I think Acanthus can regenerate from tiny bits of root (much like dandelion) so efforts to remove it, or do any gardening in the vicinity of it, simply spread it all over the place.
Another PITA for the list, Festuca amethystina. I don't mind some grasses self seeding, but this one really is something else. I had one plant of it last year and now all the pots within the vicinity of it are topped with a fuzz of baby fescues.
"What is hateful to you, do not do to your neighbour".Â
Alliums....the white one (neapolitanum?) in particular, impossible to get rid of and swamps everything else at this time of year. Even A. sphaerocephalon is spreading everywhere and in thick clumps looking like an invasion of spring onions. Acanthus mollis....seemed a good idea at the time, looks stunning in flower but now fighting to keep it within the area I want it. A hardy geranium the builders planted (might be Gravetye?), I ended up hoiking most out but mistakenly kept a clump or two.....springs up all over the place and gets hit by geranium sawfly. Beginning to regret the japanese anemones...apart from Wild Swan, which is very well-behaved. Oh, and verbena rigida (sent by suppliers as v. bonariensis)....planted it anyway, love the look of it but oh my does it spread! May yet regret the persicaria Darjeeling Red....stunning-looking as a carpet but needs a bigger garden than I have!!
I'm with @B3 regarding phygelius. Can't eradicate it completely - noticed shoots of it behind the gas meter on the outside wall. Deadly stuff. And the chive-like allium people are talking about - it's Star of Bethlehem. Horrible stringy leaves, the occasional flower and teeny tiny bulbs that are impossible to eradicate.  Alchemilla Mollis is very well behaved in my garden. I'm hoping the Japanese anemones in the dry shade area will do a bit more than they have so far and as for lily of the valley - they are a PITA for some but I can't get them to like my garden, no matter where I put them.
@didyw Lily-of-the-valley was here when we came, growing (or clinging, almost)Â on a very steep hedgebank. The soil is saturated in winter and early spring but bone dry in summer. I once saw it in someone's garden which had natural limestone pavement. The Lily-of-the-valley grew in one of the narrow, water-worn grikes in the limestone, completely filling it. I guess there too, wet in winter, dry in summer. But usefully confined by rock!
Spreaders can be good. If they can be easily tugged out, it's always good to have a few spreaders to fill up spaces that might otherwise be susceptible to less easily removed weeds. Or things like Aquilegia that don't really swamp other plants and can be allowed to move around a bit.
"What is hateful to you, do not do to your neighbour".Â
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Another PITA for the list, Festuca amethystina. I don't mind some grasses self seeding, but this one really is something else. I had one plant of it last year and now all the pots within the vicinity of it are topped with a fuzz of baby fescues.
I'm a big one for local plants sales. I now realise that most of the things for sale are the PITAs that gardeners have potted up and sold on.
A trouble shared is a trouble doubled
Alchemilla Mollis is very well behaved in my garden. I'm hoping the Japanese anemones in the dry shade area will do a bit more than they have so far and as for lily of the valley - they are a PITA for some but I can't get them to like my garden, no matter where I put them.
I am somewhat guilty of this myself - not giving away nightmare plants, but I do pot up spreaders and give them away.
I once saw it in someone's garden which had natural limestone pavement. The Lily-of-the-valley grew in one of the narrow, water-worn grikes in the limestone, completely filling it. I guess there too, wet in winter, dry in summer. But usefully confined by rock!