Pilea pumila (Dwarf Clearweed)
|Also known as:||Canadian Clearweed|
|Habitat:||part shade, shade; moist shaded woods, wetlands, wooded shores|
|Bloom season:||July - September|
|Plant height:||4 to 20 inches|
|Wetland Indicator Status:||GP: FAC MW: FACW NCNE: FACW|
|MN county distribution (click map to enlarge):|
|National distribution (click map to enlarge):|
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Separate male and female flowers, both small and indistinct, green to pale yellow and densely packed in horizontally spreading, irregular panicles about 1 inch long arising from leaf axils in the upper half of the plant.
Leaves and stems:
Leaves are opposite, elliptical to egg-shaped, 1 to 5 inches long, the tip pointed, the base rounded to wedge-shaped, on a stalk about half as long as the leaf blade. Leaves are thin and somewhat translucent with three prominent veins from the base. Surfaces are shiny with scattered short hairs on the upper surface. Edges are toothed, sometimes double toothed. Stems are sometimes branched but usually not, very fleshy and translucent, somewhat squarish, very smooth and shiny.
Pilea pumila is one of two native clearweeds in Minnesota, the other is Black-fruited Clearweed (Pilea fontana). They are nearly impossible to distinguish but for the size and color of their fruit (achenes), and only when fruit is present. As described, P. pumila's fruit is smaller and proportionately narrower, with scattered dark pigment spots against light green. P. fontana's achene is larger, proportionately broader, solidly dark pigmented but for a narrow green edging. Typically the difference is distinguishable with the naked eye with the larger, darker fruit of P. fontana standing out while still in the panicle, though some more darkly pigmented P. pumila may be more easily confirmed by collecting some fruit into the palm of your hand for closer inspection.
Some references note differences in the degree of translucency between the two species, but these are not reliable traits to distinguish them in the field. Even when the two are side by side the differences are too subtle for a positive ID. Of note is that Flora of North America states the two are not often encountered together, even though their habitats and distribution ranges overlap considerably. The leaves of the two Pilea species are similar to other members of the Nettle family, most closely to False Nettle (Boehmeria cylindrica), which has more spike-like flower clusters and leaves more finely serrated with more than 3 prominent veins scattered along the midrib, and Wood Nettle (Laportea canadensis), which has stinging hairs and is a considerably larger plant.
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Photos courtesy Peter M. Dziuk taken in a private wooded garden in Anoka County and at Westwood Nature Center in Hennepin County. Seed photos courtesy Rick Haug.
Have you seen this plant in Minnesota, or have any other comments about it?