Yes, Phygelius, no wonder they call it devils tears! Derek.

Not one that I planted but a neighbour - it's Lily of the Valley, stuff gets everywhere!!! Not forgetting her Kerria, that's just as bad!

Japanese anemone. We didn't plant it , we had it in the garden when we moved in EVERYWERE and now we can not get rid of it short of a flame thrower

My lily of the valley was planted nearly 40 years ago and was pretty rampant, then mysteriously disappeared for years. Now it's back with a vengeance. Just today, I was looking at it and thinking...

Geranium nodosum. It grows in dry shade - that'll be useful I thought. Flipping self-sows everywhere too :-(

Grape Hyacinth ... it pops up in the most inaccessible places under shrubs ... :o(

I haven't, but some elderly neighbors had some plants that they wished they'd never laid eyes on. They're grandson was going on holiday and asked them to look after some plants while he was away. He told them they needed lots of sun and water and so they put them on their window sill and gave them lots of TLC. They got a knock on the door one night from 2 police officers, their grandson had omitted to to tell them that they were cannabis plants.

I second the vote for Japanese anenome. It is a swine to eradicate. I've used round up on a patch several times and hoed it off at the ground and it still comes back!

Non, je ne regrette rien! Everything is a learning experience and one cannot regret learning. My only regrets are buying plants that I grow bored of and refuse to die so that I do not feel bad composting.

Thats a laugh about the Cannabis plants !

Fascinating! Thanks for so many interesting and thought-provoking comments. Phygelius: yes, that's a thug, if ever there was one. I'm still digging out stray plants but have almost eliminated it. No problem here with Lily of the Valley: am trying to get it going in a damp corner but so far it's resisted my attempts and I have just a few rather weak-looking shoots. Geranium nodosum: thanks for the tip-off; I shall politely refuse an offering of a 'few roots' from a 'friend.'
As for the cannabis plants: many years ago another friend asked me to help at his first Gardens Open day. He said he'd moved his cannabis plants to a 'quiet corner' of the garden so 'no-one will see them.' I quickly advised him to rope off that corner as I'd learnt from experience that garden visitors find their way into every nook and cranny.

Oh Yes! Acanthus. It seemed like a good idea at the time but it is rampant. Have tried to dig it up but every little root left grows. Have tried Roundup but still it keeps on coming. Every time it shows itself now I pull off its leaves, hopefully I will tire it out before it does me.

Grape Hyacinth is just as bad. I have resorted to digging up the whole of my spring bed to get out every tiny bulb. Only next year will tell if I found them all!

How long a list would you like? Last time I did this I think I got to over 30 plants.
There are 4 kinds of plants in my garden, in order of frequency: Thugs, dying, dead,desirable!

Oh Myron. :0))

euphorbia is my pet hate too....I,ve finally eradicated it....I think!

Pyracantha. Can't stand it.
Re cannabis? Planted some in the police station where I worked. Indoor planters, outside senior officers' offices. 11 days and no one noticed! Plant was 2ft tall by as much across by then. All I wanted was for someone to see it and ask who the hell had put it there! I suppose I should have expected that.

They were all probably waiting for it to mature before they harvested it!

Bouncing bet Saponaria officanalis, a lovely soft pink but my goodness does it bounce.

Well today I have spent digging out hardy Geraniums and Campanula trachelium. Both are seed weeds of the first order. Well I have had enough and am getting rid of all but 2 Geraniums. One which is a breeding of some friends and is sterile and the other is G farreri as it is so dratted hard to get hold of and keep alive. The rest are history.

Owdboggy - I grow campanula trachelium in shade on clay, and there's no way I would call it a seed weed. But I've seen it in the wild, and I can understand it could be a problem in more favourable conditions

Let's put it this way Andrew, I have removed over 100 plants of this thing and I am sure I have missed some. I did leave in the double white form which appeared a few years back and possibly the double purple one.
Campanula persicifolia alba is another thug for us.
Silene maritima...........shudder. It took over the rock garden and is still appearing even though I have used Roundup on the plants and the flame gun on the seedlings.

I have what feels like acres of campanula poscharskyana. It grows everywhere - on walls, in the beds, around the doors, up through the dining room floor (yes - really!) It is so pretty when it flowers, and fortunately is fairly easy to pull up - but not eradicate. As soon as the flowers have finished, I go round digging out as much as I can. I still enjoy it, but do wonder how many more years I will tolerate quite so much of it. The trouble is, with most of these characters, it's all or nothing.

I'm glad that someone mentioned acanthus. Acanthus mollis (the spinosus seems quite innocuous) is another 'plant noir', with which I've been doing battle for five years. You can dig out the nasty fleshy roots as much as you like - there are still other, deeper roots which put up leaves and, left to their own devices, flourish. I agree that Roundup makes little impression on it. But this is a plant that I inherited in the garden, rather than one I planted myself, although that's no comfort. Acanthus mollis is, of course, a beautiful, architectural plant and supposing you lived on a large country estate and had unlimited room to let it spread, it would be a joy and a delight. In my garden, it is not!