Giant knotweed superficially resembles Japanese knotweed (Polygonum cuspidatum), and the two
species hybridize. Giant knotweed is distinguished by heart-shaped leaf bases, pubescent leaf
undersides, and an inflorescence that is much shorter than its subtending leaf.
I have some knotweed too, a small patch near that old homestead site where the garlic mustard grows. Probably planted as an ornamental originally. When I attempt to identify it, it doesn't look exactly like the Japanese, nor the giant. It could be a hybrid of the two I suppose. It's a kissing cousin of them for sure. The knotweed I have doesn't seem near as resilient as the garlic mustard. It's not spreading and a good bit of it has died back after being driven on quite a few times while I was cleaning up that area.
I am working towards getting enough trees and rocks out of the way to be able to mow that area a couple times per summer with the bush hog. That may not kill it, but it ought to weaken it enough to die back after a couple of years.
Here is an article on giant knotweed
http://www.na.fs.fed.us/fhp/invasive_plants/weeds/giant-knotweed.pdf
From a knotweed erdaication guide (in the above link)
Cutting/Mowing: It is possible to eradicate small patches of knotweed with repeated and
persistent cutting of the plants. The patches must be mowed or cut twice a month between April
and August and then at monthly intervals until frost. Like pulling/digging this effort will need to
be maintained for at least two to three years. Using a hand pruner, lopper, or brushcutter, the
stalks should be cut as close to the ground as possible. The regrowth should not be allowed to
exceed six inches in height before the stalks are again cut to the ground. The cut stalks need to
be stacked where they will dry out and not root (away from moist ground). When using a
brushcutter, it needs to be ensured that scattered plant parts do not land in moist areas where they
can take root.
Habitat®, an aquatic labeled formulation of imazapyr has been approved for use in Washington
in 2004. Because imazapyr is known to translocate readily to rhizomes, this non-selective
herbicide will likely play a role in knotweed management. There is information showing highly
effective results obtained with a glyphosate/imazapyr mixture.
Renovate®, an aquatic labeled formulation of triclopyr has been approved for use in Washington
in 2004. Triclopyr will control Japanese knotweed, but there are no specific control
recommendations for Japanese knotweed on the Renovate® label. For successful translocation
to occur, the herbicide should be used at the lowest effective concentration in order to avoid
damaging the above ground tissues of the plant before the herbicide is well dispersed in the root
system. Triclopyr (Garlon 3a) at five percent solution appears to give good top-kill on Japanese
knotweed but results in mediocre long-term control on large patches. However, there are reports
of successful control using triclopyr at rates as low as ¾ percent in high volume application.
Foliar application, using backpack sprayers or similar methods, is more efficient on larger
monoculture stands of more than a few plants to several acres in size. To achieve the best
chance of complete kill, apply herbicide in the spring to plants that are less than 4 feet. The
plants need to be large enough to ensure that there is adequate leaf surface. Spray to wet and try
to avoid dripping of the herbicide from the leaves and to minimize off target damage. Although
the late bud stage of growth is considered to be the most effective time for herbicide application
for knotweed species, waiting that long also means dealing with a huge plant.
Cut stem application can result in up to 95 percent mortality. In the summer or fall, cut each
stem within one to three joints of their base (internodes). Add herbicide into the exposed hollow
stem cavity following label recommendations. Cut stem application is labor-intensive, both to
cut each stem and to apply herbicide, but it will assure that the herbicide is only applied to target
weeds and not to other desirable vegetation. Dispose of the cut stems away from moist
environments where they might take root. There is need to apply with care to avoid causing off
target damage from the undiluted herbicide.
The stem injection application instrument injects a metered dosage of glyphosate into a stem
while poking a hole in the other side of the stem. The second hole allows any liquid in the stem
to escape as the glyphosate is injected. The plant takes up the herbicide within 20 minutes of
injection. Since each stem appears to be supported by a separate rhizome, each separate stem
must be injected to kill an entire stand. For large stands, injection can be conducted several
times working towards the center of the stand or injection can be used to treat regrowth after a
foliar treatment. Although, as with the cut stem method, injecting is labor intensive, 100 percent
kill has been reported. Care is also needed with this treatment to avoid off target impacts.
Web site videos on knotweed control here
Invasive knotweed control video
This is one of the world's most invasive weeds. Mowing will not work.
NUKE IT with broad-leaved brush killer.
SC
Pretty benign compared to some of the invasives.