Not to be semantically confused with :
Knotweeds such as
Japanese Knotweed (Fallopia japonica) [a plant with similar name].
The Genus name of Knotgrasses, Polygonum can easily be misinterpreted as Polygonatum which is the genus name of Solomon's-seals such as Angular Solomon's-Seal (Polygonatum odoratum).
Easily mis-identified as : other Knotgrasses :
Equal-leaved Knotgrass (Polygonum arenastrum) which has equal-sized leaves.
- Ray's Knotgrass (Polygonum oxyspermum) which grows only on the coast and has larger flowers, a sturdier stem, is less sprawling, and the fruits visibly emerge from the dead flowers.
Sea Knotgrass (Polygonum maritimum) very rare occurring only in very few places on the southern coast of the UK. Larger, more-open flowers and stouter even that Ray's Knotgrass.
Northern Knotgrass (Polygonum boreale) but that now occurs only in the North of Scotland and is very rare at that. Leaves in two differing sizes, the larger size wider than any other Knotgrass. Flowers emerge well clear of the silvery sheaths.
Cornfield Knotgrass (Polygonum rurivagum) which mainly occurs scattered around the southern half of the UK and grows only in limy arable fields.
Some similarities to :
Water Peppers such as
Water Pepper (Persicaria hydropiper),
Small Water-pepper (Persicaria minor) and
Tasteless Water-pepper (Persicaria mitis) which also have 5-petalled flowers in small groups appearing in whitish papery sheaths but these are usually upright rather than sprawling.
No relation to :
Knotted Crane's-bill, Knotted Hedge-Parsley,
Knotted Pearlwort or Knotted Clover which all belong to differing families. [plants with similar names belonging to differing families].
Knotgrass is the commonest knotgrass and like Ray's Knotgrass (Polygonum oxyspermum) and
Sea Knotgrass (Polygonum maritimum) it grows near the shore and on gull colonies but also, un-like those, on bare ground or cultivated fields as far from the sea as you can possibly get.