Hi everyone,
I like REO's approach, however, that is too nice for most of my tenants -- they respond better to the professional 2 x 4, with a velvet glove, of course

I use PropertyManager's approach. It's clearly spelled out in my lease how the yard is to be maintained (including irrigation) and that they will be billed accordingly if I need to hire someone to take care of their responsibilities.
Today I'm sending a certified letter reminding a tenant of exactly this problem. My letter includes the paragraph in their lease that spells out their obligation and the penalties. And, photos of how I expect the property to look on the exterior. This way they know exactly what to do to get me off their back. You'd be surprised at what a tenant thinks "looks maintained," vs. my standards (which are high, but reasonable because I always have easy-to-maintain landscaping in place). This is my property and my investment they are occupying.
When I finish a rehab, I take photos of everything. This has served me well on more than one occasion.
Take care all!