My landlord has had the whole yard mowed for the past year and a half, out of no where we see the grass being cut only on their side. No words said to us, no responsibility agreement in lease,, can we be held responsible if they don’t warn us nor tell us of the recent change up 

I sold my duplexes, partially because i got tired of mowing them.  He should mow it. Especially if there are no garages.

I would reach out and ask.  Could be as simple as a miscommunication with a new tenant or new landscaper.  If lawncare isn't in your lease, I wouldn't expect a tenant to handle it.  

Better to ask now though than after the city or neighbors start complaining.

With my side to side duplexes the tenants handle it. With my up/downs, I hire it out.

@Jake Poronsky

If there is nothing in the lease and they have been taking care of it.  Give them a call. Something is odd.  I use to have the tenants cut the grass on my two units.  It just wasn’t worth it.  Grass to long, storing the mower (no garage) etc.  include the fee between the two tenants in the rent.  I pay $35 to cut the grass every two weeks.  

Speak with him first and advise it isn't in the lease contract. 

Agree, see what the lease wording is.  It should address who is responsible for mowing and landscaping.

In our area any assessments by the city for a junky yard are the responsibility of the owner to pay, so you could wait it out for awhile.

I would never expect the tenants to do lawncare for a duplex.

Start with a conversation.

@Jake Poronsky  How long have this been happening? I mean you just said that its almost 2 years that you got this setup. Perhaps the landlord just overlooked it at that time. If it is on the lease then you have to bring to his attention.