Just about any time the bulbs are so invasive!

I divide up my montbretia( now known as crocosmia) as soon as I see new shoots in spring but as Moongrower says you can do it at virtually any time.... when there isn't frost in the ground

As MG says, whenever you like. Montbretia's unkillable, even if you want to.

We are still trying to prevent the invasion from next door Beattie... Bulba digs it all up and a wee while later there it is again!

*whisper this* but I'm hopeful that I may have almost eradicated a patch. I dug it over and over like someone with OCD, removing every last root and speck. I incinerated tarpaulin-loads. Then did it again a few weeks later, then again, and again. Now I just get a few seedlings from the seeds that must still be there and I get them out when they're about an inch or two high.
But I know that if I turned my back and ignored it for a year or two we'd have the whole patch back again...

I have the same problem, it grows through my lawn from the old border i had there even tho a bulldozer had been in to dig the garden up hahaha, I cut the grass twice a week so they dont have time to grow much..
If you dig them up Martha, dont spread them around your borders, unless your wanting a garden FULL of them lol..

I love them, one of my favorites.

I have 6 types all growing in large containers.

In containers than can be okay as you can keep them under control - a bit like mint! Beattie the wretched things are growing up through other plants and shrubs we don't want to, and in some cases can't, move... so it is a slow process. As they appear out from under the hedge B is onto them :-)