To get rid of a flea infestation, you must do more than treat your pet. Fleas may bite your cat or dog, but they live and breed in the area your dog frequents most. If your pet spends a lot of its time in your yard, this means that you will have to treat it to get rid of the problem for good. But that also means that you'll want to get rid of them safely to ensure the health of your pet.
Clear away any unnecessary debris or structures in your lawn. Pull up any tall weeds and other unnecessary plants and cut the grass short. Fleas thrive in the cool, shady areas of your lawn. They cannot thrive in open areas of lawn with access to direct sunlight.
Sprinkle food grade (not pool grade) diatomaceous earth (DE) over the areas that your pet frequents. This includes your pet's house, popular routes, napping and feeding areas, favorite spots to use, etc. DE is made of fossilized diatoms and is perfectly safe to use - you can even rub it into your pet's fur. Re-treat the areas after it rains and until all signs of the fleas are gone.
Introduce predatory nematodes to the soil in your yard and garden. Predatory nematodes are commercially available in freeze dried form. Mix them with water according to the label's instructions and then spray them onto your yard. Make sure that the species of nematode that you purchase is compatible with the climate in your area and has flea eggs listed as one of its food sources. Continue to keep the soil fairly moist with bi-weekly watering. Nematodes need moist soil to survive.