Hydrangea-paniculata.pdf

RHS PLANT TRIALS BULLETIN

Number 23 December 2008

Hydrangea paniculata

Neil Lancaster

Botanist, RHS Garden Wisley

Neil Lancaster

Botanist, RHS Garden Wisley

Wendy Wesley

Trials Ofﬁcer, RHS Garden Wisley

Wendy Wesley

Trials Ofﬁcer, RHS Garden Wisley

The RHS Trial of Hydrangea paniculata

Backgroundtothetrial

Although Hydrangea paniculata has been cultivated in

European gardens for well over a century, for most of that

period there has been little choice beyond a handful of

very early introductions. H. paniculata , the panicled

hydrangea, has invariably played second ﬁddle to the more

colourful H. macrophylla , or hortensia, even though it is

generally easier to grow and more reliably hardy. The ﬁrst

edition of Michael Haworth-Booth’s The Hydrangeas ,

published in 1950, included over 300 hortensias, but only

3 types of H. paniculata , namely the cultivars ‘Floribunda’,

‘Grandiﬂora’ and ‘Praecox’.

H. paniculata inthewildandin

cultivation

H. paniculata is native of cool temperate and sub-tropical

regions of Japan, China and Sakhalin, where it grows as a

deciduous shrub or small tree to 8m in height, at altitudes

to about 1300m. It is often stated to be a very variable

species in the wild, but not all authors agree on this.

H. paniculata was ﬁrst described and named in 1829 by the

German physician and botanist Philipp Franz von Siebold

while working for the Dutch East India Company based on

Deshima Island within Nagasaki harbour, Japan. Granted

limited access to the mainland to carry out his medical

duties, at a time when other foreigners were excluded,

Siebold used the opportunity to collect and describe the

native plants, including H. paniculata . Siebold’s

determination to explore this closed country eventually

resulted in him falling foul of the authorities and he was

banished from Japan before the plant could be exported to

Europe. Fortunately, he was allowed to return 30 years

later, when he succeeded in sending back the cultivated

form of H. paniculata now known as ‘Grandiﬂora’. This

cultivar, also known as the pee gee hydrangea, is probably

still the most widely grown.

After the 1950s, the range of cultivars started to increase

with the appearance of the ﬁrst de Belder introductions

and slowly gathered pace in the 1970s and 1980s as new

generations of seedlings were raised from existing cultivars.

Because of the long lead-time for raising and introducing

new plants, only eight cultivars could be listed in the ﬁrst

edition of the RHS Plant Finder in 1987. A decade on, there

were still only twelve cultivars, four of the early de Belder

selections having become more widely available, but

subsequent years saw a more rapid increase. In the past 10

years, interest in H. paniculata has taken off with many

promising new introductions from breeders in Belgium,

Holland, France and the USA.

Other early introductions from Japan include ‘Floribunda’,

discovered by Carl Maximowicz in a Japanese nursery in

the 1860s, ‘Praecox’, raised from seeds collected on

Hokkaido Island by Charles Sargent in 1897, and ‘Kyushu’,

introduced as a wild-collected seedling from Kyushu Island

by Collingwood Ingram in 1926.

With so many new cultivars coming on to the market, the

RHS decided to hold a trial to compare the new cultivars

with the old and to select the best ones for the Award of

Garden Merit. More than 40 cultivars were assembled in a

trial that was considered by many of those involved to be

among the most successful woody plant trials yet

undertaken by the RHS. At the same time, the ﬁrst Eurotrials

were conducted in the UK, France, Germany and Holland,

providing an interesting opportunity to compare trial results

between different sites and growing conditions.

In the 1950s, at the embryonic Kalmthout Arboretum and

later at Hemelrijk, Robert and Jelena de Belder began to

experiment with raising and selecting seedlings of

H. paniculata , derived initially from an old plant in their

garden of ‘Floribunda’. The ﬁrst and most famous of these,

Front cover:

Hydrangea paniculata

‘Limelight’

Right: Hydrangea

paniculata Trial

August 2008

RHS Plant Trials Bulletin

Inﬂorescence of Hydrangea paniculata

showing pink late season colour

‘Unique’, was reputedly the sole survivor of a batch of

young seedlings that had been ravaged by blackbirds. It

turned out to be an exceptional plant whose offspring

yielded many good new cultivars, including ‘Papillon’,

‘White Lace’, P INK D IAMOND , ‘Burgundy Lace’, ‘Brussels

Lace’, ‘White Moth’, ‘Green Spire’ and ‘Melody’. Further

generations were raised in the 1970s and 1980s by open

pollination of these plants, with P INK D IAMOND and ‘White

Moth’, in particular, producing some notable cultivars. The

arboretum at Hemelrijk still contains many unnamed

plants, and seeds from these are still collected and

distributed to nurseries in the hope of raising new cultivars.

Breeders in Europe and the USA have built upon the de

Belder legacy, producing cultivars with inﬂorescences of

subtly different shapes and shades of pink, white and

green, as well as cultivars of more compact habit. Some of

the most striking new cultivars have been raised by Peter

Zwijnenburg at Boskoop, including ‘Big Ben’, ‘Phantom’,

‘Limelight’, ‘Dolly’, ‘Silver Dollar’ and ‘Pink Lady’, while

signiﬁcant contributions have also come from Darthuizer

in the Netherlands and Michael Dirr in the USA.

As a garden plant, H. paniculata has a number of

advantages over H. macrophylla . It is relatively drought-

tolerant, does well in both full sun and partial shade and

grows in acid or alkaline soil, as long as the ground is well-

drained. Importantly, it is very hardy (to about -30˚C in

the USA) and as it always ﬂowers on the current season’s

wood, it is not susceptible to late spring frosts.

