I had this problem in my garden years ago. I was advised to dig or pull it out when you see it but take care to get rid of every piece that you remove as it grows back from the smallest piece. This sounds tedious and it is! but it works.
I had tried weedkillers etc without success. It took a few months of vigilance - but success!! Good luck!
We covered our garden for over a year thinking that blocking out the light would kill it off, not so! it survived and as soon as we uncovered the ground it just popped up again, a friend of ours managed to get rid of it using Roundup, but it took about 3 applications, we still have it sadly, but we keep digging it out and it is getting less and less, but it's a slow process...........................welsh
i have the same problem, its a nightmare, apparently the marestail in your garden can be the same plant as in a garden a mile away as it spreads rapidly and deeply underground, throwing up shoots to collect light and food.
although you cannot ever get rid of it you can control it. do try the weedkiller approach i.e. round up, but be aware that marestail has a waxy coating on the stem that repels the weedkiller, get a knife first and scratch the stem and then spray the whole plant with weedkiller. after a few days then when the weedkillers done its job pull up all the weed and keep repeating this whenver it comes back, gradually it will weaken the plant and make the problem more manageable