Hydrangea paniculata Siebold

Hydrangea paniculata is unmistakable in almost all its forms, and makes a superb, amenable garden plant for full sun or lightly shaded situations. A medium to large deciduous shrub, its elongated panicles of white flowers ageing graciously to pinks and reds over a long summer season and remaining intact well into winter, are the chief attraction. In Britain flowering typically peaks in August. Hardiness (this is arguably the most cold-tolerant species) and an ability to flower very well even after heavy pruning, are important virtues. Grown in the West since the 19th century, and for much longer in East Asia, cultivar selection for inflorescence form, size, or precocious reddening, as well as for plant form and vigour is a much more recent, ongoing phenomenon.

This species’ closest relative is H. heteromalla along with those taxa in its orbit (such as H. xanthoneura), a link supported by both morphology (McClintock 1957) and molecular data (De Smet et al. 2015; Yang et al. 2024). Apart from leaves sometimes being in whorls of 3 rather than in opposite pairs, especially on vigorous shoots (Dirr 2004), inflorescence form is the simplest identification feature for the gardener. In H. paniculata there is always a single, clear main axis (apparently indeterminate, and sometimes very long) with much shorter, spreading side branches more or less paired or in whorls of three. Leafy bracts subtending the branches are conspicuous at least in the early stages of flowering. The main axis is typically erect, but may be spreading. Side branches are apparently determinate and sometimes themselves branched. Flowers are carried on the side branches; in wild-type plants many branches, especially the lower ones, end in a long-stemmed sterile flower. These stems carry the sterile flowers out to the periphery; if the main axis is short, the inflorescence appears quite flat and ringed with sterile flowers, much like a more conventional hydrangea inflorescence, though the branching structure is quite different. If the axis is longer, the inflorescence becomes conical or ovoid, fringed along at least part of its length by sterile flowers. In some wild collections the axis is elongated but usually spreading, to give a rather different ‘flat panicle’ look (M. Foster pers. comm. 2023), but the underlying construction is the same (pers. obs.). Some wild finds, and many cultivars have increased proportions of sterile flowers, giving quite different effects (see below). In H. heteromalla, by contrast, there is very often no single main axis, rather several basal branches, although sometimes there is a single axis, short and apparently determinate (see for example images of diverse wild Chinese material on the Plantplus website – Chinese Academy of Sciences 2023): the inflorescence never becomes conical or ovoid. The terminology of inflorescence form is plagued by ambiguity and conflicting definitions (Prenner, Vergara-Silva & Rudall 2009). However, it seems reasonable to apply the terms ‘panicle’ and ‘thyrse’ to H. paniculata, but not to H. heteromalla.

The ovate or elliptic leaves also vary in horticulturally significant ways. In some plants they are hairier, or glossier, on the upper surface than others; some are distinctly acuminate. In some the leaves tend to hang rather than spread (Bean 1981).

Reasonable light levels are crucial for good flowering. In a Japanese Beech (Fagus crenata) forest, Kanno & Seiwa (2004) found Hydrangea paniculata growing as an understorey shrub throughout the forest cycle, but flowering only in gaps created by falling trees. In the deeper shade of the building and mature phases of the cycle, seedlings established during the gap phase grew alongside older plants, but none flowered; rather, they persisted and spread vegetatively through layering, to flower when they next found themselves in a gap. Similarly, the species flowers reasonably in partial shade in the garden, especially at lower latitudes with stronger sunlight, but is most floriferous in full sun (Dirr 2004). This is the most drought-tolerant of the commonly grown hydrangeas (Mallet, Mallet & van Trier 1992) and thrives in most soils, although dry summers reduce vigour, and out-and-out waterlogging might prove problematic. This is a self-incompatible species (Reed 2004) attracting plenty of potential pollinators (various bees, flies and wasps, pers. obs.). While Dirr (2004) noted that it seems rarely to self-seed in garden conditions even when other clones grow nearby, many seedlings appear among plantings at the Yorkshire Arboretum, UK (J. Grimshaw pers. comm. 2023).

