Tenant Mowing the Lawn March - November Only

Hi Everyone! 

Hope everyone is doing well!

I recently had a tenant moved in (July 1st) in the lease it states that I ( landlord) will be taking care of the lawn. But a week later i've realised i could drop the $30 off the monthly rent and let him mow it. Provided i will add an addendum stating the change to the lease. 

I want him to mow only March thru November as in winter months there is really no need to move. Does anyone else do that?

I have a lawn mower in the shied, i would just have to prove the key to it. Liability? if he breaks the mower or injures himself?

Any insight would be greatly appreciated!