When does the ragweed allergy season start and end in california?

Don't worry about it. I don't know if any body has taken it over the rockies. But it is not supposed to be in california. If some body did take it over then it should still bloom around august 15th .In areas of no frost in southern california, it would behave like in southern florida where it blooms again in the end of april.
Western Rageeed. Western ragweed is found throughout much of california. Ragweed season begins most usually in july or august. The further north the earlier.