our garden was full of it when we bought our flat. we did what mangoose did, trampled/crushed it and then nuked it with triple doses of glyphosate and other random chemicals people suggested. killed the top growth then we dug the whole garden out to around a foot deep removing ANY roots or bits of roots we found. we pulled out about 3 black bin liners of roots!!! 8O sounds full on but it wasn’t too bad as at its not too big and at the same time we were taking out tree stumps/old shrubs/old air raid shelter wall etc and needed to get the levels down anyway as it was about a foot higher than the path/house floor level
we’ve since turfed it and got borders around it with new plants/veg’s and trees which are all doing fine. however the marestail is still coming back, bit by bit. luckily not in the lawn itself just in the borders – we just pull it out and dig the root up every time we find some. after speaking to plenty of gardeners/doing research its apparent it’ll never go away, and not helped by the fact next door neighbours garden is a jungle of marestail so without killing theirs off no doubt we’ll never get rid of the epicentre of the problem (theirs is a rented house – occupier and landlord have no interest in sorting it or even letting us take care of it!)
i’ve heard a good way of getting totally rid of it is to cover the affected area in black tarp to cut out sunlight for a year or so – obviously not great for kids/bbq’s. the main thing with it is any part of the root will grow into a full plant, and it survives composting too so important to dig it all out and burn/plastic bag and bin it