Highly invasive Japanese knotweed is difficult to control

You are likely seeing pretty hedge-like plants with mounds of white flowers growing along streams and highways this time of year.

This is the highly invasive plant called Japanese knotweed (Fallopia japonica or Polygonum cuspidatum), a shrub-like herbaceous perennial member of the buckwheat family that was introduced from Japan in the late 1800s as an ornamental and to stabilize streambanks.

Knotweed quickly naturalized across the country. Once established, populations are extremely persistent and difficult to control.

Growing up to 11 feet tall, Japanese knotweed’s stout, hollow, bamboo-like stems have red or purple nodes where the leaves are attached. Portions of the stem bearing leaves appear to zigzag from node to node to form dense thickets that exclude native vegetation and greatly alter natural ecosystems.

The large (3 to 6 inches long), spade- or heart-shaped, alternate leaves are distinctive. Tiny white or greenish-white, five-petaled, aromatic flowers develop in August and September and grow in numerous linear clusters that form a mass of white over the plant when in full flower.

A Japanese knotweed seedling (Courtesy of Dave Jackson)
A Japanese knotweed seedling (Courtesy of Dave Jackson)

The plant is insect pollinated. Frost-killed stems turn bronze colored and may remain upright through winter. Emerging in early spring, the young growth is especially bright red or purple and tipped with many furled leaves that are distinctly triangular.

Manual, mechanical and chemical methods are all useful to varying degrees in controlling knotweed. Removing or killing plants will provide increased light at the site, which may lead to a surge of sprouts in the following year. Prepare to monitor and control these outbreaks for multiple years.

As with all invasive species, Japanese knotweed is most effectively controlled by recognizing its early appearance and removing isolated plants before they begin spread. Manual control takes special precautions because any live plant part (1/2 inch or larger) may sprout. Plant parts must be disposed of properly and must not be allowed to enter waterways. Stems and roots must be contained or dried with little or no soil contact or they may sprout.

Do not compost plant parts. Digging or pulling (uprooting) will eliminate a portion of the root system but not all. Pull the root crown and as much root as possible. Each time new sprouts are seen (look after a week and at least 20 feet from the plant) uproot them and get as much root as possible. This method is only feasible on very small patches.

Cutting alone is not an effective removal approach. However, cutting prior to an herbicide application can be very helpful. Cut in June about 2 inches above ground level (between the lowest nodes) and wait at least 8 weeks after cutting to treat the resprouting plants with herbicide; knotweed regrowth will be much shorter than if it had not been cut, and the rhizomes will be forced to redirect their energy reserves toward resprouting instead of expanding their underground network.

Typically, knotweed regrows to 2 to 5 feet tall during the 8-week window after cutting, but this waiting period is critical — if you apply herbicide such as Round-up too soon after cutting, the herbicide will not be effectively translocated to the rhizomes . (Always read the herbicide label and wear protective clothing when handling any dangerous chemicals)

Hand cutting, mowing or other methods are not recommended due to the plants ability to spread from fragments. If this is the only option, be careful not to spread plant pieces and expect to cut multiple times a year for several years. This method is highly ineffective and should only be used if no other options are available

Dee Dee Kerscher is a Penn State Extension Master Gardener volunteer and horticultural program assistant in Berks County.

September spotted lanternfly update

Don’t panic: Spotted lanternflies have shown a tendency to swarm in September. Research has shown that this is possibly due to the need to find food. With prolonged heavy feeding on the same trees, such as tree of heaven, the food resource may be exhausted and this motivates them to move to find other suitable food. This behavior is short-lived, lasting a few days.

Facts to remember:

• They don’t bite, sting or attack people or pets.

• They are not poisonous if ingested by pets.

• They do not try to enter your homes to over winter.

• Research shows that they don’t kill trees.

• They feed on plant material, not fruit or vegetables.

• Examine all vehicles and equipment for insects before moving.

• Egg laying starts in September, look for gray putty like masses.

• Scrape off egg masses into plastic bags containing alcohol or hand sanitizer and destroy.

• Adults will persist until a hard frost.

• Avoid home remedies.

To report spotted lanternfly sightings, even in the quarantine zones, call 888-422-3359.

For all the latest updates, visit https://extension.psu.edu/spotted-lanternfly-management-guide.

The Berks County Extension offices at the Agricultural Center, 1238 County Welfare Road, Suite 110, Bern Township, are open for walk-ins and phone calls (610-378-1327) from 9 am to 4 pm The Garden Hotline (berksmg@psu .edu) is always ready to answer all gardening questions.

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