I have this, too, everywhere, but given that it grows through everything, I find the best way to keep on top of it is to keep pulling it out. The good news is that it comes out easily in 'ropes' of weed. Just keep pulling it out, I would say.

So would I, Jackie, because if you use a weedkiller, it will indeed kill other plants that it touches or drifts onto.

Don't let it seed itself if you can help it.

Totally agree with the above, just keep pulling out using a weedkiller will kill all the plants it lands on. It is very easy to pull out and can even go in the compost if it isn't seeding

Wear long sleeves when you pull it out, I discovered last week! I believe it is an annual but self seeds very freely so its best to pull it out before the little burrs form. I burned mine to be on the safe side as I had a good fire going anyway. ( I roasted potatoes in foil for tea in the heap of hot ash when it finally died down - best I've had since we used to roast them on an open fire when I was little.