knotgrass

A low-growing, weedy grass (Paspalum distichum) with spikelets arranged in two rows along the rachis.

Any of several weedy plants of the genus Polygonum having stems with nodes.

A weed of almost world-wide distribution, Polygonum aviculare: so called from the numerous nodes in its stems and its thickly spreading habit.

By extension, any plant of the genus Polygonum, properly knotweed.

In occasional use, a plant of some other genus more or less similar.