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Killing Fallopia Japonica With Roundup

Using Roundup and Other Glyphosate Weed Killers to Kill Japanese Knotweed

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What does Japanese knotweed look like? Picture.

What does Japanese knotweed look like? This photo shows an example of how the new shoots can look.

David Beaulieu

But as suggested on Page 2, there's no one "silver bullet" to be used in the war on Japanese knotweed. A second tactic used to kill Japanese knotweed, or Fallopia japonica (Polygonum cuspidatum), in your landscape or garden focuses on weed killers for Japanese knotweed. The weed killer recommended for use against Fallopia japonica is glyphosate. Trade names for weed killers containing glyphosate include Gallup, Landmaster, Pondmaster, Ranger, Roundup, Rodeo, and Touchdown. I am most familiar with Roundup.

Roundup is usually applied as a foliar spray; i.e., the weed killer is sprayed on Japanese knotweed's foliage with a sprayer after being mixed in a tank. I use Green Gorilla's pump-free garden sprayer. However, you can also inject glyphosate herbicide into Fallopia japonica, using a product that I have reviewed. By general consensus, the best time to use this weed killer as a foliar spray on Fallopia japonica is late summer or early fall, when the plant is flowering and the foliage is conducting the most nutrients to the rhizome to build food reserves. But some (myself among them) have been successful spraying glyphosate repeatedly throughout the growing season -- essentially never giving the plants a chance to put on much height.

Thirdly, Japanese knotweed can be suppressed (but not eradicated) by cutting it back throughout the summer, so that its photosynthesis is never allowed to operate at high levels. Since cuttings of Japanese knotweed easily sprout new roots and re-entrench themselves in the landscape or garden, pick up the cuttings and bag them afterwards. Don't rely on the cutting method in isolation, though. Cutting back Japanese knotweed regularly is a tactic only meant to be used in conjunction with injections of weed killer into the cane stumps. But this is a lot of work -- and certainly not my preferred method.

Finally, dig into the ground where the bamboo shoots come up most vigorously in your yard. In these areas you will probably discover the rhizome-clumps from which spring Japanese knotweed's roots and shoots. In stands of Japanese knotweed that have flourished for many years, these rhizome-clumps are very woody and can easily reach widths of a foot or more.

Japanese knotweed rhizomes can be dug up and bagged. Do not, however, expect immediate results from implementing this tactic. For no matter how careful you are, some of the rhizome roots will snap off. And from even the tiniest root of Japanese knotweed left in the ground, a new plant will eventually emerge. But remember: this is a long-term war. In this case, the nourishment your enemy requires to fight you most vigorously is stored in its rhizomes. Think of the rhizomes as fortresses. Although enemy soldiers will fan out and hide after their fortress has been destroyed, the loss of the fortress makes their long-term success more problematic.

As with cutting (tactic #3), don't rely on this eradication tactic in isolation. A good reason for wanting to dig up the woody clumps of Japanese knotweed, however, is preparing the ground for the laying of tarps, so that the tarps don't get damaged (see Page 2 for a description of the tactic that employs tarps). For, no matter how close to the clumps you trim the old canes, sharp edges are still likely to protrude through the soil from these clumps. Those sharp edges will puncture your tarp. So to get a nice, smooth surface for the laying of smothering tarps, it may be best to dig out some of the bigger clumps.

These eradication and control strategies are not mutually exclusive. In fact, waging an all-out, multipronged offensive will increase your chances of successful eradication of Fallopia japonica from your landscape or garden. For instance, you may wish to keep a tarp over the bulk of the problem area during the warm weather months, slashing/poisoning along the perimeter as necessary. Then, in late autumn/early spring, dig up as many of the rhizomes as you can (again, if only to ensure even ground for your tarps, lest they be punctured). Afterwards, place the tarps back on -- even though winter is on the way. You want the tarps to be already in place for the next growing season. That way, in case you get busy with your gardening in the spring and find yourself pressed for time, you don't have to worry about remembering the tarps.

The four tactics for eradication outlined above, whether used singly or combined, will require several seasons before satisfactory results are achieved. Think "persistence," not "silver bullet." Fallopia japonica can be eradicated from your lawns and gardens only if you are consistently vigilant. Plan on making its eradication your new hobby.

As a companion piece to the article above, please consult "Tips From Japanese Knotweed Specialists."

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