Japanese knotweed is one terrible plant. I've hacked away at it for years, fruitlessly, and finally found the way to exterminate it. In the fall, right after the first frost, the plant begins to draw all of it's life force back into the roots. That's when I hit 'em with Roundup and it worked!!! The next spring the plants didn't come back and any roots I found were almost liquified. A few very weak sprouts attempted to surface here and there but I just hoed 'em down and sprayed again the following fall, although there wasn't much left to go after. Now, in compost I'd wait 'till they sprouted and pull them out one at a time 'till all were removed. I don't think hot compost will kill 'em, maybe.

This is one of those weeds to bury in steel drums with nuclear waste. In the future, dry out the pieces on pavement till thoroughly dead and then bag and remove. This is not my normal advice for weeds but for this one I think it is best to be very safe. I don't know what it will do in your compost pile but I suspect that only a very hot pile will take care of it.

Here is a link that might be useful: Japanese Knot Weed

Eeeuch. :( Japanese Knotweed is evil, evil stuff. It's the only organic material from the yard I put right into the trash bin, instead of composting. Three years of regularly weeding the infestation here have slowed it down slightly, but not much.

In your situation Queuetue, I'd be tempted to go through the compost carefully and remove all the Japanese Knotweed you can find -- especially the roots -- and then set the remaining compost aside for a year. Maybe even inside sealed plastic garbage bags, to help anaerobic conditions occur which might help kill and break down any remaining knotweed roots or stems.

Or, you could just spread the possibly-contaminated compost as a surface mulch on garden areas which already have knotweed, then watch those areas closely.

In the part of the garden here which has knotweed, I don't ever till any more, or even rake the soil. Each year the new sprouts come up slower and fewer, but they still come up. One time last summer when I'd neglected weeding for two weeks, there were fifty of them. And in the summer of 2005, before I found out what kind of plant it was, a single stem grew over my head -- in one month.

Good luck Queuetue and all the best,
-Patrick

Here is a link that might be useful: Wikipedia article on the evil weed

This isn't the good news "you're overreacting" message I was hoping for, y'all. :)

Japanese knotweed is the devil. It's horrendously invasive, and here in Wales it is EVERYWHERE!! It seems that any untended piece of ground ends up with vast amounts of the stuff, and it's so hard to get rid of. Here it's illegal to propagate it or to dispose of it carelessly (you'll be fined if you put it out with regular garden waste). It's really bad. I'd recommend you try not to take any chance of spreading this stuff... if it were me I'd abandon that compost pile and only use it on areas already containing knotweed. This is the only weed I would react to so strongly... horrible stuff!

Having fought Polygonum Cuspidatum over a 3 acre area and WON, I feel qualified to offer these comments.
You can compost the stalks and leaves.
You CANNOT compost the roots. Every tiny piece of root will make a new shoot. As mentioned upthread, sequester the roots and let them dry, then burn or dispose of. Do not return to any growing medium like dirt, compost or apparently, concrete.
Hopefully you will have cut the plant down and poisoned it long before it flowers, but DO NOT compost them either.
The primary means of transmission is rhizomatic. Long established stands are very hard to kill since root heads will establish networks of roots to other heads. Some of the heads I excavated ( yes, a backhoe) were eight inch diameter. Persistent poisoning/cutting off growth will eventually deplete the plant's reserves though you may be batty by then.

90% of the infestation is in my neighbor's yard, right along a chain link fence with a deeply set stack of railroad ties on my side. He likes it and the privacy it provides, although I don't know if he planted it intentionally or not. I'm just dealing with it where it comes onto my side, and there are really only two patches. One of them, I've dug until no more roots could be found and put a 12" raised bed over cardboard full of next year's rhubarb. (Maybe I'll dig back up and put landscape cloth there.) The other, I'm stacking container plants over cardboard and mulch to make it unhappy - this one is right against the fence, so it'll be a life of containment there. Maybe I'll put down gravel.

Ideally, I'd convince him to help fight it, but a) Like I said, he likes it, b) he doesn't do his own yard work, and c) I've moved into Quebec and there is a significant language barrier - I'm not at all convinced I'm going to be able to convey what I'm talking about.

So, I continue the fight on my side - including laying my entire compost pile out on plastic and cardboard and seeing if I can dig those roots out of it and put them on the barbecue until they're dead.

He may speak English, too. If not, perhaps the posters on the French forum could give you the location of a site to show him. Or they could just show you how to write what you need to tell him (translate your message into French for you.)

Here is a link that might be useful: Garden Web French Gardening Forum

There is something very educational about tearing an entire 5'x3'x3' compost pile down, spreading it onto cardboard and plastic and sifting through every inch of it. The good news, I've reclaimed about the mass of knotweed root I put in.

After a week in a warm (not hot, but less cold than I thought it would be - maybe 95-100 deg) pile:

a) There was no growth on the root sections.

b) There was considerable breakdown on the cut ends - the crawlies seem to like them a lot.

c) What were very very solid pieces of root are now limp and soft, like a carrot left out on the counter for a few days - which I suppose is a very apt description.

I put them on cement and chopped them well - pulped them, really - with a shovel. Within sight of the mother plant, so it knows I mean business. Bagged and out with the trash, with a handful of salt for half an hour where I did the chopping before rinsing the patio into the storm drain. Wife stopped me from getting the lighter fluid, because the neighborhood was watching by now.

This link has info in French including the following. (Tell your neighbour it's in red in the original article!)

'Cette espèce a été nommé parmi les 100 espèces exotiques envahissantes parmi les plus néfastes au monde'

Here is a link that might be useful: Infos sur Fallopia japonica

Which basically translates to...

'This species has been named among the most exotic determined invasives of the world.'

My Mother is from Quebec...