three times a week. Consider them gift plants: they probably will never bloom a second time indoors and are best either chucked after blooming or, in milder climates, planted outdoors.
The hardiness of bigleaf hydrangeas is limited to USDA hardiness zones 6 to 11. Although they’ll often survive in colder zones, dying to the ground in winter, they won’t readily bloom there without special protection, such as a circle of chicken wire filled with leaves or straw. This protection needs to be left on until temperatures warm up: even the slightest touch of frost readily kills the delicate flower buds.
Typically, bigleaf hydrangeas planted in the garden bloom in early summer on new stems appearing from old wood (stems of the previous season): if the old stems die to the ground, there will be no bloom that year. Some cultivars, most notably those of the popular Endless Summer® and Forever and Ever® series, have the capacity to rebloom later in the season, from new wood, extending bloom over the entire summer, but this effect is often an elusive one, not seen under all conditions. In colder climates, due to winter damage, and in hot ones, due to heat stress, “reblooming hydrangeas” rarely rebloom.
The bigleaf hydrangea is best adapted to spots with morning sun and afternoon shade, plus good drainage. It is not drought tolerant and may need considerable watering if planted in sandy soil. A good mulch at least 3 inches (7.5 cm) deep will help keep the soil at the proper level of humidity. Avoid planting bigleaf hydrangea on hot, dry, exposed sites.
This hydrangea ranges widely in size, reaching 10 feet (3 m) or more in height and diameter in mild climates, but barely scratching 2 feet (60 cm) in colder ones where winter dieback is severe.
Mountain hydrangea (H. serrata) is similar to bigleaf hydrangea and likewise comes both lacecap and mophead varieties in a similar color range. It’s a smaller shrub, 2 to 4 feet (60 to 120 cm) in height and spread, with smaller flowers and leaves, and is generally considered an easier plant to grow, being more tolerant of less than ideal conditions, although it is not really any hardier (USDA hardiness zones 6 to 9). Like it the bigleaf hydrangea, it may survive in zones 4 and 5, but considerable winter protection will be needed to get it to bloom.
Smooth Hydrangea (Hydrangea arborescens)
The classic white hydrangea, ‘Annabelle’, is widely planted in temperate climates all over the world. It is most definitely a mophead variety, with huge rounded flowerheads of white blooms gradually turning to lime green. The species itself has lacecap flowers, with an open circle of sterile white flowers surrounding smaller fertile ones in a flat cluster, and some cultivars, such as ‘Haas’ Halo’, maintain that form.
For years, smooth hydrangea was only offered in white, but newer pink flowered cultivars, like Invinicbelle® Spirit II and Invinicbelle® Ruby, are becoming very popular, as are white mopheads with stronger stems than the floppy older ‘Annabelle’ and ‘Grandiflora’ varieties, like Incrediball® (‘Abweto’) and dwarf varieties like Invinicbelle® Wee White.
This is one of the few North American species, native from New York to Florida and from the East Coast to Oklahoma. It produces gray-brown stems with peeling bark and egg-shaped leaves with toothed margins. It flowers on new wood.
It’s one of the hardiest hydrangeas, growing perfectly well in zones 3 to 9, and is perfect for full sun or dappled shade, even shade in hotter climates.
Panicle Hydrangea (Hydrangea paniculata)
Panicle hydrangeas, originally from eastern Asia, have gorgeous cone-shaped blooms that emerge white in midsummer and age to various shades of lime, pink and red as fall nights cool down, finally drying and turning tan for their winter-long display. These hydrangeas are some of the most cold-hardy and indeed, grow best in cooler climates (USDA hardiness zones 3 to 8). They adapt to most soils, but prefer rich, moist, well-drained ones, and grow best in sun or partial shade. They bloom on new wood.
They can be big plants, up to 15 feet (4.5 m) high and 12 feet (3.5 m) in diameter if allowed to grow on their own, but can easily be maintained at one third that size by regular pruning. There are also dwarf varieties, like ‘Bobo’, for smaller gardens. Panicle hydrangeas are sometimes pruned into a tree form.
The classic variety is H. paniculata ‘Grandiflora’, often called just PG hydrangea, seen in older gardens everywhere, as it is a very long-lived plant, but its heavy flowers cause the stems to splay out and down, making the plant very broad indeed; not a good choice for a tiny garden. Most modern cultivars, like Pinky Winky®, have sturdy, upright stems and are preferable in gardens where space is at a premium.
Oakleaf Hydrangea (Hydrangea quercifolia)
Aptly named, this hydrangea is accented by leaves that resemble those of an oak tree (quercifolia means oak leaf). The incredible foliage also gives an amazing and long-lasting fall coloration: shades of purple and red.
The flowers are equally attractive: large pyramidal panicles of mixed fertile and sterile blooms that turn pinkish over time. It is one of the earliest blooming species of hydrangea. Many cultivated varieties are showier than the species, with dense panicles of sterile flowers and sometimes even double ones, like Snowflake® (‘Brido’).
Although native to fairly mild climates in the Southeastern United States, from North Carolina west to Tennessee, and south to Florida and Louisiana, oakleaf hydrangea will grow in USDA hardiness zones 5 to 9. It can be grown in even colder climates, but then suffers winter damage and may flower infrequently if at all.
