Shrub (0.2-)0.75-1 .5(-2) m tall, erect (or diffusely branched) with branches erect to ascending, forming rounded or irregular bush. Stems green, 4-lined and ancipitous when young, soon 4-lined and rounded, becoming reddish brown and terete in 2nd or 3rd season; cortex exfoliating in strips or flakes; bark pale grey, smooth, thin. Leaves sessile or rarely with pseudopetiole to 3(4-6) mm long, sometimes clustered; lamina 30-70 x 6-15 mm, narrowly oblong to narrowly elliptic-oblanceolate, plane? or with margin recurved, paler or somewhat glaucous beneath, deciduous at basal articulation, apex rounded-apiculate or rarely retuse to acute, base attenuate to narrowly cuneate (and then pseudopetiolate); venation usually clearly visible beneath: c. 10-16 pairs main laterals with subsidiaries and densely reticulate tertiaries, only midrib prominent; laminar glands dense. Inflorescence ( 1 )3-7(-9)-flowered, rarely with paired accessory flowers, with paired single flowers or triads or 1-3(-7)-flowered branches from 2 nodes below, the whole broadly to narrowly cylindric; pedicels 1-2 mm long; bracts foliar or reduced, oblong-elliptic. Flowers 15-30 mm in diam.; buds broadly ovoid. Sepals 5, 4-8 x 1.5-4 mm, enlarging in fruit, imbricate, unequal to subequal, broadly or narrowly elliptic to obovate-spathulate or oblanceolate, obtuse or apiculate-obtuse to acute, plane or with margin recurved, basal veins 3-7, not or obscurely branching. Petals 5, golden yellow, becoming incurved-deflexed, 7-15 x 3-6 mm, 1.75-2 x sepals, obovate to oblanceolate-spathulate, with apiculus lateral, obtuse or apiculate-obtuse to acute. Stamens c. 150- 500, longest 10-11 mm, 0.7-0.85 x petals. Ovary 3(4-5)-merous, (3-)4 5.5 x 1.2-3 mm, narrowly ovoid to narrowly ellipsoid, acute, placentation incompletely axile; styles 3(4-5), 4-6 mm long, 1-1.3 x ovary, remaining erect, separating slightly only as fruit matures. Capsule (6-)7-l 3 x 4-7 mm, narrowly ovoid-conic to ovoid or rarely ellipsoid, acute to subacute, trigonous to rounded, exceeding sepals, thickly to thinly coriaceous. Seeds blackish brown, 1.5-2 mm long, ecarinate; testa linear-reticulate to subscalariform.
2n = 18 (Nielsen, 1924; Adams in Robson & Adams, 1968).
Rocky slopes, embankments, dry river bottoms, open woodland (in north), usually on limestone but sometimes on granite; 90-over 600m.
Eastern U.S.A., Canada (southern Ontario): north to Iowa, Michigan and New York, west to Missouri, Arkansas and Oklahoma, south to Mississippi, Alabama and Georgia.
H. prolificum is the pivotal species in subsect. Centrosperma, linking 1. H.frondosum with 3. H. kalmianum and the H. densiflorum (Spp. 4- 8), H. nitidum (Spp. 9-11) and H. fasciculatum (Spp. 12-14) groups respectively. H. kalmianum differs by the shorter habit and the restriction of the inflorescence to the terminal node, and usually by the 4-5-merous gynoecium and narrower leaves; and the other groups differ by the smaller flowers and (usually) fruits and usually narrower leaves. Most of them also have less intrusive placentae. H. prolificum is very variable, the most primitive forms occurring in Arkansas and Oklahoma, i.e. on the other side of the Mississippi valley from the major area of H. frondosum. Thence it has apparently spread mainly northward and eastward round the north of the Mississippi Embayment to Iowa, Wisconsin, Michigan, New York and New Jersey and south to Mississippi, Alabama and Georgia. It is rare in the eastern Coast Plain and absent from, although (according to Adams, 1962: 15) hardy in, Massachusetts. For hybrids with 1. H. frondosum, 3. H. kalmianum, 4. H. lobocarpum and 5. H. densiflorum, see discussions under these species respectively.
For discussions of Svenson's Hypericum prolificum L. lectotype choice, see Fernald & Schubert (1948), Svenson (1952), Adams (1959, 1962) and Gillett & Robson (1981).
Coulter (1886a) treated M. ledifolia as a synonym of H. prolificum, but, as Adams (1962: 50) pointed out, the description is not diagnostic; it could refer to H. densiflorum. According to my own records, however, the specimen does belong to H. prolificum.