Legally, you can't make the tenant do anything to the garden, including mow the lawn, if they don't want to. You can put clauses in the AST but, to be honest, they are pretty meaningless - there's no real way to enforce them (you won't get an eviction based on someone not cutting a lawn). So long as the property, including the garden, are returned in the original state, less any fair wear and tear, at the end of the tenancy then the tenant has the right to keep the property as they wish in the meantime.
If it really concerns you, then employ a gardener once a month and stipulate in the AST that the gardener will require access on a given day each month. Of course, even if the tenant agrees to this initially they can still, legally, deny access to the gardener if they wish. The tenant's right to 'quiet enjoyment' will override anything you write, and they sign, in the AST.
At the end of the day, so long as the property comes back to you in an acceptable state, I wouldn't worry about what happens in the meantime too much. The tenant is allowed to live as they wish in their own home while they rent from you. Take photos of the garden (and the rest of the property) and include copies of these with the inventory so you have a record of what the garden etc look like at the start of the tenancy.