Legal Question in Real Estate Law in California

First, be careful to distinguish between legal activity (e.g., medical use) and illegal activity; and between use of drugs and dealing drugs. The consequences and remedies available to a landlord differ substantially.

Next, you probably need more than another person's suspicion. You'd also need something linking the "growing and selling" activity to the property.

I'd suggest three things: (1) buy and review one or more of the paperback self-help books on landlording in California -- they have fairly detailed treatment of how to handle drug-dealing tenants; (2) gather some more evidence, perhaps by personal observation or double-checking what the manager has seen; and (3) discuss the issue with the local police or sheriff after making an appointment to come in and see the right officer.

If you do have illegal drug activity (growing, selling) on the property, you need to evict the tenants, but before doing so, you need to be pretty sure the activity is illegal and that it is taking place on the rented property.