BotanicalDescription

Hydrangea paniculata is a deciduous woody plant with

serrate leaves arranged in opposite pairs or clusters of

three, and ﬂower heads made up of showy, sterile ﬂowers

mixed with smaller, fertile ﬂowers. What sets it apart from

other species is the structure of the inﬂorescence, which is

typically a cone-shaped panicle. The only other Hydrangea

that has ﬂowers arranged in a panicle is H. arborescens and

that is distinguished by having lobed leaves.

Habit: In the wild may be a lax shrub to about 2m or a

small tree to 8m.

Shoots: Somewhat brittle. Downy at ﬁrst, becoming

hairless.

Leaves: Elliptic or ovate, serrate, acuminate, rounded or

tapered at the base, 7.5 – 15cm long and about half as

wide. Arranged in opposite pairs, or in 3’s. Sparsely

pubescent above and on the veins below.

Flowers: A mixture of small, yellowish fertile ﬂowers and

long-lasting (up to about 8 weeks) sterile ﬂowers with 4 or

5 white sepals, usually entire, often ageing to pink or

purplish-pink, particularly on the underside. Flowers are

borne in panicles on the current season’s growth, opening

from the base toward the apex of the panicle. The ovary is

semi-inferior.

Fruit: A many-seeded capsule. The seeds have no dormancy

and typically germinate in 2 to 4 weeks.

Hydrangea paniculata

The Trial

AwardofGardenMerit(AGM)

Objectives

To compare and assess new and old cultivars

To recommend the Award of Garden Merit to those

considered to be the best

To determine correct nomenclature

To obtain specimens, photographs, and descriptions as a

record to be held in the RHS Herbarium at Wisley

Hydrangea paniculata ‘Big Ben’ AGM(H4) 2008

Hydrangea paniculata ‘Floribunda’ AGM H4) 1993*

Hydrangea paniculata ‘Grandiflora’ AGM(H4) 1993*

Hydrangea paniculata ‘Kyushu’

AGM (H4) 1993

Hydrangea paniculata ‘Limelight’ PBR

AGM(H4) 2008

Hydrangea paniculata ‘Phantom’

AGM(H4) 2008

Records

Habit

Response to pruning

Stem colour

Foliage colour (summer and autumn)

Flowering period

Inﬂorescence shape

Flower colour (early, mid-season and late season)

Hydrangea paniculata

AGM(H4) 1993

P INK D IAMOND ‘Interhydia’

Hydrangea paniculata

AGM(H4) 2008

P INKY -W INKY ‘Dvppinky’ PBR

Hydrangea paniculata ‘Silver Dollar’ AGM(H4) 2008

Hydrangea paniculata ‘Unique’

AGM (H4) 1993*

* Recommended to be rescinded at the AGM Review

in 2012

Entries

68 entries, representing 47 cultivars, were submitted by

gardens, nurserymen and individuals in the UK,

Netherlands, Belgium and the USA. Details of each cultivar

are given in the Selection Table below.

The following will receive an Award of Garden Merit

when available to the public

Hydrangea paniculata ‘Dolly’

Hydrangea paniculata ‘Pink Lady’

Cultivation

Three plants of each entry were planted in May 2004. All

plants were cut down to the base after planting. The plants

were grown in the open, with no shade, and watered only

during periods of drought. Soil analyses were conducted

annually and treatments applied as necessary to eliminate

nutrient deﬁciencies and maintain approximately neutral

conditions.

Hardiness rating

H4 = hardy throughout the British Isles

JudgingCriteria

The Woody Plant Trials Subcommittee assessed

entries in the trial for the Award of Garden Merit

using the following criteria:

Habit

Bud / Flower / Faded flower

Floppiness of head

Foliage

Pruning

All plants were pruned each year in late March. The three

plants of each cultivar were used to test three pruning

regimes:

1 Hard pruning (to two buds)

2 Moderate pruning (to four buds)

3 Light pruning (removal of previous year’s ﬂowerheads

only)

The AGM is only awarded to plants that are:

Excellent for garden decoration

Available from nurseries

Reasonably resistant to pests and diseases

Of good constitution

Essentially stable in form and colour

Reasonably easy to grow

Hydrangea paniculata Trial September 2007

RHS Plant Trials Bulletin

Light

Moderate

Hard

1 Hard pruning (to two buds)

Where there was a response to pruning, the lightly pruned

plants were the ﬁrst to ﬂower and carried more, smaller

ﬂowers than the hard-pruned plants, which ﬂowered later,

and bore larger, fewer panicles. Those plants subjected to

moderate pruning were intermediate in panicle size and

time of ﬂowering. Although the hard-pruned plants tended

to grow with added vigour, their larger ﬂowerheads were not

always advantageous, sometimes producing panicles that

could not be supported and ﬂopped.

February

March

April

2 Moderate pruning (to four buds)

A further pruning demonstration using spare plants was set

up in which three plants of each cultivar were medium-

pruned at the end of February, March and April. Delaying

pruning was found to delay ﬂowering, as illustrated above.

3 Light pruning (removal of previous year’s

flowerheads only)

Both techniques proved to be of interest, the ﬁrst

producing varying bloom size and a ﬂowering delay, the

second bringing forward or delaying ﬂowering so as to

provide a succession of blooms. Either could be used to

extend the period of ﬂowering for group plantings. It may

also be worth trying more than one technique on the

different stems of a single plant, though this was not

attempted during the trial period.

Hydrangea paniculata

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