Nasturtium officinale aka Watercress

Nasturtium officinale, or watercress, belongs to the mustard family. This water-loving perennial grows fast and forms dense patches. Its native range spans Europe, central Asia, and the Arabian Peninsula. It also includes Macaronesia and parts of northern and northeastern Africa. It is native to Britain and Ireland. Wild plants grow beside streams and in ditches with moving water. Many sites lie on chalk or limestone.

In gardens, plants reach about 0.3–1.6 feet [0.1–0.5 metres] tall. They spread about 1.6–3.3 feet [0.5–1 metre] wide. The RHS also describes a low form around 0.7 feet [0.2 metres] tall. Fleshy stems trail, float, or grow beneath the water. They take root and build thick colonies. Even small stem pieces can root into new plants. Dark green leaves have an oval shape and wavy edges. Leaves may fall or remain partly through winter. Small white flowers appear from late spring into midsummer. Bees and flies visit them, and plants can also self-pollinate. Seed pods follow the flowers.

Grow watercress in full sun or part shade. It needs soil rich in humus that stays wet. Soil must at least remain moist through summer. Clay, loam, and sand can all support it. It tolerates acid, neutral, and alkaline soils. The RHS rates its hardiness H6; PFAF lists USDA zones 3–11. Plants reach their mature size in one to two years. Start plants from seed or rooted runners in spring. Pinch out shoots often to gather fresh young leaves.

Watercress suits ponds, bog gardens, and wet wildlife gardens. Its spreading cover can help hold soil in wet places. Flowers supply nectar and pollen for visiting insects. Growers raise it as a food crop in many countries. It has also escaped gardens and become a serious river weed in New Zealand. Slugs, snails, aphids, and caterpillars may attack it. Leaf spot and root rot can also cause problems. Useful Tropical Plants reports its conservation status as Least Concern.

Eat the peppery leaves raw or cooked, or sprout the seeds. The leaves provide vitamins and minerals, including iron. People have also used watercress in traditional herbal remedies. Wild plants in water draining from livestock fields may carry liver flukes. Do not eat those leaves raw; cooking kills the parasites. PFAF also warns that watercress may affect paracetamol metabolism. The ASPCA lists it as toxic to dogs, cats, and horses. Eating it may cause mild vomiting and diarrhea.