Once again, we do not know who owned the plants. The OP never stated if those plants were brought in or owned by the current tenants living there or if they were leftover from previous tenants. If they were leftover and not given to the current tenants, then they belong to the property owner.
This one incident does not automatically make this LL the worst LL ever and horrible. It may be an isolated incident. If you make one mistake and did not follow proper procedure, which I am sure you have in your lifetime, does that make you a horrible and bad person/employee/etc?
We don't know what kind of rental units these are. We don't know what the lease states. We don't even know if the tenants actually owned the plants in question.