Tenacious gardeners won't let a few heavily shaded garden spots prevent them from populating their plot with sun-loving plants. A few well-placed mirrors can bring light to darkened corners of a garden. And plants grow just as well under reflected light as they do under direct sunlight. By increasing the sunlight in your garden in this manner, you can expand your planting possibilities.
Apply waterproof silicone caulk sealant to the edge of the mirror(s) where it meets its frame. This will prevent water from seeping into the mirror, which may cause the reflective backing to peel off.
Find the areas of your garden that receive the most direct sunlight in the morning (it is better to reflect mild morning light than harsh afternoon light). Those sunlit places closest to shady areas of your garden are ideal spots for your mirrors.
Place the mirror in the sunlit area (in the morning) and angle it so that it reflects a wide band of diffuse light (concentrated beams may burn the plant) over the plants that need it. Once the mirror is in place, support it by affixing it to an existing structure (a fence or even a boulder will do) in your garden with epoxy or non-rusting wire. Or introduce a functional or decorative support (something as simple as a metal pole or something as decorative as a bird bath) and affix the mirror to that.