Lawn Maintenance Liability Situation

[Autumn Brooke]

I have three separate tenants, each renting room + common areas (2 of the tenants are friends) in a single family home. One tenant has an injured knee and is due to have surgery In July. I'd like to make tenants responsible for lawn/yard maintenance and provide a self propelled push mower. I have .23 acres. 

Is this a potential liability with one of the tenants having an injured knee? Not sure how to go about the situation.