Part of our border has always had mombretia. One year I dug down 2-3 ft & took out every speck of the blinking plant. Over time we got a lot more ...its all leaf...its stifled & killed off a Hydrangea & surrounded a Sarah Bernheart Peony...It is an old strain so the new showy ones put the flowerless ones to shame. The border is
just over 4m by 1.2m. A conifer marks the cut off on the left & an amelanchier on the right small fence behind. Border is west facing...Cheers
Do you mean "Montbretia" ???
Many of the Crocosmia's can be hard to erradicate if they've been growing in one area for a long time. They multiply like crazy (obviously !) and some of them have a runner type root that connects the corms to one another...so when you dig up a clump of them, there's probably more you didn't get that are still in the vicinity.
If you've dug down 2 to 3 feet and still have them the only thing you can do is just remove them the minute you see them poking through the soil.
The ones I have travel underneath the lawn and come up five feet away, so I share your frustration !
I to share that frustration, I was making veg beds several years ago and wanted it to resemble the old fashioned Potager type layout that was found in old gardens / cottage gardeners used this layout, other name for this was kitchen garden, anyway, did all the hard work, got the wood laid to outline the shaped beds, laid black expensive woven ground cover that was supposed to keep the Devil himself at bay, barrowed tons of lovely stone chips for the pathways around the beds 4 inches depth, but 2 years later, that darn Mountbretia found it's way to poke through the woven fabric, and the 4 inches of stone chippings, Believe me, it's even harder to try eradicate that pest of a plant out from the stones, I've raked the stones away, tugged /broke / stamped and scarped these new shoots of Mountbretia to no avail. I think it's a case of every year try remove whatever you can. I know the newer types of this plant are NOT as invasive as the older type we have found growing 3rde generation of families gardens. and still some folks are asking for some of this plant, I refuse to offer it but give the new named form of the plant to help them enjoy the shape/form and they have been happy with that.
Wish I had a magic wand to remove this but maybe someone else will come in with another method.
Good luck and kindest Regards.