Persicaria hydropiper aka Marsh Pepper Smartweed

Persicaria hydropiper is an annual herb in the knotweed family. Its common names include water-pepper and smartweed. The older name Polygonum hydropiper refers to the same species. Its native range spans northwest Africa, temperate Eurasia, and Australia. It also reaches western and central Malesia. The RHS lists it as native to Britain and Ireland. It grows beside ponds, streams, and rivers, and in ditches. It also grows in wet woods and on floodplains.

Plants reach about 1.3–2.3 feet [0.4–0.7 metres] tall. The stems branch freely and grow upright or sprawl. Sprawling stems may lean against nearby plants for support. Lower stem nodes can form roots and produce new growth. The smooth stems may be green or red. Leaves grow alternately along them on short stalks. They are green, sometimes with an olive tint. Their shape ranges from lanceolate to oval, with smooth edges. The leaves have a strong, hot, pepper-like taste.

Small greenish-white flowers appear in summer and fall. They hang in thin racemes or spike-like clusters. Many flowers stay partly closed and look like buds. Each flower has five sepals but lacks petals. Flowers contain both male and female parts. The plant can pollinate itself and set seed. Its small fruits are dark brown to black. Descriptions include flattened, oval nuts and dull, three-angled seeds. Plants often shed seed nearby and form colonies.

Water-pepper needs wet soil or shallow water. It grows in sand, loam, clay, and organic-rich soil. Full sun suits it, though NC State also lists partial shade. Plants For A Future reports that it dislikes shade. Its rapid growth can fill gaps in disturbed wetlands. It can also spread strongly in these sites. In gardens, it serves as an accent or grouped planting. Wet gardens and edible gardens suit its needs. NC State reports few insect problems but lists Japanese beetle.

The leaves, stems, and seeds have food uses. People eat them raw or cooked, or use them as seasoning. Small seeds provide a hot substitute for pepper. Sprouts and young seedlings also serve as salad additions or garnishes. The stalks yield a yellow-gold dye. Herbal traditions use the plant for its astringent effects. Plants For A Future advises caution with medicinal use. Useful Tropical Plants reports that people use its leaves as fish poison. These food uses therefore do not establish safety for every use.