It will have lots of roots, not just one so, as Dove says, spray regularly with glyphosate and this will eventually kill off all the roots too. If you can't spray, the alternative is to pull off all visible top growth and check every few days for new shoots reappearing. Remove these by hand or use a spot gel or paint on treatment of glyphosate on the leaves and eventually the roots will die from lack of food form above.
Either way it's a slow process and requires patience.
Digging is another option but no matter how painstaking you are, little bits of root will be missed and will grow into new plants and you'll have to remove every visible bit form the roots of plants you want to keep as well as the soil in between.
I'm going through this process myself after surgeries which have meant I've been unable to look after the garden for nearly 2 years so bindweed, couch grass, nettles, creeping buttercup and thistles have been enjoying themselves.