Overgrowth of green Japanese Knotweed leaves

Japanese Knotweed and Psyllid and on the Walnut Creek Greenway

Up close view of green japanese knotweed leaves

Perhaps you have seen a green plant along the greenway with arrowhead-shaped leaves, the zig-zag pattern of its leaf buds, and the sprinkling of red specks along its stalk. This is the invasive Japanese Knotweed, a noxious weed that can take over wet areas.  

Japanese Knotweed (Fallopia japonica) and Giant Knotweed (Fallopia sachalinensis) are both present in Raleigh. Imported as a landscape plant to New York in the 1800s, it has spread to all but seven US states. What makes it so unique is its adaptability and penchant for spreading through both underground rhizomes (underground horizontal stems) and establishing itself from a broken piece, perhaps yanked off during a flooding event. It can grow as much as 8 inches in a day and one plant can have a rhizome system that covers half a football field-sized area. It can push through concrete, asphalt, and building foundations. In Great Britain, if there is knotweed on a property, banks won’t even consider a loan, making it virtually unsellable.

To add to this, it has no known predators or controls and you can pull it up, but you still have the underground rhizomes that are ready to send up more plants. Further, it is believed that climate change may be making it easier for this weed to spread. 

Tiny black insect on green leaf

But there is hope!  Researchers at North Carolina Department of Agriculture’s Beneficial Insects Lab have been working with researchers in Europe, with promising results, to study whether a tiny leafhopper, known as a psyllid (Aphalara itadori), can eradicate knotweed. This insect feeds solely on knotweed and can decimate the plant. Because this method has been successfully used in other places, there are no known unintended consequences at this point.

In June, Nancy Oderkirk of the NC Department of Agriculture, and Cheryl Knepp of the NC Department of Transportation released a batch of these insects in a substantial patch of knotweed on the Walnut Creek Greenway between State Street and Rock Quarry Road. Two biotypes were released including one adapted to feasting on Japanese Knotweed and the other specialized for Giant Knotweed.

Will it work? How many insects will it take to make it work? Will other insect predators wipe out the carefully cultivated insects? All of that remains to be seen, but we are lucky that we have these dedicated researchers working on it!

If you are interested in learning more about invasive plants, the Natural Resources section of Raleigh Parks, Recreation, and Cultural Resources expects to host classes at Durant Nature Preserve in the fall of 2020 and at Walnut Creek Wetland Park in the spring of 2021.

To learn more, or find out when there are invasive volunteer days, check out the Invasive Species Program webpage.