Hi-
I don't know if you can do much to stop its spread other than cutting the rhizome when it spreads past your limit. - Gwenn

If there's one thing I learned growing my Costus gingers indoors , there's no stopping the rhizomes from spreading and I'd suspect even with a very deep barrier of metal or plastic or some other material they'd do just as my Costus rhizomes do, when they've reached the sides of the pots and can't spread any farther out. At that point the rhizomes turn downwards and keep going downwards til they've hit the bottom of the pot and can go no farther. I'd imagine like the bamboos, it would take a very deep barrier to turn them back around. I don't know of any chemical either , short of an herbicide , that would make the rhizomes stop spreading. You're left with the "cutting the rhizome" solution I'm afraid, and I've also learned that those costus rhizomes are very tough to cut except for the sharpest of "loping shears" or some other sharp tool. It's almost like trying to chop through wood with a butterknife , if your tool is dull.

Thanks for your answers. I'm wondering if I've let lose a monster! I have found bread knives are just the thing for cutting escaping rhizomes. They are long, thin and stay sharp seemingly forever as long as you don't drag it across rock. It has turned out to be a most valuable and versatile garden tool, excellent for prying out rootbound plants in terra cotta pots, cutting off grass, weeds etc. that grow on paving stones.

X

I must say, , even growing my Costus barbatus as a pot plant , it's a little too fast growing for indoors, and I have to constantly be dividing the darn thing, and finding lots of large pots to repot the "extras", which outgrow the somewhat smaller pots within just a few months and then need repotting again. However , since mine act more like biennials in nature , with the older shoots dieing back afer less than a year, it's always nice to have newer growth replacing the old canes all the time. Maybe it's just my indoor growing conditions that cause the individual canes to have such a short life, but I suspect that many rhizomatous growing plants have that "determinate" growth habit like cannas , which after flowering always produce new leaders, and rhizomes from which the new flower stalks can grow. It seems the eyes or remnants of the previous years shoots never grow again to reflower a second time. I wonder, if anyone else has thought about this habit or perhaps I'm mistaken about my assessment of their "habits".
Another change that my ginger has undergone, is the failure to produce young pups from the older stems as they died back, as they did the first year , while I was growing them.
None have produced the "odd" pups that formed along the leaf nodes next to the dieing leaves. Those came out looking a little like new branches or shoots , but quickly produced and ever enlarging bump at the bottom of the shoots right next to the mother stems, and afer a while they'd drop off naturally and would root just like a regular rhizome , when they lay on the soil. Those came in very handy for starting new plants, but perhaps keeping the plants a bit wetter or something else I'm doing since then has proven to change that habit of forming the pups?? Those plants can have some amazing and yet very "difficult" habits to figure out, and I've been gardening for fifty years.

like bamboo ,have you tried a sheet metal barrier about 12'' in the ground ?