Assuming that you know for a fact that the plants are marijuana -- and there have been more than a few cases in which landlords confused a tenant's tomato plants with marijuana plants -- you need to act carefully. It would be sensible to consult a lawyer in the state in which this is occurring, ASAP, to discuss your options for responding to the tenant's illegal activity while avoiding the possibility that the state might attempt to seek forfeiture against your property. Also, people running grow-ops often to do other illegal things, like stealing electricity from neighbors, and also have a history of trashing homes, and any delay in acting may affect your effort to obtain insurance coverage for any damage they have caused.