Mirrors, made to amplify space and reflect light, come to the rescue of outdoor gardens that are cramped or have dark spots. The trick lies in knowing which mirrors to use and where to place them.
Appraise your garden to determine where more light is needed. Problems often occur along walls and fences and next to the house, where thick shrubbery or ivy can seem impenetrable, and under trees, where deep shade subdues everything
The rule of thumb is that if a window or opening of any size might logically be there, it's OK to use a mirror. Don't plan to use an expensive hand-carved mirror, though. You'll want one with a metal frame that can weather gracefully or perhaps one in a rustic wooden frame. Or select a new acrylic mirror; these are lightweight and less breakable than glass. Thrift shops or salvage stores often sell inexpensive framed glass mirrors that work very well for most purposes.
Outdoor mirrors, unless they are large, look more stylish in pairs or even in threes. Hang them fairly close together at the same height, tilted slightly upward, and they will reflect the sky to look like windows.
When weartherproofing your mirrors, if the frame will rust, remember to spray the back surface with polyurethane to avoid rust stains on nearby surfaces. You'll usually need no special treatment for mirrored glass; a weathered look is attractive. If you like, though, you can seal the back by spraying it with several layers of exterior polyurethane. Use clear silicon to seal the grooves between the glass and the frame---on both front and back---for an even more weatherproof treatment, especially if you live in an area with severe winters.
Walls and fences lend themselves well to the mirror treatment because they provide plenty of hanging space. If your hedge or a cover of ivy is overgrown, trim it back enough in spots to provide glimpses of the surface behind it. Hang the mirror, using a rustproof nail or hanger. Keep shrubbery or vines trimmed to reveal the mirror.
Aesthetics, as much as lack of light, determines where you would want to hang a mirror on a house. Next to the back door or on a large blank wall are popular spots. Hang the mirror as you would hang it on a fence.
A dark area under trees or in a corner presents a problem because there is no place to hang a mirror. A canopy of tree limbs provides an intimate setting, so consider placing a couple of chairs and a small table, complete with mirrored candleholder, beneath it. Tuck a sparkly mirrored lawn ornament into a dark corner to liven it up.
Replace the glass panes in an old window frame with mirror. Hang a mirror behind a flowering plant to create the illusion of more blossoms. Or do as people did in the 18th century: Use outdoor lanterns with mirrored backs to reflect the light and double its effect.