Even in northern California, the recommendations are to purchase the essential oils of whichever plant(s) you want to use: eucalyptus, pennyroyal, etc., and mix/dilute them with whichever recipe you prefer for flea control.
For years and years I resisted the chemical spot-ons for my dogs, trying to keep them on a "least-toxic" regimen. I've always flea-combed my cats, so no problem there.
But my 5-year-old boy dog is so severely allergic that if even ONE flea bites him, he starts itching absolutely everywhere. He's been miserable for months--fleas aren't his only allergy trigger, and despite three years of carefully testing several potential triggers, we haven't removed everything possible from his life.
He is on spot-on flea control, and so is my 9-year-old girl dog. Cats are still flea combed.
We did get a hard freeze in December, but we needed one again in February, and it didn't happen.
Bad flea season starting up.
Sunset Zone 17, USDA Zone 9