Knotweed Research

Upper Delaware River Knotweed Research and Action

Japanese Knotweed is present everywhere along the Upper Delaware River and its tributaries. Left unmanaged it continues to spread along the river and everywhere it can find suitable conditions crowding out native riparian vegetation leading to soil loss and bank destabilization.

We are undertaking research to document the extent of knotweed colonization and its impacts on soils and aquatic habitat. We are also establishing 3 demonstration sites where landowners can see different options for you to manage, control or even eradicate knotweed using a variety of low-cost techniques. You will also be able to see what kinds of native riprarian vegetation can be seeded or planted to actively compete with knotweed once it is managed.

Management Techniques

Mechanical Cutting Knotweed Management Method

Mechanical cutting includes mowing, weed whacking with hard blades, or cutting with machete or sickle on a regular schedule through the growing season.

Each time the plants are cut the knotweed becomes weaker. Plants need to be cut every two to four weeks for multiple years during successive growing seasons with the goal of controlling its spread to adjoining areas and its ability to form seeds for future generations.

Covering Knotweed Management Method

Knotweed can be eliminated from an area by long term covering with an impermeable geotextile membrane or heavy duty tarp.

Mature knotweed plants first emerge in the spring as very strong, densely packed, sharp spears. To effectively cover the patch, the knotweed plants need to be cut down to the ground and then covered. The cover needs to be held in place along the edges and ballasted in the middle to prevent the material from moving due to wind or flowing water.

Foliar Spraying Herbicide Method

We highly recommend employing a licensed herbicide applicator for this method and do not recommend this method for landowners.

There are two methods for applying herbicides to eradicate knotweed.  One method is called “foliar spray” which means spraying the leaves and stems with the herbicide. The knotweed is cut first in the spring to make herbicide treatments easier, as knotweed can grow up to 10-13 feet tall. It is important to let the knotweed regrow for 8 weeks after cutting, so the plants are developed enough to properly absorb the herbicide. They will still be much shorter than 10 feet.  In August the licensed herbicide applicator will spray Rodeo 2%, which is specifically formulated for use near aquatic areas, on the leaves of any knotweed that has regrown in the area. A third application 2-3 weeks later may be necessary. This method, while effective, can damage or kill other vegetation in proximity if not used carefully according to directions on the label.

Herbicide Stem Injection Method

Directly injecting herbicides into the stem of each knotweed plant is a very effective method to eradicate knotweed from an area. This method highly targets the knotweed and avoids damaging other nearby vegetation compared to foliar application of herbicides. We highly recommend employing a licensed herbicide applicator for this method. They will use hand-held injectors to administer about 3ml of this highly targeted treatment of Rodeo (100%) to each stem of knotweed at the site. The company should revisit the site 2 months after the stem injection treatment to spray the leaves of any remaining plants with Rodeo.

Goat Grazing

Goats will eat many types of plants. They have been known to have a taste for knotweed which has good protein, calcium and high moisture content. Attempting to effectively manage knotweed by foraging will require the goats to revisit the knotweed patch repeatedly during the year and over more than one year. It is important to contain with fencing and closely manage the goat herd to avoid damage to streambanks and other vegetation.

To report the presence of one of the three species (Japanese, Giant or Bohemian) of Knotweed on your property go to our project page on iMap Invasives and go to your location and create a report. If you don’t already have an account on iMap Invasives, create one. For your record please tag it to the Upper Delaware Knotweed Project.

If you would like to be added to our mailing list for this project please send an email to info@fudr.org