Why Late Summer and Early Fall Are the Best Times to Treat Japanese Knotweed

Japanese knotweed can quickly become one of the most difficult invasive plants to manage on a property. Once established, it can spread aggressively, creating dense patches that overwhelm surrounding vegetation and become increasingly difficult to control.

David Anderson of Mayer Tree Service recently visited a cemetery in West Roxbury, Massachusetts, where the property managers asked for help controlling a significant Japanese knotweed infestation.

The timing of the treatment is an important part of the strategy.

What Is Japanese Knotweed?

Japanese knotweed is an extremely aggressive invasive plant that can spread rapidly once it becomes established.

Although it may look like a large shrub or weed above ground, much of the real problem lies beneath the soil. Japanese knotweed spreads through an extensive underground root and rhizome system. New shoots can emerge throughout an area, allowing the plant to continue expanding year after year.

Once a patch becomes established, simply cutting down the visible growth is usually not enough to eliminate the plant.

In fact, Japanese knotweed can grow extremely tall during the growing season, sometimes creating thick stands that are difficult to walk through or properly treat.

Why Mowing Can Be Part of a Japanese Knotweed Treatment Strategy

At this West Roxbury property, the Japanese knotweed had become extremely tall earlier in the season.

David recommended that the area be mowed down in late spring or early summer.

The goal wasn’t simply to remove the knotweed. Instead, mowing helped make the next stage of treatment much more manageable.

After being cut back, the plants regrew to approximately three feet tall.

At that height, technicians can more easily access the plants and thoroughly apply the appropriate treatment compared with attempting to treat a dense stand that may be 10 to 15 feet tall.

When Is the Best Time to Treat Japanese Knotweed?

Late summer into early fall is often an ideal time to treat established Japanese knotweed, particularly when the plants are flowering.

During this part of the growing season, the plant is preparing for winter and transporting energy produced by its leaves back toward its underground root and rhizome system.

That makes proper herbicide treatment especially valuable because the goal isn’t simply to damage the foliage above ground.

The treatment needs to affect the underground system that allows the knotweed to return.

At the West Roxbury property, the plants had regrown after mowing and were approximately three feet tall and blooming. This created a much more manageable opportunity to treat the infestation.

Why Cutting Japanese Knotweed Alone Usually Isn’t Enough

One of the biggest challenges with Japanese knotweed is its ability to return after being cut.

You may remove everything you see above ground, only to discover new shoots appearing later.

That’s because the underground system remains alive.

Effective Japanese knotweed management often requires a longer-term strategy that may include:

• Cutting or mowing at the appropriate stage
• Allowing controlled regrowth
• Treating the foliage at the right time of year
• Monitoring the area for new growth
• Performing follow-up treatments when necessary

Because Japanese knotweed is so persistent, controlling a mature infestation may require more than one growing season.

Don’t Let Japanese Knotweed Take Over Your Property

If you’ve noticed Japanese knotweed spreading across your landscape, waiting can allow the infestation to become larger and more difficult to manage.

The right treatment strategy depends on the size of the infestation, the location of the plants, the surrounding vegetation, and the time of year.

At Mayer Tree Service, our team helps property owners throughout the North Shore of Massachusetts and Southern New Hampshire identify and manage invasive vegetation while protecting the health of the surrounding landscape.

If you’re concerned about Japanese knotweed or another invasive plant on your property, contact Mayer Tree Service to schedule an evaluation and discuss the best approach for managing it.

Watch David Explain Japanese Knotweed Treatment in West Roxbury

In this video, David Anderson of Mayer Tree Service explains why this Japanese knotweed infestation was cut earlier in the growing season and why late summer into early fall provides an important opportunity for treatment.