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How to Kill Japanese Knotweed?

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Japanese knotweed is a significant invasive plant that can spread in many environments. It is notorious for being one of the most challenging weeds to kill. It can be a stubborn plant that might harm your garden. You might also have a tougher time selling your house if there’s a Japanese knotweed growth in the area.

Japanese Knotweed can be killed by using a multi pronged approach of the following 5 methods.

  1. Digging out the rhizomes. This removes the food source for the plant to continue to grow.
  2. Smother The Plant. Laying tarps over the plant removes the light. The plant cannot photosynthesize.
  3. Cut the plant regularly. Each time you cut the plant, it has to use energy to produce top growth. This weakens the root system.
  4. Inject herbicide directly into the rhizome. This will help destroy the plant’s root system; a licensed company should perform it.
  5. Burn all clippings of Japanese Knotweed. The smallest piece of this plant has the ability to infest new locations.

But as you will discover in this guide, you can kill Japanese knotweed if you know what to do with the plant. The process takes a while and requires multiple steps, but it will be effective if you handle it well.

What Is the Japanese Knotweed Plant?

Japanese knotweed, or Reynoutria japonica, is a perennial plant in the Polygonaceae buckwheat family. The plant is native to Japan and other parts of Eastern Asia, but it is considered an invasive species in North America and Europe.

Japanese knotweed features hollow stems and slightly ribbed leaves. The leaves feature slight oval shapes. It is not related to the bamboo plant, but it does have an appearance similar to bamboo.

The plant has stems that can grow to about ten to twelve feet during a growing season. The leaves can be about five inches long on average.

You’ll also notice small white flowers in the plant. These flowers appear near the stems and produce racemes that can get to six inches long. These flowers are the most noticeable during the summer and fall seasons.

How Does the Japanese Knotweed Plant Grow?

The Japanese Knotweed plant spreads mainly through seeds and stem fragments. The seeds that produce the plant are typically spread by water, wind, animals, or soil contamination. It is hard to identify Japanese knotweed seeds, making it harder for the plant to be identified in many cases.

The plant isn’t likely to spread in heavily forested areas. It will develop mostly in places with various disturbances, including construction, animal activities, and other things where the soils experience significant shifts.

The Japanese knotweed plant will start growing in the spring season. It will keep growing during the and eventually into the middle part of the autumn before it dies off in the winter. The growing season might last longer if the winter is mild enough and if the prior summer was damp.

How Long Does It Take To Remove Japanese Knotweed?

Do not assume that you’ll immediately clear out Japanese knotweed when treating the growth. It can take a few growing seasons to remove the Japanese knotweed infestation from your yard. The goal is to remove the visible growth and clear out the roots, seeds, and other items that might develop.

The plant may become less visible after a while, but it will still take an extended period before the seeds, roots, and other pieces under the ground can die out.

You’ll have an easier time killing the plant in the summer months. The plant will start flowering in the summer and bring in more nutrients towards its roots. Limiting the plant’s access to water and sunlight will be critical.

You might require multiple efforts to remove the plant throughout the year. You might start by digging a space around the roots and then smother the plant with tarps, for example.

How Do You Dig Out the Japanese Knotweed?

One way to kill Japanese knotweed is to dig out the rhizomes near the base stem. These rhizomes are plant stems that produce roots while promoting the upward growth of each plant. Removing these parts becomes harder for the knotweed to continue to grow, making an additional effort to kill the plant easier to follow.

Digging these parts out helps you inhibit the plant’s growth, but it works best if you smother the plant after you finish trimming everything.

You can use these steps to remove the Japanese Knotweed rhizomes:

  • Look for the rhizome pieces that are producing the shoots at the base. Some of the more mature clumps can be up to a foot wide and feature a woody texture.
  • Use a small garden shovel to remove these rhizomes. Target spaces a few inches away from the rhizomes to improve your grip over these spots.
  • Add your rhizomes in a separate bag for disposal. Do not leave any bits of these rhizomes behind, as they can produce new plants if you aren’t careful.

