Arrowleaf Tearthumb (As in Tear, Tore, Torn)

— Written By Thomas Glasgow and last updated by Jami Hooper
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Arrowleaf tearthumb (Polygonum sagittatum) is a widespread native annual that you’ll want to handle with gloves on, if you handle it at all. (This was a new plant for me, and thanks to Extension Specialist Rob Richardson for making the ID). Our local Soil & Water office recently brought in a sample taken from a drainage ditch that was overgrown with a variety of weedy plants. Tearthumb likes to sprawl across or through other vegetation, and there was a good representation of this species in the ditch. The images below should help in your identification efforts. The genus Polygonum includes the non-native invasive Japanese knotweed (Polygonum cuspidatum), as well as Polygonum densiflorum, which is native but can be a significant problem in ditches and other waterways.
Stem is square in cross-section

Stem is square in cross-section, and lined with downward-pointing prickles. These prickles can be
quite irritating to exposed skin. The prickles also help the stems stay attached to other plants.

Foliage in November

Foliage in November. Leaves can grow up to 4 inches long and 1 inch across.

Flower clusters

Flowers are borne in clusters (short racemes). In this image you can just make out the tripartite
character of the style, where it darkens in color just inside the corolla (floral envelope).

Developing fruit

The developing fruit, an achene. Polygonum is in the buckwheat family, Polygonaceae.