Pilea pumila aka Canada Clearweed

Pilea pumila produces small, inconspicuous greenish-white to yellowish flowers from mid-summer to early fall (July-September), with blooming lasting about one month. The flowers are less than 3mm long and develop in narrow clusters (panicles) from leaf axils. Male and female flowers are separate but occur on the same plant (monoecious). The flowers lack petals and are wind-pollinated, so they don't attract pollinators. After flowering, the plant produces small, flattened teardrop-shaped achenes (seeds) that are green with distinctive purple to black spots.

Pilea pumila propagates exclusively by seed. The plant is an annual that completes its life cycle in one growing season and dies after setting seed. Seeds require cold stratification (8 weeks of cold treatment) to break dormancy and germinate. In nature, the plant self-sows readily and can form large colonies.

Pilea pumila is wind-pollinated (anemophilous). The flowers lack petals and produce no nectar, so they don't attract insect pollinators. The plant is monoecious, meaning individual plants bear both male and female flowers, which allows for self-pollination when pollen is carried by air currents. This wind pollination strategy is effective in the shaded forest understory habitats where the plant naturally grows.

Three varieties of Pilea pumila are currently recognized. Pilea pumila var. pumila is the most widespread, found throughout North America and eastern Asia. Pilea pumila var. hamaoi occurs in China, Japan, and Korea. Pilea pumila var. obtusifolia is restricted to China. In North America, plants with more rounded leaf bases and sharper, more numerous teeth have sometimes been called var. deamii, though this variety is poorly defined and not widely accepted as distinct.

Pilea pumila is a native outdoor plant in eastern North America and eastern Asia. It thrives outdoors in USDA hardiness zones 2b-10b, making it adaptable to a wide range of climates from southern Canada to Florida and Texas. The plant prefers shaded to partially shaded locations with moist to wet, rich loamy soil. It naturally grows in deciduous forests, stream banks, seeps, and wetland margins. While it tolerates temporary standing water, it doesn't require permanently wet conditions. Due to its weedy nature, it may spread aggressively by self-seeding in favorable garden conditions.

Pilea pumila tolerates a remarkably wide temperature range, hardy from USDA zone 2b (minimum -40°F/-40°C) through zone 10b (minimum 35°F/2°C). As an annual, it completes its life cycle during the growing season and survives winter through its cold-hardy seeds. The plant grows actively in typical summer temperatures and the foliage turns bright yellow in autumn before dying back. Seeds require cold stratification to germinate, indicating adaptation to cold winter conditions.

Pilea pumila is generally pest and disease resistant. Illinois Wildflowers notes that 'the foliage is little bothered by disease and is usually quite attractive.' Rather than being damaged by insects, the plant serves as an important host for several butterfly caterpillars including Milbert's Tortoiseshell, Comma, Question Mark, and Red Admiral. Various aphids and leafhoppers may feed on the plant, and a moth larva mines the leaves, but these rarely cause significant damage. Unlike its relative stinging nettle, clearweed lacks stinging hairs.

Pilea pumila is technically edible but not commonly consumed. The young leaves can be eaten raw in salads as a mild green, making it a potential survival food. Unlike its relative stinging nettle, clearweed lacks stinging hairs and can be handled and eaten without cooking. However, the plant is not widely cultivated for food and has limited culinary applications. Always ensure positive identification before consuming any wild plant.

Pilea pumila has traditional medicinal uses among Native American tribes. The Cherokee rubbed the stems between their toes to treat itching, while the Iroquois squeezed liquid from the stems and inhaled it to treat sinus problems. In Asia, the plant has been used as a diuretic. A 2022 phytochemical study found Germacrene D-4-ol, a compound with antifungal activity, which may explain its traditional use for treating itching (potentially athlete's foot). However, the plant is not widely used in modern herbal medicine, and more research is needed to validate its therapeutic properties.

Pilea pumila is notable for its educational applications. The plant's translucent stems make it excellent for demonstrating transpiration in classroom settings—when stems are placed in dyed water, students can observe the colored water moving through the transparent tissues. Ecologically, the plant serves as a host for butterfly caterpillars including the Red Admiral, Question Mark, Comma, and Milbert's Tortoiseshell. It also supports various aphids, leafhoppers, and leaf-mining moths, making it valuable for wildlife gardens.