Minnesota Seasons - clearweeds (Pilea spp.)

Overview • Description • Distribution • Taxonomy

Overview

Pilea is a very large genus of flowering plants. It is the largest genus in the Nettle (Urticaceae) family.

Pilea occurs nearly worldwide in the tropics, the subtropics, and the warm temperate zones. It is not native to Australia or New Zealand, but it has been introduced into both countries. It occurs in the United States east of the Great Plains. It also occurs in southern Canada from Nova Scotia to Ontario. It is common in Minnesota.

Photo by Greg Watson

There are currently 882 described Pilea species worldwide according to International Plant Names Index, but many species remain undescribed. The total number of species is expected to be about 30% higher. There are 27 species in North America north of Mexico, but most of these occur only in Puerto Rico, the Virgin Islands, or Hawaii. There are just 2 species in Minnesota.

A few species are grown as garden ornamentals or potted plants. Aside from that, they have little economic importance.

Description

With rare exceptions, most Pilea species are shade-loving, woodland-dwelling, herbs, shrubs, or subshrubs.

The stems are hairless.

The leaves are opposite and stalked, and there is a single, strap-shaped, leaf-like appendage (stipule) between each leaf stalk and the stem. Paired opposite leaves are usually unequal in size. The leaf blades are egg shaped, and they usually have 3 veins. They are hairless, but they have scattered, conspicuous, crystaline, calcium carbonate inclusions (cystoliths).

On almost all species, the inflorescence is a branched and flat-topped, convex, or pyramid shaped cluster of minute flowers rising from upper leaf axils. Male and female flowers occur on the same plant, usually on different branches of the same cyme or panicle. Male flowers have 4 petal-like sepals (tepals) and 4 stamens. Female flowers have 3 tepals, two opposite lateral tepals, and one enlarged, hood-like, middle tepal.

The fruit is a dry, flattened, one-seeded capsule (achene). It is partly covered by the hood-like tepal. In some species, the lateral tepals are under tension, and they eject the achene when it is mature.

There are two Pilea species that occur in Minnesota, and they are very similar in appearance. According to Flora of North America, they are best distinguished by the color and texture of the mature achenes.


Distribution
		Sources 2, 3, 4, 5, 7, 24, 28, 29, 30.
10/9/2024


Taxonomy
Kingdom	Plantae (green algae and land plants)
Subkingdom	Viridiplantae (green plants)
Infrakingdom	Streptophyta (land plants and green algae)
Superdivision	Embryophyta (land plants)
Division	Tracheophyta (vascular plants)
Subdivision	Spermatophytina (seed plants) / Angiospermae (flowering plants)
Class	Magnoliopsida (flowering plants)
Superorder	Rosanae
Order	Rosales (roses, elms, figs, and allies)
Family	Urticaceae (nettle)

Subordinate Taxa
artillery plant (Pilea microphylla) Antilles clearweed (Pilea rivoriae) artillery plant clearweed (Pilea trianthemoides) Canada clearweed (Pilea pumila) Caribbean clearweed (Pilea herniarioides) Chinese money plant (Pilea peperomioides) cliffside clearweed (Pilea sedimentata) creeping Charlie (Pilea nummulariifolia) depressed clearweed (Pilea depressa) friendship plant (Pilea involucrata) Krug’s clearweed (Pilea krugii) Liquillo Mountain clearweed (Pilea multicaulis) Margarett’s clearweed (Pilea margaretiae) Moon Valley pilea (Pilea mollis) mosgo clearweed (Pilea tenerrima) mountain clearweed (Pilea yunguensis) Navassa clearweed (Pilea perfragilis) Pacific Island clearweed (Pilea peploides) Puerto Rico clearweed (Pilea leptophylla) Richard’s clearweed (Pilea richardii) rockweed clearweed (Pilea microphylla) silver tree (Pilea spruceana) springs clearweed (Pilea fontana) verdolaguilla clearweed (Pilea parietaria) virdrillo (Pilea hyalina) Virgin Island clearweed (Pilea sanctae-crucis) West Indian clearweed (Pilea inaequalis)

Synonyms
Adicea Adike Chamaecnide Dubreulia Neopilea Sarcopilea Therebina

Common Names
Clearweeds

Glossary

Achene

A dry, one-chambered, single-seeded seed capsule, formed from a single carpel, with the seed attached to the membranous outer layer (wall) only by the seed stalk; the wall, formed entirely from the wall of the superior ovary, does not split open at maturity, but relies on decay or predation to release the contents.

Cyme

A branched, flat-topped or convex flower cluster in which the terminal flower opens first and the outermost flowers open last.

Panicle

A pyramidal inflorescence with a main stem and branches. Flowers on the lower, longer branches mature earlier than those on the shorter, upper ones.

Stipule

A small, leaf-like, scale-like, glandular, or rarely spiny appendage found at the base of a leaf stalk, usually occurring in pairs and usually dropping soon.


Visitor Photos

Share your photo of this plant.
		This button not working for you? Simply email us at info@MinnesotaSeasons.com. Attach one or more photos and, if you like, a caption.


MinnesotaSeasons.com Photos

Camera

Slideshows

slideshow


Visitor Videos

Share your video of this plant.
		This button not working for you? Simply email us at info@MinnesotaSeasons.com. Attach a video, a YouTube link, or a cloud storage link.


Other Videos

Camcorder


Visitor Sightings

Report a sighting of this plant.
		This button not working for you? Simply email us at info@MinnesotaSeasons.com. Be sure to include a location.