If, at any of the homes where you have lived, you have had a stand of Japanese knotweed (Polygonum cuspidatum) growing in the yard, you are aware of several important facts revealing why it is so important to remove this weed from your property. For example, you know that:
- The area consumed by such a stand is useless. You can't grow a lawn there, you can't plant a garden there.
- If this invasive plant offers any beauty at all, it is only in the autumn, when it is in flower. The fuzzy look of its floral display (from a distance) is what has given it its alternate common name of "fleeceflower." But the plant's stalks in spring and summer offer no ornamental value. Worse still, when the stalks morph into dry, brown, woody, dead canes in winter, they become downright ugly. Since it takes these canes years to break down, they saddle your landscape with a perennial eyesore.
- Japanese knotweed can even reduce home values. In the U.K., mortgage lenders increasingly are refusing a mortgage on a property with Japanese knotweed growing on it. Elsewhere, the impact may be more subtle, but it can still be felt.
Removing Japanese knotweed is easier said than done, and arguments can be made against whatever weed control method you try.
Smothering Japanese Knotweed
One way to try to kill Japanese knotweed is by smothering it with tarps. The rationale behind this approach is simple enough:
- Japanese knotweed needs sun and water to grow.
- By cutting it down and covering the area where it is growing with impervious material, you deprive the weed of what it needs to grow.
- Eventually, the Japanese knotweed will be starved out.
No chemical treatments are used.
It should not cause much hardship to people who can afford to be patient.
It will take a long time.
Being a rhizomatous plant, this weed is perfectly capable of sending up shoots beyond the edges of your tarps.
This project is labor-intensive. Before you can lay the tarps, you must prepare the ground for them.
Jim Glaister of The Knotweed Company, LTD observes that a stand of Japanese knotweed threatened with an attack as serious as that posed by smothering will go into a state of dormancy for up to 20 years. During those 20 years, it will live off the nutrients that it has stored up in its rhizome system.
It is the plants' way of weathering the storm until favorable conditions return (the removal of the tarps, in this case), an effective survival mechanism. If, for whatever reason, you have to remove the tarps before the 20-year waiting period is up, the Japanese knotweed will come back.
Even if you can wait for such a long time for Japanese knotweed to exhaust its nutrient supplies, you are still not in the clear. The weed will try to "beat the tarps" by popping up outside the tarp area's perimeter. You must remove these escapees when you find them because, otherwise, photosynthesis will occur (when the sunlight hits their leaves), and a reinforcement of nutrients will be sent down into the rhizome system.
On the other hand, there are definite advantages to the smothering method.
Although you can supplement by using chemical herbicides (to kill the escapees), you do not have to, so this method can be considered organic. You are killing Japanese knotweed with patience, not with chemicals. In military terms, the equivalent would be keeping a city under siege for a number of years to starve out the inhabitants, rather than going in for an all-out assault.
If you wish to stay organic and avoid spraying to control these escapees, sink bamboo barriers down several feet, all along the perimeter of your tarps. But, for a large area, this solution represents a lot of work and significant cost.
As long as there is no rush to do something more permanent with the space in question, simply plan on leaving the tarps in place for the 20-year waiting period. What makes exercising patience here more palatable is that you can simply landscape right over the tarps, using raised beds and/or container gardens. And mulch can be applied over the tarp areas outside the raised beds and/or container gardens, both to disguise the tarps and to protect them from UV rays. So, while you wait, the space can still be attractive and functional.
Spraying Japanese Knotweed
Spraying Japanese knotweed with herbicide is a popular removal method. It, too has its pros and cons.
It is less work than using tarps.
You can allow the plants' rhizome system to work against them.
If you do it right, spraying can remove Japanese knotweed more quickly.
It requires use of chemical treatments.
The herbicide most commonly used on Japanese knotweed will also kill anything else it comes into contact with.
Unless you spray in just the right way, Japanese knotweed can also use its "dormancy defense" against the herbicide.
The chemical herbicide most commonly used to kill this weed is glyphosate (sold most often under the brand name of "Roundup"). Glyphosate is non-selective: It does not discriminate over what it kills. If you get any on your lawn grass or shrubs, for example, it will harm them.
Glaister points out that exactly how you spray can be critical to your success. Many homeowners, thinking that spraying more often means they are doing "a better job," are actually making a mistake. The intense spraying causes the Japanese knotweed's dormancy defense mechanism to kick in. Yes, spraying kills the top growth, but the rhizomes live on, ready to fight another day (after a sufficient pause). The result is that the weed returns in a few years. To combat this result, Glaister recommends taking a more gradual approach to spraying.
Spraying does it have its pros, however.
Unlike with the smothering method, there is very little prep work involved in spraying. In fact, you can even buy sprays in ready-to-use form to avoid having to do any mixing.
The best time to spray is in fall. This the time of year when nutrients are most actively being stored in the plants' rhizomes. So, when you spray the herbicide onto the foliage, it gets pulled right down into the Japanese knotweed's critical underground storage system for nutrients.
Holding off till fall to spray (as opposed to spraying throughout the spring and summer) also ties into Glaister's advice about less-frequent spraying being more effective. While it is impossible to give a timetable, you should, following this method, be able to eradicate Japanese knotweed in a matter of years, rather than decades.