Platycrater arguta (Hydrangea platyargutaI) provides textural contrast with large foliaged hydrangeas and other shade loving plants.
PHOTOS BY MICHAEL A. DIRR

In 2019, before the pandemic, I started a new Hydrangea tome since seismic taxonomic, breeding and cultivar introduction advances occurred after the publication of my Hydrangeas for American Gardens in 2004. Aside from the avalanche of new cultivars, especially prevalent among H. arborescens, H. macrophylla and H. paniculata, one of the most significant changes is the merging of several Hydrangeaceae genera into Hydrangea. Decumaria, Dichroa, Pileostegia, Platycrater and Schizophragma are now Hydrangea species according to Taxon 64(4): 741-753 (2015). Though these changes will not be universally adopted, I noticed the Germplasm Resources Information Network (GRIN), maintained by the USDA-ARS, has embraced them. Through six editions of my Manual, Platycrater was not included. The Hillier Manual of Trees and Shrubs (2019, 9th edition) has never listed the species through nine editions. It is not a household garden word and checking internet sites is limitedly available. The past few years, I collected several cultivars and I’m on the hunt for others. I showed a plant in full flower to a nursery friend in 2020 and he commented it would make a great hanging basket plant. I believe the species and cultivars have much more to offer than this, so let’s dive in.

Platycrater arguta, a monotypic species, is worthy of consideration for use in partially shaded to shaded situations in USDA Hardiness Zones 7-9. Broken Arrow Nursery in Hamden, Connecticut, lists the species as Zone 5. I discussed this with the nursery and was told it dies to the ground but regenerates. It exists as a subshrub and though potentially killed to the ground in winter, it rebounds and forms a 3-feet-high by 3-feet-wide delicate, graceful flowering shrub by summer. In Athens, the plant is completely above-ground stem hardy (Zone 8). Philip Schretter, a former student who manages the campus landscape arboretum at Georgia Southern University-Armstrong (Savannah), added the species in 2005 to the collection. It is 5 feet high, 6 feet wide, never been pruned or cut back, and suffered no cold damage in 17 years. His photos show a graceful, rounded, arching shrub with willow-like leaves. There, it is deciduous, but in mild winters has remained semi-evergreen. The Flora of China records size as 1½- to 10-feet high shrub where it occurs in sparse forests and thickets in valleys, on mountain slopes, and along streambanks from 1,300 feet to 6,000 feet elevation in China and also Japan. A paper in BMC Evolutionary Biology (2014) delineated two varieties, var. arguta and var. sinensis based on geography and molecular data.

In the Dirr garden, the species overwintered without stem injury in containers after the 2019-20 winter, when the lowest temperature in Athens was 21.1°F. These plants were in leaf in late March with flower buds evident in April and in full expansive flower in June. The emerging leaves are bright green, darkening with maturity, turning yellow in November-December in Zone 8. Leaves are opposite, 3-6 inches long, 1½ to 3½ inches wide (often less in width), petiole ½ to 2½ inches long (petiole on my original plant is ¼-inch long), acuminate apex, narrow cuneate base, the blade lanceolate to elliptic, with 7-9 veins on either side of the midvein, the margins roughly serrate to serrulate.

‘Honey Moon’ has fragrant flowers.
PHOTOS BY MICHAEL A. DIRR

Flowers develop from previous year’s wood and on the new growth in June-July, continuing sporadically for a time, never overwhelming, yet elegant and refined. The original plant in my possession developed flowers from previous year’s stems as well as new growth. Inflorescence is a terminal panicle (may be corymb) with multiple semi-pendulous flowers. Each flower composed of 3 to 4 white sepals, 1-1¼ inches in diameter, broad ovate, and fused from the base to the middle, forming a triangle or square outline. Flowers may be fragrant or have no odor. The yellow stamens are positioned in the center of the reflexed sepals. Platy means flat and crater means bowl in reference to the flowers while arguta means silver, referring to the white flowers. I noticed all manner of insects visiting the flowers. The fruit is a striate (parallel lines), two-valved capsule, long styles resembling antennae, each capsule with tiny dark brown seeds.

The species is best grown in filtered light with uniform moisture and good drainage. Atlanta Botanical Garden-Gainesville grows the species in shade and full sun where it is in leaf 2-3 weeks ahead of H. macrophylla. I observed the Atlanta Botanical Garden-Gainesville plant in full flower in early June; the foliage as fresh as a daisy (no flowers) in October. The species has exhibited significant heat tolerance which is unexpected considering its native habitat in the wild. Foliage is exceedingly clean with no evidence of mildew or Cercospora at Premier Introductions, Inc. Cuttings root in 4-6 weeks when treated with 0.15% KIBA quick dip and placed under mist. On June 24, 2020, single-node cuttings were treated with 0.15% KIBA quick dip, placed in medium under mist and had rooted by July 28, 2020. Seeds should be processed similarly to H. arborescens and H. macrophylla, i.e, well-drained, peat-based germination mix with seeds surface sown and watered thoroughly. I was told seedlings require two years from seed sowing before flowering.

Emerging leaves are bright green, darkening with maturity, turning yellow in November or December. Flowers develop from previous year’s wood and on the new growth in June or July.
PHOTOS BY MICHAEL A. DIRR

Cultivars

‘Honey Moon’ (from Issima Plant Works, Little Compton, Rhode Island) has fragrant flowers three times as large as ‘Kaede’. Foliage is significantly larger than ‘Kaede’ and ‘Rosea’. My plant is yet to flower but the balloon-shaped buds (March 28, 2021) are three-times as large as the species type. ‘Kaede’ has larger, darker green foliage than the species and flowers twice as large. Leaves are slightly larger and stems thicker than the typical species. Ozzie Johnson (Atlanta) has a 6-foot by 6-foot plant that was developing fall color when I visited in mid-November. Cuttings he shared with me rooted at this late date. Nurseries Caroliniana (North Augusta, South Carolina) offers the plant. Owner Ted Stephens was gifted the plant during one of his trips to Japan.

‘Pink Moon’ from Issima Works (Compton, Rhode Island) has larger flowers, larger number of sterile flowers, and deeper pink sepal color than ‘Rosea’. The color is on the underside of the sepals so somewhat minimized for the viewer. Flowers are fragrant.

Process seeds much like you would H. arborescens and H. macrophylla — with well-drained, peat-based germination mix with seeds surface sown and watered thoroughly.
PHOTOS BY MICHAEL A. DIRR

‘Rosea’ has pink flowers and is offered by Broken Arrow Nursery, (Hamden, Connecticut). Was sent a few cuttings by Adam Wheeler, horticulturist, and the buds are bright pink.

Variegated form is not exactly dramatic as leaves are blotched/streaked/stippled gray and green.

I procured the species for possible hybridization work with other Hydrangeaceae members. Was not sure this is possible until reading Rinehart et al., Proc. SNA Res. Conf. 50: 656-659 (2005), where the authors showed affinities among Platycrater arguta, H. involucrata and H. aspera. Kind of an intriguing idea. Was told successful crosses were made with H. aspera but have yet to confirm. The large-flowered white and pink cultivars lead me to believe it could be a stand-alone ornamental flowering shrub. There are breeding opportunities using the cultivars listed above. It could prove a pretty garden plant as a textural contrast with large foliaged hydrangeas and other shade loving plants and would function as an elegant container, and perhaps hanging basket plant.

In 2005, the landscape arboretum at Georgia Southern University-Armstrong added the species to its collection. It’s not suffered cold damage in 17 years.
PHOTOS BY MICHAEL A. DIRR