Japanese knotweed (Polygonum cuspidatum) is an Asian plant that has invaded and
degraded miles of riparian areas and wetlands in the East. It should come as no
great surprise, but unfortunate news for Wisconsin regarding JK is that I have
recently documented its ability to produce viable seed in our upland settings,
seed that readily germinates here (at least in the Madison area) and can spread
to otherwise "clean" nearby streamsides. Consequently, JK will likely be much
harder to control than we'd hoped since seed dispersal gives it the ability to
appear anywhere in the landscape where it's damp enough for germination. New,
distant upland sites will increase the chance of it spreading to more wetland
and riparian sites. I've been noting new upland clones in woodlands in several
counties the last couple seasons, and it's likely that these have come from
dispersed seed.

In any invasive control planning that you do, or may advise on, consider that
all JK should be enough of a concern that its ability to produce seed should be
determined, and quicker removal (of at least any female plants) than we thought
may be required to protect other areas on the watershed, especially lower,
riparian sites where JK growth can be especially rampant and destructive, and
from which vegetative spread downstream is highly likely.

Keep in mind:
JK is still low on everybody's radar screens, but that means it's still one of
those plants we can relatively easily keep out of most of our wet areas.
We should all make an effort to recognize it and take steps to control it,
especially female plants, at least where it seems likely to get into riparian
areas.
It's a distinctive plant, given its height, bamboo-like stems, clonal habit, and
incredibly fast growth, though in summer it can be missed until blooming in
August.
It's especially recognizable in winter (even early spring) as the very rusty
color of its dead stems stand out against the white snow (or gray backgrounds).
It's still sold in local shops under a variety of names, such as "dinosaur
plant" (large, fast-growing exotic look?) and "French lace" (based on its
lace-like sprays of cream-colored flowers?). (Other reported names: Fallopia
japonica, Pleuropterus zuccarinii, Polygonum japonicum, P. zuccarinii, P.
sieboldii de Vriese, Reynoutria japonica, Japanese bamboo, Mexican bamboo,
Japanese polygonum, Japanese fleeceflower, false bamboo, Kontiki bamboo,
bombascus.)
NR 40 should help with the sales problem, but keeping it from taking over large
stretches of streamsides (as in the East) will be more of a challenge for us
all.
See Wisconsin's brochure at
http://dnr.wi.gov/invasives/publications/pdfs/japanese_knotweed_brochure.pdf
Report its locations at ninvasiveplants@...


Here are more of the details about local seed production and spread if you want
to read more about it:

This news may not come as much of a surprise to you, but much research on JK has
suggested that it does not spread through sexual reproduction and seed. Emphasis
has instead been on its vegetative spread, either by rhizomes (that can extend
60 feet and grow under paved roads) or by pieces of the plant that sprout new
roots wherever they lodge on moist soil (such as where flood damaged plant parts
root downstream). All JK plants in Europe and Britain are (or have been) female
(in fact, genetically the same plant!) so research findings from there logically
dwell on asexual spread. (Vegetative dispersal along riparian areas will almost
certainly remain our biggest concern.)

To our detriment in North America we have both genders and fertilized female
plants here can produce typical, winged seeds. Research at the University of
Pennsylvania and in Washington State has shown that the plant's seeds are viable
there, germinating both in research plots and natural sites. Reports of
seedlings are still few, and I know of no previous reports of JK seed production
or seedlings here in Wisconsin (or the Midwest).

My recent sighting of the plant in upland sites suggested likely spread through
seed, especially considering U Penn's work. Consequently, I located JK plants in
Madison that produced seeds last fall, harvested some, and have successfully
germinated them in near natural conditions. Scouting specifically for seedlings
this spring under some of these plants has also shown that seeds can germinate
totally unaided. Finally, I have also located JK seedlings at least 10 meters
away from one parental clone, totally out of its shadow, and growing along the
sides of a nearby stream--our worst fears confirmed!

See the seeds at http://www.flickr.com/photos/cpurrin1/2244040247/ and learn to
recognize the seedlings and other life stages at
http://www.oardc.ohio-state.edu/weedguide/singlerecord.asp?id=230

The fate of JK seedlings has not been well studied, and it's probable that most
die under the severe shading produced by the huge JK clones growing above most
of them, but those that escape this shading may do well, even under the heavy
shading typical at ground level in many of our mesic forests and highly
competitive wetlands. Seeds from one Washington State clone that germinated near
the clone resulted in several healthy, three year old seedlings. It's been
suggested that a seedling needs 4 to 6 leaves to make it through its first
winter. How they fare in our climate is unknown, so I will watch those I've
documented here, but also encourage you to keep an eye out for them and let me
know if you see seedlings that appear to be successfully overwintering and
establishing new clones where you live in the state.

(BTW, a location reported to me where an established clone does not seem to be
"moving downhill" as one might expect with seed, does not necessarily suggest JK
doesn't spread by seed. Rather, seed production or its viability may be
nonexistent or low since fertilization is probably still rather uncommon. JK is
not yet well spread throughout our landscape and finding both genders in close
proximity is probably still rare. Let's try to keep it that way...)