Mares Tail........

This is one of the very worst weeds to get rid of, I'm afraid. One thing to try is to trample all over the patch to bruise the shoots and then try glyphosate weedkiller - but the best way, if a slow one - is to lay black plastic sheets on top of it, weighted down, to smother it. This will take a LOOOONG time - over a year, up to two years. Even then, it might re-appear as the roots go down a long way. Sorry, but there really isn't a quick and easy solution.

We had Japanese Knotweed in our previous garden - and believe me, that's worse!

How big is your 'large patch'? What are you prepared to do to remove it?

Mare's tail is covered in a protective 'cellophane-like' sleeve which will protect it from spraying hence the bruising 'Spritzhenry' speaks of.

Blanketing the area is fine if it's not outside your back door nor does it cover most of your garden, otherwise you need to be a lot more proactive.

I had a 120 ft section of it in my garden when I arrived. During my second winter I dug it out, fortunately it has very distinctive roots. By spring it was gone, but not 'sorted', and stems would reappear for a couple of years. This is the time you most convince this weed to go elsewhere! In my case, I used a small garden trowel to dig out every recurring stem down to a depth of about 4" or more WHEN IT APPEARED. In this way the plant obtained no sustenance from the daylight AND was forced to expend valuable energy each time to grow back those 4" to reappear. One of us would 'win' and I sure was not going to give up!

This is attrition. No two ways about it. But the weed has not reappeared for some years now.

I agree with Muddywellies, if you can, just keep digging it out every time you see it, it may take some time but very rewarding in the end.