10. Prunus spinosa L. (blackthorn, sloe)

Pl. 537 a, b; Map 2480

Plants shrubs to 4 m, usually strongly suckering to form dense thickets. Branches strongly thorny. Twigs minutely hairy when young, usually glabrous at maturity, producing pseudoterminal winter buds (these usually in a cluster of 2 or 3 at the tip). Petioles 4–6 mm long, minutely hairy, occasionally only on the upper side, glandless. Leaf blades 1.5–4.0 cm long, 1.0–2.2 cm wide, less than 2 times as long as wide, elliptic to obovate, broadly angled to rounded at the base, angled to a broadly or bluntly pointed tip, the margins simply toothed or scalloped, the short, blunt to rounded teeth mostly gland-tipped, the upper surface glabrous or sparsely short-hairy along the midvein near the base, the undersurface short-hairy along the main veins, at least when young. Inflorescences produced before the leaves, of solitary or less commonly paired flowers per bud, the flower stalks 1.5–6.0(–8.0) mm long, glabrous or rarely short-hairy. Flowers with the hypanthium 1.5–2.5 mm long, hemispheric, glabrous. Sepals 1.5–2.5 mm long, reflexed at flowering, ovate to triangular-ovate, the margins glandular-toothed, the inner surface glabrous. Petals 4–8 mm, elliptic to broadly elliptic or broadly oblong-elliptic, white. Fruits 10–15 mm long, globose, shallowly longitudinally grooved on 1 side, the surface bluish black, glabrous, glaucous, the fleshy layer well-developed but sometimes relatively thin, the stone subglobose, somewhat flattened, the surface smooth or nearly so. 2n=32. April–May.

Introduced, uncommon, known only from two historical specimens from Jasper County (native of Europe, Asia, Africa; introduced in the northeastern and northwestern U.S., also Missouri, Tennessee). Open, disturbed areas.

This shrubby species produces abundant thorn-tipped branches and often forms dense thickets. It flowers abundantly and the small, bluish black fruits are attractive but inedible. Apparently it is no longer as commonly cultivated in the Midwest as it once was.

Trees of the common European plum and Damson plum (Prunus domestica L. var domestica and var. insititia (L.) Fiori & Paol.) occasionally persist at old home sites, but thus far have not escaped into the wild in Missouri. Members of the P. domestica complex are superficially similar to P. spinosa, differing among other characters in their mostly paired flowers and glabrous twigs, and are somewhat larger trees with larger fruits. These taxa are no longer as commonly cultivated in the Midwest as they once were. Currently, most of the plums available as fresh fruits in markets in the United States are derived from the Japanese plum, P. salicina Lindl. However, cultivars of P. domestica are still the source of most prunes. Fresh damson plums occasionally are offered for sale in midwestern markets, and are recognizable as relatively small, bluish black, glaucous plums. Damson plums also are the basis of plum brandy (slivovitz).