The Plant That’s Eating New Hampshire: Japanese Knotweed


An aggressive, hard-to-kill weed that chokes riparian buffers in the Northeast could soon be under attack from one of its natural enemies.

USDA plans to release the knotweed psyllid in the United States, the agency said Tuesday.

The Japanese insect feeds on the sap of the giant, Japanese and Bohemian knotweed.

The invasive plants, which can grow 10 feet tall, were introduced from Asia in the 1800s. They are most common in the Northeast, the Pacific Northwest and eastern Canada.

Sometimes mistaken for bamboo, knotweed can grow in many habitats, but it flourishes along streams, where it can overrun conservation tree plantings and grasslands.

Knotweed does a poor job of protecting soil from erosion in these delicate environments, and it offers little to wildlife.

The plants are also strong enough to crack concrete and asphalt.

Current knotweed controls are expensive and temporary.

Multiple years of mowing and herbicide treatment can keep the infestation at a manageable level but may not be enough to destroy the plants, according to Penn State Extension.

Without a biocontrol program, these unsatisfying containment measures would likely be needed permanently, according to USDA.

The agency has little hope of eradicating knotweed from the United States, but it expects the psyllid will at least reduce the severity of knotweed infestations.

Potential biocontrol organisms undergo extensive testing to make sure the new species won’t become problems themselves.

USDA tested the psyllid’s preference for 70 plant species and varieties.

Of particular concern was buckwheat, a crop that is related to knotweed.

The psyllid preferred the knotweed to buckwheat, and it showed little aptitude for establishing itself in the crop.

Moreover, buckwheat is grown all over Japan in close proximity to knotweed, but the psyllid is not listed as a pest of buckwheat there.

USDA determined that the insect is highly specific to knotweed.

The knotweed psyllid has already been released in the United Kingdom and Canada.

The 2010 introduction to Britain was the first time a natural predator had been deployed for weed control in the European Union, according to the BBC.

The USDA public comment period on the psyllid release ends June 27.

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