DE might be a good thing to try (Diatomaceaous Earth - finely ground
microorganism fossils), though you'll want to read up on whether it's
effective for outdoor use. It's not a poison, the fine crystaline
edges of the powder actually cut the ants' exoskeleton as they walk
through it and they basically "bleed" to death. It's nontoxic to
humans and pets and usually is found in garden centers.