Windsor Conservation Commission

Japanese Knotweed Control Experiments in Windsor

Japanese Knotweed control: Japanese knotweed is a particularly invasive weed in Connecticut. Large stands can be often seen along woodland borders. It forms large colonies of erect stems that can reach 4 to 8 feet in height. It is capable of crowding out all other vegetation. This perennial plant is difficult to control because it has extremely vigorous rhizomes that form a deep, dense mat. Cutting it down does little good, because it easily grows back from the mat of rhizomes. Some control strategies involve covering the cut down stems with large pieces of carpet or black plastic, but this is particularly difficult with large stands.

The Windsor Conservation Commission with help from the Connecticut Agricultural Experiment Station in Windsor experimented with herbicidal controls over the summers of 2011 and 2012 and we are reporting on the progress here. On a large stand of Japanese knotweed (approx. 9000 sq. ft.), the stalks were cut down in early July, 2011. When they had grown back to about 4 feet tall in mid-August, the Agricultural Experiment Station sprayed them with two different herbicides. They tried Roundup (glyphosate) and Habitat (imazapyr). Roundup is widely available. Habitat is not available in the typical retail outlets and the property owner would probably need to hire a professional applicator to spray Habitat.

When the area was examined in the spring of 2012 it was apparent that much of the knotweed was not returning although some did. Variations in the amount of sunlight reaching the plot, and variations in when the cutting in 2011 took place, made it difficult to judge whether one herbicide was more effective than the other. The owner of the property has applied a subsequent Roundup treatment; this time hitting the few plants that have re-emerged while they are small. It is expected that very small re-treatments of the area may be required for several more years, but this appears to be an effective approach.