They are still in common areas though... Who is responsible for maintaining the common areas? Usually it's the LL. I'm just picturing a hodgepodge of pots with weeds, dead plants and some new growth. If you don't want things to be thrown out then don't leave them out in public.
I still imagine them as rare plants and priceless pots. But still the landlord should have put them in storage for a while to see if some one claimed them. This action just does not lead to good landlord tenant relationships. It looks like some turn over will be in the future for this landlord due to such unprofessional behavior.
I can't think of much worse things a menacing tenant can do then have potted plants in the yard. This must stop! What can be next! Maybe the tenant will want to plant flowers next ! The horror, the horror!