By Mixared
United Kingdom 
I am have terrible problems with a weed in my garden. It looks like a very small christmas tree and the roots go on forever. I have tried various weed killers but they always come back after a few weeks.
3 Jul, 2011
Answers
You will never get rid, but if you constantly pull them up they do get thinner and weaker till you end up with just a few weak ones. They like poor soil and unfortunately have this habit of coming up in the centre of the plants making weedkiller unusable. If they are really invading a plant then the only way is to dig it up to remove the weed from in between the plant roots.
3 Jul, 2011
It sounds like marestail .
It is notoriously difficult to eradicate and mnost wed killers don't touch it.
Kibosh kills the top growth within a few days but is very expensive and I'm not yet convinced it kills all of the root
Here's something I wrote on the subject
http://www.flickr.com/photos/31559373@N00/4757456972/
3 Jul, 2011