Legal Question in Landlord & Tenant Law in California

If the addition is made when the contractual agreement is entered into, sure the landlord can exclude activity which is legal. Many landlords do not allow water beds, structures or play items that would increase their liability insurance, smoking, etc. There are sound reasons for an owner not wanting pot to be grown -- increased electrical and water bills, likelihood of others breaking in to steal the plants, personal dislike of drugs, etc.

Mr. Shers is right regarding adding it into a rental agreement before it is signed. I would add that since it is still a federal crime, and the landlord could be subject to forfeiture of the property for knowingly permitting it to be used for that purpose, if an existing tenant is growing pot, they should be given a 3-day eviction notice for breach of the lease terms prohibiting illegal activity. You do have a lease that prohibits illegal activity, right?

If you want to add a pot-specific clause to an existing tenant's contract, or if heaven forbid you have to add a "no illegal activity" clause, you still can do that with a month-to-month lease by giving 30-days notice of the change. If the tenant remains in the premises after 30 days, the change is legally binding.