The method described by Tegwyn works very well....I used the same method to rid myself of a very large (30x50') area of goutweed except that I ran it over first with a lawnmower with a bagger on it.
I then sprayed the new growth about a week later (it grows up quickly) with Roundup. Cutting it down allows you to spray new growth which takes up the Roundup spray quickly thru photosynthesis. It also allows you to use less spray because you are spraying short new growth instead of all that tall older growth.
It helps if you know how Roundup works....it will only kill plants that are sprayed...it works by being drawn into the root from the leaves by photosynthesis. The plant must be actively growing and have foliage. Saturating the soil will not kill plants and will only be a waste of spray.
It will not kill neighbouring plants that the spray has not touched...it will not be picked up by those plants from the ground thru their roots. If you have desirable plants mixed in with the gout weed, you can cover them with a plastic bag or nursery pot temporarily while you spray.
I recommend buying the jug of concentrate and mixing your own spray according to directions. It seems to work better than the spray bottles of premixed stuff. Perhaps they are a weaker mixture.
Then just remember to wear plastic gloves (I use surgical gloves that can be thrown away), rubber boots that can be rinsed after (not fabric shoes) and last but not least....begin spraying at the farthest point and walk backwards as you spray so you don't walk thru the sprayed foliage. You don't want to end up with footprints of dead lawn where you walked after with Roundup on your boots!
Don't laugh....I've seen many people do this! :)
Don't allow your pets or children to run thru the area while it is wet. It is safe when dry and breaks down in the soil quickly. You can plant in the soil in 4 days although you won't see the gout weed dying for about 2 weeks.
Be patient...it does work!
I only had to respray some tiny pieces the next year, up against rocks or desirable plants that got missed the first time.
Q
Coral Bark Japanese Maple
Q