Dr. T's Snake-A-Way Snake Repelling Granules
These granules contain naphthalene (7%) and sulfur (28%), they are designed to repel both both poisonous and non-poisonous snakes. The manufacturers tests found that 91% of garter snakes and rattlesnakes were repelled effectively. You should spread the granules evenly across the borders where you wish to stop snakes. Re-apply the granules every couple of weeks and especially if you experienced heavy rain.