This woodland species is well adapted to landscape use. In northern or cooler climates, oakleaf hydrangeas do well in full sun to light shade but in southern or hot climates, some afternoon sun protection is advised. It will grow best in rich, well-drained, slightly acidic soils. At full maturity, it can be from 6 to 8 feet in height and diameter, but can be kept smaller by pruning. Also, there are many dwarf cultivars of smaller stature.
Climbing Hydrangea (Hydrangea petiolaris, syn. H. anomala petiolaris)
This very different hydrangea is a climbing shrub from central Asia. It first forms a mounding shrub, then after several years in this juvenile form, starts to produce climbing branches that cling to rock faces, tree trunks and walls through dense aerial roots. They can reach up to 50 feet (15 m) high after any years. These climbing stems in turn produce, after several years, horizontal secondary branches that arch out, giving the plant more depth than most climbing plants. It is from those branches that clusters of scented white flowers appear in late spring-early summer. The flowers show the typical lacecap form: a frothy, flattened, very large cluster (up to 10 inches/25 cm in diameter) of fertile flowers surrounded by an open halo of large sterile ones.
The climbing hydrangea is covered with heart-shaped green leaves turning yellowish in the fall. There are also cultivars with variegated foliage. The exfoliating, reddish-brown bark offers considerable winter interest.
It’s yet another hydrangea that is easy to grow, adapting to just about any well-drained soil, although it prefers rich, fertile, moist conditions. It’s best known as a shade plant, adapted to part to full shade, but does fine in full sun if grown in consistently moist soil. It’s adapted to USDA hardiness zones 4 to 8, but worth trying in zone 3.
If there is nothing for it to climb, climbing hydrangea will also grow as a ground cover.
Considerable patience is needed with the climbing hydrangea. It may take 3 or 4 years before starting to climb and yet another 3 or 4 years before producing the first blooms. From then on, though, it will normally flower abundantly every year.
There is considerable confusion between this plant and another climbing hydrangea, H. anomala, and indeed, H. petiolaris is still sold under the name H. anomala petiolaris. However, authorities recently accorded H. petiolaris full species status. H. anomala is a smaller species with smaller flowers and reduced hardiness (USDA zones 5 to 8 rather than 4 to 8). It is rarely seen in gardens.
More popular is the Japanese hydrangea vine (Hydrangea hydrangeoides), still sold under its former botanical name, Schizmophragma hydrangeoides. It’s quite similar to H. petiolaris, with flowers that appear identical from a distance, but differ by the fact that the sterile flowers produce only a single paddle-shaped white bract rather than a bloom with 4 sepals. It is also less hardy (USDA zones 5 to 9).
Two cultivars are popular. ‘Moonlight’, with silvery-blue leaves with dark green veins and ‘Roseum’, with pink bracts. Rose Sensation™ is similar if not identical.
Pruning Your Hydrangea:
How you prune your hydrangea depends on the species:
Bigleaf Hydrangea (H. macrophylla): This species blooms mainly on old wood. It requires very little trimming, as too much pruning removes potential blooms. Any pruning should be carried out immediately after blooming by cutting back flowering stems to a pair of healthy buds. Of course, you can prune also out weak or winter-damaged stems in early spring.
Smooth Hydrangea (H. arborescens): It blooms on new wood, so can be pruned back harshly in late winter or early spring and will bound back to flower the same summer. Pruning encourages new growth and tends to produce stronger stems and better blooms.
Panicle Hydrangea (H. paniculata): It also blooms on new wood and can be pruned in late winter or early spring. It does fine with no pruning at all, but is often cut back to control its size. For a shorter shrub, perhaps 4 feet (1.20 m) in height, cut to the ground in early spring; leave the stems 1–3 feet (30–90 cm) long for taller plants.
Oakleaf Hydrangea (H. quercifolia): It blooms on old wood (previous season’s growth), so prune right after flowering.
Climbing hydrangea (H. petiolaris): It rarely requires pruning, but when it does, cut back immediately after it blooms, as it flowers on old wood.
Pink or Blue Flowers?
One of the most common questions about bigleaf and mountain hydrangeas is how to get blue flowers. You can change a pink hydrangea flower to blue by changing your soil’s pH … and vice versa. (You cannot change the color of white or green flowered varieties.)
To get blue flowers, there must be aluminum in the soil and indeed, it is present in most soils. In acid to neutral soils, the shrub will absorb it readily and blue flowers will result. But in alkaline soils, the roots can’t take up the aluminum and the flowers will be pink.
Do a soil test to find out what conditions you have. If your soil is acid to neutral, and your flowers are pink, but you’d like blue, apply some granular aluminum sulfate, sulfur or iron sulfate to acidify the soil. If your soil is alkaline, you can try to acidify it by working in lots of organic material and adding elemental sulfur gradually over several years.
Ask a local expert for advice on what works best in your area. Always follow package directions carefully. It may take several months or a season for these soil modifications to work.
Uses in the Garden
Hydrangeas can be used in so many ways in your garden, from a focal point to establishing a border and filling the foundation around your house. And they don’t just have to be planted in the landscape anymore either. Many hydrangeas, especially the newer varieties, are more compact and do incredibly well in a decorative pot. And don’t forget to cut some blooms for an indoor flower arrangement.
You’ll definitely enjoy the Year of the Hydrangea!
Unless otherwise noted, all photos are courtesy of the National Garden Bureau.