Shrub up to c. 1-5 m tall, with branches widely spreading. Stem orange, 4-angled but not ancipitous when young, soon 2-lined, eventually terete; internodes 10-60 mm long, shorter than leaves; bark reddish-brown. Leaves rather broadly petiolate, with petiole 1-2.5 mm long; lamina 30-75 x 8-20 mm, lanceolate, acute to apiculate, margin plane, base cuneate, paler beneath, not glaucous, chartaceous; venation: 4-5 pairs main laterals, all free or subdistally closed, with subsidiaries not prominent and obscure tertiary reticulum; laminar glands dots and short streaks; ventral glands absent. Inflorescence 1-9-flowered, from 1-3 nodes, subcorymbiform; pedicels 5-10 mm long; bracts reduced, ± narrowly elliptic, persistent. Flowers 40-50 mm in diam., cyathiform; buds narrowly ovoid, acute. Sepals 6-9 x 3.3-4.5 mm, free, imbricate, subequal, erect in bud and fruit, ovate-lanceolate or elliptic to oblong- spathulate, acute to apiculate-obtuse or rarely rounded-apiculate, margin entire to minutely denticulate, hyaline, sometimes reddish; midrib ± distinct, veins not prominent; laminar glands linear, c. 12. Petals golden yellow, ± shallowly incurved, 20-30 x 15-19 mm, 2.5-3 x sepals, broadly oblong-obovate, with apiculus subterminal, obtuse; margin entire. Stamen fascicles each with c. 45 stamens, longest 9-13 mm long, 0.3-0.4 x petals; anthers yellow. Ovary 7-8 x 4-5-5 mm, ovoid-conic; styles 4.5-5 mm long, c. 0-6 x ovary, free, suberect, outcurved near apex; stigmas truncate. Capsule c. 20 x 10 mm, ovoid-conic, ridged. Seeds not seen.
2n = 30.
Known only in cultivation.
H. x cyathiflorum was apparently introduced to cultivation by Jackman's of Woking, Surrey, England, about 1952 as H. patulum 'Gold Cup'. The name was later (c. 1960) changed to H. hookerianum 'Gold Cup', probably on account of the terete mature branches; and I (Robson, 1970), on account of its acute to subacute sepals and buds, placed it under H. beanii. The source of Jackman's material has not been located. It may be significant, however, that this plant is grown at Hidcote Manor, Gloucestershire, under the name 'Lawrence Johnston', after the late owner of Hidcote Manor and maker of its garden. A chance seedling originating there might well have been thus named. The chromosome number (2n = 30) suggests that it is a triploid on the base x = 10, and its relatively poor fruiting would tend to support this hypothesis (see also 49xx. H. 'Hidcote'). I have not seen any wild-collected material of this plant, which is strikingly intermediate between H. addingtonii and H. hookerianum. It is therefore very likely that it arose in cultivation in western Europe, probably in Britain after the introduction of H. addingtonii by Forrest (?). As other hybrids between H. addingtonii and H. hookerianum may be (or may have been) produced, I proposed that the plant described above to be known as H. x cyathiflorum 'Gold Cup'.