Hydrangea Intersectional Hybrids

One intersectional hybrid (H. × versicolor) has been known for many years, though not recognized as such for most of that time. It is probably significant that it involves Section Dichroa, for too long a botanical and horticultural backwater. The potential value to breeders of wider, less traditional crosses has only been explored seriously since the late 20th century. Most of the few which have reached the market are still known mainly to hydrangea cogniscenti. However, the success of ‘Ushyd0405’ (RUNAWAY BRIDE®, FAIRYTRAIL BRIDE® etc.) puts a spotlight on the garden potential of hybrids between members of the Macrophyllae and the Chinenses, surely an area to watch.

Section Macrophyllae × Section Chinenses
Suspected natural hybrids between H. serrata and H. luteovenosa have been found in Japan (e.g. H. ‘Iyo-no-samidare’ and H. ‘Seto-no-tsuki’, below): these drought tolerant, hardy small shrubs may have some axillary as well as terminal inflorescences. Deliberate crosses made in the West between H. serrata or H. serrata/macrophylla hybrids and members of Section Chinenses include ‘Saturnalia’ and ‘Irish Lace’, neither of which has found much of a distribution. Two seriously commercial crosses involving H. macrophylla-based plants and members of Section Chinenses have reached the market to date, ‘Ushyd0405’ and ‘French Bolero’. Both make mound-forming plants with small axillary lacecap inflorescences down the length of their stems, giving a very distinctive ‘look’ as well as extended flowering. US Plant Patent applications show that a number of similar crosses are in the pipeline, including a mophead ‘Wnhydfbwmh’ (US Plant Patent 35,612, selected 2018 by Ushio Sakazaki, H. aff. macrophylla ‘Sugar White’ × H. luteovenosa) with white flowers streaked green, from green buds. A general, non-trademarked term for this emerging group of cultivars is still lacking. On the basis of the first two to reach the market, John Grimshaw suggests Mound Hydrangeas (pers. comm. 2024), although it is probably too soon to know how appropriate to a wider range of such hybrids this will prove.

Section Macrophyllae × Section Dichroa
These Sections are both in the Hydrangea II clade. Hydrangea × versicolor, which reached Western gardens from Hong Kong, has been shown to be H. macrophylla × H. febrifuga (in the widest sense); similar crosses have occurred in cultivation without recourse to laboratory techniques. For more details see Hydrangea × versicolor.

Section Macrophyllae × Section Hydrangea
Ovary culture has been used in making viable hybrids between H. macrophylla and both H. arborescens (Cai et al. 2015; Bak & Han 2024) and H. quercifolia (Bak & Han 2024), with no known impact on garden plant breeding to date.

Section Macrophyllae × Section Heteromallae

Experimental hybrids between H. macrophylla and H. paniculata required embryo rescue and in vitro propagation; one 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 breeding dead-end. Bak & Han (2024) also produced H. serrata × H. paniculata using similarly heroic measures.

Section Macrophyllae × Section Calyptranthe

French nurseryman Henri Cayeux successfully crossed H. macrophylla with H. petiolaris before 1922; the resulting plant was intermediate between the parents, but it was apparently never distributed, and all material was destroyed in the devastating bombing of Le Havre in 1944 (Haworth-Booth 1984). This is perhaps the first documented hybrid between climbing and shrubby hydrangeas. Bak & Han (2024) demonstrated that H. macrophylla × H. anomala is possible using embryo rescue, so long as H. anomala is the pollen parent. No information on plant morphology was published.

Section Calyptranthe × Section Cornidia

Hydrangea ‘Inovalaur’ (SEMIOLA®) – see below – is the result of one such cross, using conventional techniques. These are sister sections in the Hydrangea I clade.

Section Hydrangea × Section Asperae

These Sections are both in Hydrangea I. H. arborescens × H. involucrata proved possible using conventional techniques (Jones & Reed 2006); only crosses using H. arborescens as the seed parent produced seedlings, and then only a few, using H. a. ‘Dardom’ (‘Annabelle’ crosses yielded nothing). Early, uninspiring reports of flowering were published, and nothing more has appeared in the literature or nursery trade. One of the hybrid plants had some pollen fertility.

Section Hydrangea × Section Heteromallae

Using embryo rescue, seedlings were produced from purely experimental H. paniculata × H. arborescens crosses (Bak & Han 2024).

Section Hirtae × Section Chinenses

Wild plants from Honshu known as H. × amagiana Makino (probably not grown in our area) have been interpreted as H. luteovenosa × H. hirta (Ohba 2001).