I wasn't sure what "fishwort" was but I found the latin name, Yikes, "houttuynia cordata" or chameleon plant is very, very difficult to eradicate. It's considered highly invasive and it's rhizomes will regrow if you toss them aside. Do a gardenweb search on "Houttuynia" or "chameleon plant." (Get a glass of wine 1st...and a box of tissue:)

I once planted bishop's weed, not knowing what it was. I thought I'd never get rid of it! And I don't think it's as bad as fishwort. Reportedly it takes several treatments of Round-Up to kill it. Even if you could spray it. The only other alternative is to dig it out. I know, I know! Either way, it's a major task.

BUT...IF you choose to use an herbicide you can wrap the plants in plastic trash bags to protect them from the spray....all the way to the bottom of the stems. I really don't know how many sprays it will take to do a complete kill. Then you might think it's dead and it might rear it's ugly head again.

Another thing, if you use Round-Up don't get the pre-mixed stuff. Go for the "super concentrate." Make sure you spray on a very hot day when there's no wind. I'd also use a "spreader sticker" to help the spray adhere to the plant leaves. Read about it here;

http://www.plant-care.com/1271-wetting-agents.html

You can find it in most garden centers.

BTW, the Chinese name for the plant means "smelly fish."
I also found this on the web.
http://gurneys.com/product.asp_Q_pn_E_11050

I wish you luck.

Here is a link that might be useful: Houttuynia Cordata