Beware of Periwinkles

I just finished tearing out the periwinkles I planted around my big mint bed last spring. I chose them because I thought they were one of the few flowers that might be able to hold their own amongst the rampant mint. They did more than hold their own; they nearly choked my poor mint to death, making it necessary for me to plant three small supplementary patches in order to keep supplying the Green Goddess. When I pulled up the periwinkles just now, they had gone from innocent-looking little p...