January 2010 Plant of the Month — Hydrangea Print
There are about 70 recognized species of Hydrangea.Traditionally the Plant of the Month explores a genus of herbaceous perennial, but in a daring departure this month’s subject is the genus Hydrangea and a closer look at some of the newer cultivars. Lately, has been a wonderful resurgence of interest in these beautiful woody shrubs. Not surprisingly this genus falls within the family Hydrangeaceae which also includes the well-known genera Deutsia and Philadelphus and the perhaps less familiar but eminently garden-worthy, Schizophragma, Decumaria, and Kirengeshoma.  There are about 70 recognized species of Hydrangea, most of which are native to Asia, but there are a few that call North America home. Generally the foliage is opposite on the stems and the seeds are contained within a capsule or berry. The name Hydrangea comes from the Greek for “water vessel” in reference to the shape of the fruit.

The flowers of Hydrangea are generally arranged in large compound inflorescences such as panicles or more flattened corymbs such as mopheads and lacecaps.  The interesting form of the blooms comes from the fact that the fertile flowers are generally small and rather inconspicuous, often resembling buds that never open, while the showy more obvious flowers have large papery bracts but are most often sterile.  This is how the lacecap effect is achieved — fertile flowers in the center of the inflorescence with the flashy ‘petals’ towards the outside. Many of the cultivars in production are selected specifically for their abundance of these showy but sterile flowers, imparting the benefits of an extended season of bloom, as the plants are not trying to produce seed, and a distinct lack of ‘weedy’ seedlings that attempt to establish themselves in parts of the garden of their choosing rather than the gardener’s.

Growing and pruning

Hydrangea are generally easy to grow, preferring full sun to partial shade and a moist location, they do not like to sit in water but drought conditions are definitely not appreciated. The flowers are of course the reason that the shrubs are grown and can sometimes be a point of frustration and despair for the gardener. The inflorescences are produced at the terminal ends of the stems and can often be large and heavy. If the plant has grown too soft or too fast the stems may not be strong enough to hold the blooms upright and flopping may occur. Caution with fertilization and location can often ‘cure’ this problem as well as choosing cultivars that are noted for strong upright habits. But perhaps the greatest frustration is the complete lack of flowering that some gardeners experience, especially in colder, more northerly hardiness zones. Whereas the roots and the plant itself are perfectly able to withstand harsh winter conditions, the flower buds are often susceptible to damage or death from late spring frosts or other unseasonable conditions. In addition inappropriate pruning is often the culprit as the gardener removes the year’s flower buds in the guise of shaping the plant. Depending upon the species, Hydrangea form their flower buds in the late summer and autumn and overwinter them to bloom on what is called ‘old wood’, or the previous year’s growth. Autumn or Spring pruning will obviously remove these buds and thus the floral display for that season. Conversely some species develop their flower buds during the spring and early summer blooming on ‘new wood’ and are thus unperturbed by winter conditions. These are best pruned in the spring if it is necessary.

Native to North America are H. arborescens and H. quercifolia. H. arborescens is sometimes known as the Smooth Hydrangea, the well known and much loved (or maligned) cultivar ‘Annabelle’ is a selection from this native species. Whereas H. quercifoila as the name suggests is the Oakleaf Hydrangea known for its attractive foliage in addition to the large cone shaped panicles that put on such a show in late summer. The flower buds are formed on old wood and thus require a sheltered location in northern gardens, being reliably flower-hardy only to zone 6, but it is quite possible to enjoy the blooms in zone 5 with careful siting. However the foliage is so magnificent in the fall turning shades of burgundy, purple and bronze that even without the flowers it is well worth growing.  The famous American plantsman and explorer John Bartram is responsible for the naming of this species and the introduction of it to European gardens.

Of far more interest to growers and gardeners are the two species H. macrophylla (Big Leaf Hydrangea) and H. paniculata (Panicled Hydrangea) both of which originate in the far east (China, Japan) and have provided ornamental horticulture with some of the most beautiful and popular woody shrubs grown today.

Blue or pink?  Often it can be your choice.

H. macrophylla is also known as Mophead, Lacecap or Florist Hydrangea, depending upon which of over 600 cultivars is being grown. Because they flower on old wood it is these bigleaf cultivars that often sustain damage and thus fail to flower.  Winter protection can help, as well as careful choice of cultivars as newer releases are being selected specifically for hardiness. It is also H. macrophylla that are adaptable in their flower color depending upon the soil conditions in which they are grown. Their natural color is a creamy white but the flowers are blue in acid soils and pink in alkaline soils. It isn’t the pH of the soil per se that causes the shift it is actually the presence of aluminum. In alkaline soils, or those with an abundance of phosphorous, the aluminum is tied up and unavailable to the plants causing the flowers to be pink, but in acid soils the aluminum is readily available and able to affect the pigment color turning the petals blue. Treating the soil with aluminum sulphate will both acidify the soil and add to the availability of aluminum ions. The degree of effect and the depth of the pigment is dependent upon the cultivar, many of the selections are based on this ability.
 
