You are walking across Iowa State's lovely campus on some warm sunny spring day between classes. Two slow-walking students are in front of you, and the traffic coming at you is heavy. You briskly step off the concrete sidewalk and tred on the grass for a few steps, then return to the sidewalk in front of the people you passed. You have just selected for knotweed, created a habitat for the knotweed species-group association to thrive in: compacted and untilled soil.
This selection and habitat can be dramatically seen in the picture left. This is a grass roadway that has pickup truck traffic on it. The trucks have compacted the soil, driving out the less competitive grasses and allowing the knotweeds to take over.
The knotweeds usually co-exist as an association of two species, prostrate and erect knotweeds. Prostrate knotweed emerges first, and as it flowers and wanes in the spring and summer, the taller, larger leaved erect knotweed becomes apparent. The picture on the left shows this, the taller, lighter green plants are erect knotweed.
Erect knotweed leaves are broader, more oval (below: left, center), than the slender, smaller prostrate knotweed (below: right) leaves:
Erect knotweed often seems heavily infested with powdery mildew (left), a fungal growth that looks severe but doesn't seem to slow the knotweed down much. Although this might be an approach for biological control, knotweeds are very poor competitors with crop plants, within which they rarely infest.
Prostrate knotweed is very similar to erect knotweed. They differ primarily by leaf shape. Prostrate knotweed leaves are relatively narrow and long (left).
All members of the Smartweed family have a distictive structure at the stem-branch junction called the ochrea (erect, below left; prostrate, below right). The ochrea appears as a membranous sheath enclosing the junction.
Flowers of both erect and prostrate knotweed are very similar (left).
Flowers of both erect and prostrate knotweed are very similar, and quite small.