- Name also: Knotweed
- Family: Dock Family – Polygonaceae
- Growing form: Annual herb.
- Height: 10–40 cm (4–15 in.). Stem ascending–erect, up 1 metre long.
- Flower: Regular, 3.5–4 mm (0.14–0.16 in.) wide. Perianth of 5 or 6 segments, united at base, clearly overlapping, with white or pink margins. Stamens 5–8. Pistil formed from 3 fused carpels. Styles 3. Flowers in small cymes.
- Leaves: Alternate, hairless, short-stalked. Stem leaves longer than those of the respective branches. Blade narrowly elliptic to obovate, end rounded to tapering tip, margins entire. Stipules fused into a stem-enclosing, membranous sheath (an ochrea) that is fibrous and silvery.
- Fruit: Deep brown, conical, finely knobbed, 3-edged achene, shorter than dry perianth, with 3 equally wide sides.
- Habitats: Fields, yards, roadsides, seashores.
- Flowering time: July–September.
Knotgrass is a very polymorphous annual of bare ground. At least five subspecies races thriving in different conditions are distinguished in Finland: ssp. aviculare, ssp. boreale, ssp. neglectum, ssp. microspermum and ssp. rurivagum. The seashore variant ssp. boreale var. crassifolium is native to Finland. It prefers maritime sands and shingle. It can be distinguished from the type by its somewhat fleshy and broadly ovate leaves that have rounded tips.
Knotgrass withstands trampling well because it is able to regenerate destroyed tissue easily. Its seeds cannot germinate without light, a characteristic that partly explains the habitat preferences. Knotgrass can remain on a site for years, stored in the seed bank. This makes it a troublesome weed that is not easy to root out.
In Russia knotgrass has been used as a herbal medicine. This species can be distinguished from its close relative, small-leaved knotgrass (common knotweed, P. arenastrum) by a few characteristic details. Small-leaved knotgrass is generally smaller, has shorter internodes, smaller leaves and shorter leaf-stalks as well as smaller flowers whose perianth is further fused than that of knotgrass. The fruits also differ, as those of small-leaved knotgrass have one side narrower than the others. These differences probably derive from different chromosome numbers. Knotgrass is triploid whereas small-leaved knotgrass is diploid.