Use mirrors that are weatherproof. You can use plastic mirrors available at most DIY super stores.
Secondly, if you have birds visit your garden a lot, be sure you know where the flight paths of the birds are that visit your garden. This is because mirrors could confuse the birds, so don’t put them in a place where birds could mistake it for more flying space!
While the reflecting property of mirrors can really help increase the amount of light that reaches certain areas of the garden, this can also be a problem. You don’t want mirrors to reflect direct sunlight into your home or onto your outside seating areas, so keep that in mind when you’re positioning them. Alternatively, angle the mirrors down to avoid it.