Like other hydrangeas, H. paniculata grows away strongly after pruning, but unlike most these shoots flower well in their first season; indeed, their inflorescences tend to be larger than in unpruned plants. Heavy annual pruning is therefore perfectly feasible (Lancaster & Wesley 2008). Unpruned or merely dead-headed plants build to make large, upstanding shrubs or ultimately small trees. Pruning allows smaller, bushier plants over the medium to long term. Initially, stems are cut back only to form a permanent skeleton, short or tall, broad or narrow as desired; thereafter, annual growth is cut back to two buds in late winter. This results in the largest panicles and the most vigorous shoots, although their weight and brittleness can make them vulnerable to wind damage in exposed positions (Lawson-Hall & Rothera 1995). Thinning out the shoots as they grow can further increase panicle size (Bean 1981). Pruning to four buds gives intermediate size and vigour, while postponing pruning until as late as the end of April can delay flowering where this is desirable (Lancaster & Wesley 2008).

The Panicle Hydrangea grows wild across a swathe of southern China, Taiwan, most of the Japanese islands and into the Russian Far East. Real variation in hardiness might be expected, but garden material in general seems very tough. When H. p. ‘Grandiflora’ was pushed experimentally to its limit in Finland, LT₅₀ (the temperature resulting in 50% death from cold) proved to be –37°C (Suojala & Lindén 1997). This makes Panicle Hydrangea suitable for gardens across much of Europe and North America, away from areas with seriously dry summers. Dirr (2004) lists a string of cultivars flowering reliably in Zone 4 Minneapolis, MN. This general hardiness might simply reflect how little of the potential variation is represented in garden populations.

Like several other Japanese hydrangeas, H. paniculata became known in the West through Phillip von Siebold, who grew it in his Japanese botanic garden and described it scientifically (Siebold 1829). Like many of Siebold’s plants, introduction was delayed by his banishment from the country; it had reached Europe by 1861 (Bean 1981). It was already well known in Asia, leaf extracts being used widely in southern China as an anti-inflammatory. This has been linked to coumarins in the plants, and there is a significant strand of research into its possible use in acute kidney damage (Wang et al. 2022). Siebold (1829) made it clear that the species was already a garden ornamental in Japan, with cultivars known.

A few early introductions seem to have dominated the garden gene pool. The essentially mop-headed ‘Grandiflora’ had arrived in Europe from Japanese cultivation by around 1870 (Bean 1981), bringing with it the genes for future varieties of this type, though ‘Grandiflora’ itself was among the most important garden hydrangeas on both sides of the Atlantic for much of the 20th century. ‘Floribunda’, with more sterile flowers than usual, spread across the inflorescence, again came to Europe from a Japanese nursery in the 19th century (Hoffman & Houtman 2007) and many contemporary cultivars share it as an ancestor. The distinctive, early flowering ‘Praecox’ derives from a seed collection made on Hokkaido in 1893 by Charles Sargent (Bean 1981). ‘Kyushu’ was collected in 1926 on the eponymous island by British cherry specialist Collingwood Ingram (van Gelderen & van Gelderen 2004; Association Shamrock 2022). 20th century collections are too many to list, but almost all are from Japan. The few mainland Chinese introductions include plants of Jiangxi provenance (by way of Shanghai Botanic Garden) at the Arnold Arboretum (Arnold Arboretum 2023) and Tom Hudson’s ‘Big A’ and ‘Big B’ (Association Shamrock 2022). Taiwanese material is equally sparse but includes Crûg Farm’s BSWJ 3542, 3556 & 3802 (Taipingshan, 1996) and Dan Hinkley’s DJHT 99157 of 1999 (Crûg Farm Plants 2023; Reed & Rinehart 2009). Russian-provenance plants are apparently unknown.

Polyploidy is significant in this species, and ploidy seems to correlate with geography. Across the Japanese islands, Funamoto & Ogawa (2002) found diploids (2n=36), tetraploids (2n=72) and hexaploids (2n=108). Tetraploids are the norm in Shikoku, Kyushu and W & C Honshu, while hexaploids were found in C & N Honshu, and in Hokkaido. Diploids were localized in a few mountainous areas of W Honshu and Shikoku. Interestingly, almost all ploidy estimates from plants in Western gardens show tetraploids (reviewed in Reed & Rinehart 2009). However, ‘Praecox’ (Hokkaido provenance) is a hexaploid as might be expected, as is ‘Dharuma’ (Beck & Ranney 2014). These two plants have much in common, including early flowering and rather flat inflorescences, and are so distinctive that a hybrid origin for at least ‘Dharuma’ had sometimes been mooted (van Gelderen & van Gelderen 2004). Three pentaploid cultivars have been identified, presumably the result of a tetraploid crossing with a hexaploid (perhaps ‘Dharuma’): these are the very significant ‘Bulk’ (= QUICK FIRE, = EARLY SENSATION) as well as ‘Smhplqf ‘(= LITTLE QUICK FIRE) and ‘Wim’s Red’ (Beck & Ranney 2014). Experimental crosses showed that pentaploids are easily made and are fertile, producing offspring with a wide range of chromosome numbers including aneuploids and higher ploidy levels. We have seen no ploidy estimates for non-Japanese plants.

