Planting in containers will prevent spread under most conditions. I did find out that beaver dig up and eat the rhizomes, and also use them to clog drain pipes. If beaver or muskrat "transplant" your lilies, then some could escape, but that would be a rarity. My family of 6 beaver have now been eliminated, but not before really trimming back my lily beds. It appears that they may have decimated my white and yellow lilies while leaving the pinks less disturbed.