I don't know what happens about renewals in the US. In Australia, a lease usually automatically reverts to a periodic lease at the term, unless stated otherwise in the contract. So we don't need to notify the tenant unless we are going to raise the rent- in that case, there is a notice period required.
If you do send a letter, making sure that it is in accordance with the lease, make it so that an action is required if they want to change.
"The current lease will end in two months, and will revert to a month by month agreement, as specified in the lease. If you do not wish to renew, please notify me in writing at least x weeks before the end of the lease"