I can't speak to which crocus species to plant to naturalize easily, but as many of them ripen seeds in pods at or below ground level, crocus dispersal is going to go more slowly than in a species with explosive pods. Impatiens comes to mind for the latter. Ants might help with dispersal, but most gardens do not have healthy ant hills. Digging a few up in a patch that is crowded, and planting them where you want your great gorgeous swathes of flowers will help hasten the process. You could also look for seed and scatter it around, as an ant would do. Much easier to do with plants in pots than in the ground, but possible. Kathleen