Apart from the early introductions mentioned above, almost all cultivars grown in Europe and North America have been bred here since the mid-20th century. Jelena and Robert de Belder of Arboretum Kalmthout, Belgium, began the process by raising seedlings from ‘Floribunda’ in 1958 (Hoffman & Houtman 2007). Disturbance by birds on a hot day led to the loss of all but one seedling, the celebrated but now superseded ‘Unique’. Three more generations of seed-raising produced a set of widely grown cultivars, including ‘Brussels Lace’ and ‘Interhydia’ (PINK DIAMOND). More recently commercial breeders have taken up the species, first in Europe, then in North America; ‘Limelight and ‘Bulk’ were early commercial successes from this period. Very many (too many?) cultivars continue to appear. Inflorescence size, compact form, the stem strength needed to support huge flower heads, early flowering (partly to lengthen the season, partly to catch customers at the peak of garden centre visiting) and precocious pinking of the sepals have all been breeding aims (Dirr 2021). Cultivars can be classified by the proportion of sterile flowers in the inflorescence, as follows (Lancaster & Wesley 2008): Open (<10% sterile flowers, e.g. ‘Kyushu’); Half Open (10–50% sterile, e.g. ‘Big Ben’); Dense (50–80% sterile, e.g. ‘Dolly’); Very Dense (80–100% sterile, e.g. ‘Limelight’). The great majority of new cultivars are dense or very dense.

While most H. paniculata inflorescences age eventually to reddish tones, some cultivars develop stronger, clearer or earlier pinks or reds than others. ‘Dvppinky’ (= PINKY-WINKY) was a commercially important early example, now arguably superseded by cultivars such as ‘Renhy’ (= VANILLE FRAISE). Environmental conditions are clearly important for performance of these pink cultivars. Cooler temperatures, at least at night, are usually implicated in colour development, and water stress is thought to work against colouring (Proven Winners 2023). In practice, the relative performance of these varieties can be unpredictable, with different cultivars colouring best in different years in an Ohio research collection (Wessel 2023).

Further characters differentiating cultivars are stem colour (deep reds are usually considered a good thing) and stem strength, especially important in varieties with large, heavy heads with many sterile flowers. With perhaps 200 named cultivars to date, and more appearing yearly, we present a large selection based on distinctiveness or unusual features, quality as judged in trials, commercial importance today or in the past, or historical importance as a parent. Arguably the best place in the world to compare serious numbers of H. paniculata cultivars is the near-comprehensive UK National Collection, in Darley Park, Derby (Hydrangea Derby 2023). Reports of several trials, at Wisley, SE England (‘Wisley trial’ – Lancaster & Wesley 2008), across W Europe in the Netherlands, Germany, France and Britain (‘Euro trial’ – Hoffman & Houtman 2007), and Chicago Botanic Garden, United States (‘Chicago trial’ – Hawke 2022) are invaluable. A new British trial is currently under way at RHS Garden Bridgewater in NW England, and the text here will be updated when its results are known.

There are now too many H. paniculata cultivars, appearing at too fast a rate, for an attempt at a comprehensive listing to be useful here; this is the role of the International Register (Association Shamrock 2022). We make a large selection, including the more common established cultivars, those of historical significance, varieties singled out in trials as especially good, along with a sample of those too new for any judgement to be made, listed on alphabetical pages (Hydrangea paniculata Cultivars A–D etc.). More will be added as time goes on.

Well documented hybrids with other species are few. ‘Dharuma’ has sometimes been listed as a possible hybrid with H. heteromalla (van Gelderen & van Gelderen 2004; Edwards & Marshall 2019) but its status as a Hokkaido hexaploid seems a better explanation for its unusual features, and most specialists today treat it as unhybridized H. paniculata. Experimental hybridization with H. macrophylla involved embryo rescue and in vitro propagation; a single clone made it as far as a rooted plant, a sterile white mophead (Reed, Riedel & Pooler 2001; Reed 2004), and this seems to have been a dead-end.

Hydrangea paniculata is grown as a speciality cut flower crop in both Europe and North America (Leeson et al. 2004; Full Moon Blooms 2020).