Pilea fontana (Black-fruited Clearweed)

Plant Info
Also known as: Lesser Clearweed, Bog Clearweed
Genus:Pilea
Family:Urticaceae (Nettle)
Life cycle:annual
Origin:native
Habitat:part shade; wetlands, moist shaded woods, lakeshores, moist prairies
Bloom season:July - September
Plant height:4 to 20 inches
Wetland Indicator Status:GP: OBL MW: FACW NCNE: FACW
MN county distribution (click map to enlarge):Minnesota county distribution map
National distribution (click map to enlarge):National distribution map

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Detailed Information

Flower: Flower shape: indistinct Cluster type: panicle Cluster type: whorled

[photo of flowers] 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: Leaf attachment: opposite Leaf type: simple

[photo of leaves] 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.

Fruit: Fruit type: seed without plume

[photo of fruit] Fruit is a flattened, teardrop-shaped dry seed, up to 1/16 inch long and 75-85% as wide.

[photo of seed] Surfaces are unevenly textured, solid deep purple to black with a narrow band of green all along the outer edge.

Notes:

Pilea fontana is one of two native clearweeds in Minnesota, the other is Dwarf Clearweed (Pilea pumila). They are nearly impossible to distinguish but for the size, shape and color of their fruit (achenes). As described, P. fontana's fruit is broader than P. pumila, and evenly dark pigmented across its surface except for the green edging. P. pumila's achene is proportionately narrower, with scattered dark pigment spots against light green, usually denser towards the ends. As its common name - Black-fruited Clearweed - suggests, the difference is discernible with the naked eye 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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More photos

Photos by K. Chayka taken at Orway Prairie, Pope County. Photos courtesy Peter M. Dziuk taken at Long Lake Park, Ramsey County, Westwood Hills Nature Center, Hennepin County, and at Ordway Prairie. Seed photos courtesy Rick Haug.

Comments

Have you seen this plant in Minnesota, or have any other comments about it?

Posted by: Craig - Chatfield, N Fillmore County
on: 2016-08-13 11:36:09

I have large large patches of this. It seems to grow where I've killed wild garlic. There are a few wild garlic plants mixed in with these making it harder to kill/find all the wild garlic.

Posted by: Frank - western Hennepin County
on: 2016-08-29 14:25:22

These come up mid to late summer -often in zones where I've seen Jewelweed. I thought they might be an invasive, so hard to tell when it's a new plant to me and they are wide spread. They grow in our woods, along the edges mostly, very little sun, but some dappling or an hour or two tops. They are flowering now, and grow about 24 inches tall. Glad to see we don't have another invasive on our hands.

Posted by: jerry k - On Rice Creek, Fridley, Anoka County, Minnesota
on: 2017-09-03 19:38:46

I too found that this plant seems to profit from the eradication of garlic mustard. It must be that the allelopathic effects from the garlic mustard at least benefit this one native species. It wasn't very common before the garlic mustard invasion, but after eliminating the garlic mustard, it now totally blankets the forest floor in my back yard. Glad to discover it's native at least! Hopefully the native microbiota in the soil will recover and other native species will return.

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