Description from Flora of China
Herbs, perennial, with rootstock; stems erect and rather stout, to 50 cm tall, quadrangular, sulcate, densely shortly hirsute to hispid. Leaves in whorls of 4, sessile; blade drying papery, broadly elliptic-oblong, ovate, or sometimes suborbicular, 2-4.5 × 1-3 cm, both surfaces densely hairy and ± scabrous, base obtuse to subrounded, margins shortly aculeolate-ciliate, apex acute to weakly obtuse; principal veins 7-11, palmate. Inflorescences thyrsoid, paniculate, with terminal and axillary, many-flowered and to 10 cm long cymes; axes shortly hairy, aculeolate to glabrescent; bracts leaflike or lanceolate to linear-lanceolate, 1-2 mm; pedicels ca. 1 mm. Ovary 0.4-0.5 mm, subglabrous. Corolla pale yellow, rotate, 2.5-3 mm in diam., hairy outside, glabrous or scabrous inside, fused basal part 0.2-0.3 mm; lobes ovate or lanceolate, 1-1.2 mm, apex ± recurved. Immature mericarp berry 3-5 mm in diam.
Rubia polyphlebia belongs to the R. mandersii group (see there) of R. sect. Oligoneura. We have seen no material of this species, but there is a good drawing in H. S. Lo (in FRPS 71(2): 304, t. 67, f. 1-7. 1999).