H. paniculata on the other hand is often referred to as the PeeGee hydrangea due to the almost overwhelming presence in the marketplace of H. paniculata ‘Grandiflora’ a great plant but there is much more to explore. As the name implies the flowers are held in large cone shaped panicles (along with H. quercifolia the only species with this shape – the others are much more spherical). Flower color is not influenced by the pH of the soil, they usually start out a creamy white and mature to deeper shades and hues of pink. H. paniculata are considerably hardier than H. macrophylla cultivars especially because they bloom on new wood. Pruning in the spring is quite acceptable.  H. paniculata was first formally  described and named by Philipp von Siebold while he was living in Japan. This is just one of the many plants he sent back to Europe in the early 19th century. He is recognized for his contributions to horticulture with every plant that bears his name; Hosta sieboldii, Sedum sieboldii, Malus sieboldii to name but a few.

All Hydrangea make beautiful cut flowers either fresh or dried. Dying the blooms — even after they have been dried for a couple of years is a wonderful way of bringing color into an arrangement. Recently there has been a veritable spate of introductions of new and garden worthy cultivars, a few of which are discussed below.

Hydrangea EDGY™ Hearts (H. macrophylla ‘Horheart’ PPAF)

The trademark EDGY is meant to signify that these selections are pushing the envelope. These are the first blooms with a picotee edge to the heart-shaped flowers. In alkaline soils the flowers are deep pink (intensified with the addition of lime to the soil) with a clear, distinct white border. In acidic soils the pigment will be more blue. Flowering occurs in June and July with late July-early Aug the best time to prune. Foliage is a clear dark glossy, green and stems are strong and well able to support the large flower heads.  Autumn brings lovely shades of red to the foliage. Reaching a mature height and spread of 2’-4’ (60cm-120cm) this is a great selection for containers, but is still compact enough to provide protection for the sensitive buds. Originally developed in Germany due for retail release in N/A in 2010. Zone 5-9

Hydrangea EDGY™ Orbits (H. macrophylla ‘Harbits’ PPAF)

The second in the series, this one developed in Michigan, USA. Same great coloring and habit but with double lacecap inflorescences. Also to be released 2010. Zone 5-9

Hydrangea CITYLINE™

A series of 6 (and counting) beautiful little dwarf shrubs. They will reach a maximum height and spread of 1’-3’ (30cm-90cm) with a clean tight habit. Foliage is a glossy dark green. In most of North America the flowers will be varying shades of pink but will be pushed into the blues in acidic locations. Their small stature makes these plants incredibly easy to protect with a layer of mulch, but don’t be fooled into thinking that the display will be petite to match the plants, flowers are large and vibrant.  Zone 5-9

CITYLINE™ Berlin (H. macrophylla ‘Berlin Rabe’ pp# 10,912)

The largest of the series, this quick growing cultivar has flowers that open clear green and soon mature to a deep pink.

CITYLINE™ Mars (H. macrophylla ‘Ramars’)

More on the compact side with bicolor flowers, each petal having a central splotch of pink. Blooms age to an attractive green. Excellent mildew resistance as well.

CITYLINE™ Paris (H. macrophylla ‘Paris Rapa’ pp# 10,906)

The large flowers are green and red as they emerge and age to a beautiful dark pink with green centers.

CITYLINE™ RIO (H. macrophylla ‘Raga’)

Large flowers have a green ‘eye’ especially prominent at emergence. Petal edges are a magical shade of blue with pink tones.

CITYLINE™ Venice (H. macrophylla ‘Venice Raven’ pp# 10,928)

Very compact habit with large clear pink flowers. A little aluminum sulphate develops them into a rich blue.

CITYLINE™ Vienna (H. macrophylla ‘Vienna Rawi’ pp# 10,930)

Flowers emerge green and age to a clear pink. Effect is of a green centered flower with pink petals.

Hydrangea PINKY WINKY™ (H. paniculata 'DVPinky' pp#16,166)

Winner of the 2008 RHS Award of Garden Merit AGM. A wonderful addition to the PG type of Hydrangea. Large inflorescences (12”-16”, 30cm-40cm) appear in midsummer on strong red stems and continue the show right through until frost. New flowers emerge creamy white while the older blooms have aged to a delicious deep pink. Since the panicles continue to grow as the flowers emerge a stunning two-tone effect is achieved. This coloration is not effected by the pH of the soil. Blooming occurs on new growth so spring pruning is acceptable and no winter protection is required. Foliage is a strong green becoming a clear golden yellow with hints of burgundy in the autumn.  Height is a magnificent 6’-8’ (2m-2.75m). Full sun is preferred but moisture is not as big an issue, some drought can be tolerated once established. Zone 3-8

Hydrangea paniculata ‘Limelight’ pp12,874

The unique chartreuse flowers cause this magnificent shrub to really stand out in the summer and fall garden. Blooms emerge in mid summer as a cream to soft lime green but quickly mature through chartreuse to their fall coloring of a deep rich pink. By late summer all shades are present on the one plant – what a show!  The panicles are a little smaller than PINKY WINKY™ but are still a respectable 6” -12” (15cm-30cm). Excellent as a cut flower either fresh or dried. Plants will quite quickly reach their mature height and spread of 6’-8’ (2m-2.75m) and being a H. paniculata they require no winter protection as they bloom on new growth. Zone 3-8.
 

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