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I thought I would include this information here because I am about to embark on trying to eradicate 160' x 40' of this terrible weed from my yard. I was surprised to see this planted in some people's gardens, and I just want to get the word out about the dangers of propagating this plant. There are many views on this, but the following information seems to be the most common. I don't claim to be any expert! I'll follow up later with my successes (hopefully)! - Doreen
What is it? Japanese Knotweed (Polygonum cuspidatum) is one of the most extraordinary examples of an invasive plant known. Resembling bamboo, (but not related), it grows with frightening speed in the spring to a height of 7 to 10 feet. In Europe and North America, there are only female plants. Not only is it a single sex, it is also a single clone! In total biomass terms, it is probably the biggest female in the world! Left unchecked, it will smother all native vegetation in its path and can grow into dense stands as large as three acres. How does it reproduce and spread? It reproduces by underground roots and fresh pieces of stem. The root (rhizome) system may extend from a parent plant up to 23 feet laterally and to a depth of 6.5 feet. Very small pieces of root (as little as 0.2 ounce) give rise to new plants. Fresh stems will produce shoots and roots when buried. Stems in water produce viable plants within 6 days. Most knotweed invasions occur not by seed, but by humans transporting pieces of the plants which exist in soil. Why is it so important to eradicate it? Japanese knotweed is extremely invasive, crowding out all native vegetation and eliminating wildlife food and habitat. To many homeowners' dismay, it can break up driveways and sidewalks. Once established, it is a monumental task to eradicate it. It has quickly spread across most of the the United States where it has no natural insect and disease enemies to keep it in check. In 1981, The U.K. passed the Wildlife and Countryside Act which made it illegal to spread Knotweed. How do you get rid of it? Be ready for several years of constant vigilance. The extensive roots sustain it even when all top growth is cut off. To kill it, you must kill the roots. This is best achieved using repeated applications of glyphosate-based herbicides such as Roundup Brush Killer. (I don't advocate the use of herbicides, but gee whiz, it seems to be the only thing that will work!) These can be applied as spray (on calm days) or painted on individual plants. Some people have tried injecting Roundup into hollow stems immediately after cutting. It is best to cut and/or spray in early summer, then spray again in late summer when the plants are storing energy in the roots for the next year. Never dig up Japanese Knotweed. This breaks the roots into pieces which will propagate into new plants. There are opposing opinions on the use of black plastic "tarp" so I won't comment until I experiment with it. For short term control, Japanese knotweed may be cut using a lopper
or brush-cutter. In some areas, prevention of invasion from adjacent
stands may be achieved by frequent mowing, but be aware that mowing
may spread it further by cutting up the stems. Stem nodes when in contact
with the ground have been known to root. NOTE: Please let me know if there is any misinformation in here. This is based on discussions with local people and readings on the internet, including newsgroups. University of Connecticu t- Japanese Knotweed (PDF) University
of Wisconsin - Japanese Knotweed
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NOXIOUS
WEEDS: JAPANESE KNOTWEED 
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