Here's a site which covers the basics:
Landlord/Tenant Rights: Encyclopedia of Everyday Law
You could also ask a lawyer. There's an online site for that too.
You might want to consider the cold snap in January which took out a lot of plant life.
If tenant was paying for water perhaps they couldn't afford to water the back yard and if it was not specifically mentioned in the rent that they do so you may not have any recourse. If there's damage to the yard from dogs digging or something, then you may have a case.
Hope that helps.