Lawn Maintenance tenant's responsibility

[Krista Manna]

So in the lease it is my tenants' responsibility to maintain the lawn/yard. Neither of the tenants in the building have done so. The lawn is now a foot high. I have even provided a reel mower for them to use but they needed to use it before the grass grew a foot tall.  It will not work very well when the grass is that high.  How would you handle letting them know that the grass that high is not acceptable.  I don't want to seem like a jerk but it's gotten to an unacceptable level.  How would broach the subject.