Shared backyard space with duplex tenants

[Joseph Jovel]

We just placed 2 tenants in our duplex within the last 3 months and there have been some issues with the shared backyard space.  Tenant 1 is in construction and will pile up a lot of wood and other materials along their side of the fence. Tenant 2 has been growing concerned about this not only because has it been unsightly, but shes worried about pests, bugs, etc. being attracted by the piles of stuff. Pictures attached. 

Where Im not sure about things is how much say does she have in what they do with their side of the yard? As the homeowners and landlords, we obviously do not want junk piled up in the yard, but what is the best way to approach this, and how much say do we have about how they use their space? Traditionally, our other properties are single-family homes and have never had to deal with shared spaces, so this whole shared space thing is a new one for us.

Any advice or suggestions is much appreciated

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