Identifying Prostrate Knotweed (and How to Distinguish it from Spurge)

 

Recently I was asked to help identify prostrate knotweed (Polygonum aviculare) that was collected from an alfalfa field. Admittedly, the tough and wire-like specimen that was submitted to me at the end of autumn didn't much resemble the succulent seedlings that I have often observed emerging in tree and vine systems in early spring. Consequently, I dug out my 'Weeds of the West' and 'Weeds of California and Other Western States' books and double-checked with colleagues to ensure that my ID was accurate. This post is meant to build on that effort and describe the morphological traits that are characteristic of prostrate knotweed and how to differentiate the species from a similar-looking weed, spotted spurge (Euphorbia maculata).

 

 

Prostrate knotweed is an annual (or sometimes short-lived perennial) weed that is widely distributed throughout North America. The species spreads by small (1.5-2 mm wide x 2.5-3 mm long), 3-sided, brown seeds that require a period of cold-moist stratification for germination. Seeds germinate and seedlings emerge in late-winter to early-spring (in my experience, it is one of the earliest species to emerge).

 

 

Prostrate knotweed seedlings grow upright, initially, following emergence. Leaves are alternate and are lanceolate to oblong in shape (leaves on mature plants can be more ovate in appearance). At the base of each leaf, a membranous sheath (called an ocrea) surrounds the swollen stem node. As the plants mature, they become more prostrate (especially after mowing or cutting events), branched and mat-like. The stems become wire-like and exhibit longitudinal ribs. Flowers are white (often with a pinkish tinge on the margins) and are held in small clusters in the leaf axils.