90cm still growing. Thought to have been eradicated here two years ago but making a recovery
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This plant is a number one menace!
As you have found, if you dig it up, or apply glyphosinate, any pieces of root that survive will regrow but, if you leave it, it will become even harder to remove and it will eventually take over your entire garden!
Believe it or not, I first saw this around 1950 growing on a canal bank near London Zoo.What attracted my attention was its phenomenal growth rate, about 15 to 20 cm per day. One now wishes that it had been eradicated there & then.
On the bright side, the hollow stems can be used to make home-made flutes or penny whistles.
Lat/Lng: 53.6401, -2.3979
OS grid ref: SD737160
grassland adjacent to housing estate