Hydrocotyle umbellata aka Water Pennywort

Hydrocotyle umbellata is a low, spreading perennial in the ivy family, Araliaceae. Its common names include dollarweed and water pennywort. It grows in parts of North, Central, and South America. Wild plants live in swamps, wet open ground, and roadside ditches. In North Carolina, it occurs in the Piedmont and Coastal Plain. It also grows outside its native range, including Thailand and New Zealand.

Plants reach about 0.25–0.83 feet [0.08–0.25 metres] tall. Their spread ranges from 3–20 feet [0.91–6.10 metres]. They creep across damp ground or float in water. Long rhizomes help them spread and form thick mats. The green stems are fleshy, smooth, and branched. Leaves grow in an alternate pattern and have no hairs. Each glossy, dark green leaf is round or kidney-shaped. Its edge has rounded teeth and shallow lobes. The leaf stalk joins the blade at its centre. This gives each leaf the shape of a small umbrella. The foliage is deciduous.

Small flowers grow in umbels, with five to ten blooms per cluster. NC State describes white, star-shaped flowers with five petals. Each has a green ovary and two green styles. Chicago Botanic Garden lists green blooms instead. NC State gives the bloom season as April through October. Chicago records a shorter season, from July through August. The small, flat, round fruit is a schizocarp. Fruit may be green or brown and appears in summer. Waterfowl eat the seeds, and the flowers attract bees.

Grow this plant in full sun or partial shade. It suits moist ground, poor drainage, and wet soils. It can grow in clay, loam, or sand. Slightly acid soil is preferred, though neutral soil also suits it. Hardiness reports differ between the two garden sources. NC State lists USDA zones 4–10; Chicago lists zones 8–11. Gardeners use it as groundcover in bogs and water gardens. It tolerates foot traffic, and Chicago lists resistance to deer. Submerged growth shelters small fish, frogs, and other water life.

Rapid spread makes this a high-maintenance plant. Dense pond mats can change oxygen levels and crowd other plants. Some areas class it as invasive or a noxious weed. Roots and seeds can renew growth after physical removal. NC State reports no major insect or disease problems. Plants can be raised from seed or divided. Folk medicine has used it for inflammatory disorders. That use does not establish safety as food. The leaves can cause nausea; keep them away from children.