You can dig out the plant at any time of the year, although it works best before you attempt to smother the Japanese knotweed with tarps.

How Can You Smother Japanese Knotweed?

You can smother the Japanese knotweed after you remove as many of the rhizomes as possible. It is easier to smother the growth with tarps after the main pieces that can trigger the growth are cleared from the area.

Smothering the space with a tarp limits the plant’s growth and kills it. The roots and seeds will not be likely to spread as much when you keep these growth features under control.

This process works best in the spring season when the plant is most likely to grow. You can limit sunlight to prevent the plant from growing.

You can use these steps to smother the plant:

  • Cut any mature stems down as close to the ground as possible. Always remove the remaining debris before it can potentially spread seeds.
  • Check for any sharp edges left from trimming the plant. Anything too sharp might puncture your tarps.
  • Apply at least one tarp over the plant. Be sure the tarps are opaque enough to keep sunlight from entering. You can also use rocks and other heavy items to keep the tarps down in their place.
  • You can walk over the tarps if you notice any new shoots starting to pop up from under the covers. You may not see many of these shoots due to the tarps preventing sunlight from reaching the plant.
  • Keep the tarps over the space for as long as it takes for the shoots to stop popping up. The existing knotweed will die in a few weeks. You can then dig the dead plant out of the space after the seeds and other parts weaken.

How Can You Cut Japanese Knotweed?

The next option to consider when managing Japanese knotweed is to cut the plant back. This effort works best when you do it alongside other methods like smothering.

Here are some points on how to cut Japanese knotweed:

  • Start during the spring and summer seasons when the plant is more likely to grow.
  • Use a pair of garden clippers to trim the plant down as well as possible. Start with the protruding leaves around the outside, and then work towards the central stem.
  • Gather all clippings as soon as you finish cutting them to prevent new seeds and growths from developing.
  • Move towards the inside part of the plant after you trim all the outside spaces. It would be best if you had cleared the leaves and flowers from the rest of the plant at this point.
  • Repeat the effort as new shoots appear.
  • Add tarps over the area and proceed with the smothering effort.

Can You Use Herbicides to Kill Japanese Knotweed?

You could also kill Japanese knotweed by using herbicides, but this process can be risky if you don’t use the right items. Glyphosates are effective against Japanese knotweed.

Glyphosates are useful crop desiccants that promote excess drying. By drying out the plant, it becomes harder for the plant to stay hydrated and thrive. The plant will eventually die out due to its lack of moisture.

Glyphosates are useful for how they are safe near water. The compound does not enter water as much as other herbicides. But you’ll still need proper protective equipment when handling this herbicide, including gloves and a facial covering to prevent inhalation or exposure.

Avoid using glyphosate around any spaces where you plan on planting vegetables or other potentially edible items. The compound may stay in the soil and enter the food supply.

The timeframe to kill Japanese knotweed this way can take a while. It may take about three or more growing seasons to remove the plant, although it may be less if it is small enough.

Is It Possible To Burn Japanese Knotweed?

The last option you have to kill Japanese knotweed is to burn the plant. But you will not burn the plant while it is in the ground. Rather, you will burn it after you have successfully removed the plant, allowed it to dry out, and placed it in a safe space for drying. The burning process keeps the seeds and other items leftover from spreading.

You must also burn the plant in your garden and not in any other space. You may be subject to fines in your local area if you spread the plant elsewhere.

Be cautious when looking at ways how you can kill Japanese knotweed. You can remove this bothersome and invasive plant if you know what works and use the best efforts.

Conclusion on how to kill Japanese Knotweed

This is a crazy tough plant that is incredibly invasive. You may have to use more than one of the options above in order to beat this weed. It will take multiple treatments and could require a couple of years to eradicate completely.

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