They "run" underground like some of the bamboos.. My big caerulea's doing it & going under the fence to my neighbors who hates it, but he's an ass anyway so..

It's just a big root ball. Go down to the ditch & find a sprout a ways from the mother plant (or close, whatever) then dig down about 4 to 6" away from it in a circle so you've got the sprout & some of it's native soil.. Just like digging up anything else. You just want to get it & some of it's root system. Once you've got it & some of it's roots (still in the dirt- try not to disturb them if you can) toss it in a pot & take it home.

Many of mine have grown through the holes in the bottom(s) of their pots & rooted into the ground- then sent out runners & are coming up some distance away. Some don't appear to do it at all (Amethyst) and some do it alot- (incarnata) According to my neighbor, caerulea will take over the yard in seconds, but I haven't seen that on my side of the fence. Next time I see him I'll ask what he's feeding it.

I just had to lmao @krstofer , sounds like we have the same neighbor...

I dug 3 out of the ditch just before the mowers got them this year. I did not get all of the root, so I put them together in a 1-gallon pot with some potting mix and it they root then I will put them in a BIG pot on the patio and make a teepee for them to run on. This way I can enjoy them and not have to worry about their invasiveness. Thanks, krstofer. I guess you have gotten revenge on your neighbor, whether you wanted to or not!