to start, give the tenant the option of mowing themselves or paying toward your gardener. For example, rent 975/month if they mow weekly, 1000 if they don't, build in a fine of $100 if the lawn is overgrown (higher than x inches).
with the current tenant, they get a cure or quit notice to allow mowing or be kicked out.
have the gardener work with the tenant on a good time when the tenant will be out...with the cars.
YES, you have a leg to stand on even if the lease doesn't specifically say they have to move the cars. generally a lease requires a property to be maintained to a minimally decent degree....trashing the yard/not allowing the lawn to be mowed violates that.
you can't change the lease until it expires unless they agree....if you offer to make it their responsibility mid lease, you could need offer a rent reduction.
have they been given legal notice the gardener will be coming? it could be standing notice such as gardener will come every monday, if the weather is bad, it may be tuesday for example.