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:
