Constructing some simple raised beds in your garden is a great way to increase the versatility and scope of your garden, allowing you to easily control soil composition and drainage to grow plants and vegetables practically year round. However, building raised beds comes with the same environmental concerns as any other construction job, and particularly green-thumbed gardeners may wish to construct their beds from recycled or otherwise environmentally friendly materials. Happily, a wide array of green bed-making materials are available from landscaping suppliers.
These extremely simple beds are little more than a boundary of heavy, closely-arranged rocks around your raised bed, and are about as close to nature as you're likely to get. Despite their distinctly rough and ready appearance, rock beds can be remarkably sturdy, and shallow beds don't even need to be mortared in place to create a sturdy soil base. They also provide excellent drainage due to their irregular walls, and can be assembled on very short notice.
However, while simple rocks might seem to be the greenest building material around, take care where you source them from. Rocks taken from your own land or legally taken from the wild are fine, but quarried rocks carry a lot of embodied energy (not to mention a hefty price tag).
Traditional wooden raised beds are made from cedar, which is valued for its durability and natural resistance to rot. However, fresh-cut cedar is both expensive and highly damaging to the environment, especially if taken from old growth trees. For a greener alternative, look into using reclaimed timber, which can be just as durable and attractive without the hefty carbon (and dollar) price to pay. If you have your heart set on the clean look of fresh timber, choose timber from renewable plantation-grown trees. Timber beds can be constructed in a near-infinite variety of shapes and styles and are particularly useful for retaining soil moisture in hot weather.
Unfortunately, reclaimed timber has similar problems with sourcing to rocks, and where you find your timber can drastically affect how it performs. Reclaimed railway sleepers, for instance, can be a tempting choice, but the chemicals used to treat them during the manufacturing process can leech into your soil and drastically alter its chemistry. Try to source untreated timber that you can treat yourself using safer chemicals, and consider inserting a waterproof liner into your beds to protect against chemical leeching.
Choosing bricks for your beds can give your garden a very traditional, turn-of-the-century feel, and when recycled bricks are used, they can also be a boon to the environment. Constructing brick raised beds is almost as simple as constructing rock beds, but bricks can be used to create much taller and robust beds, suitable for growing larger plants such as shrubs and root vegetables. They are also immensely durable, and retain moisture extremely well in hot weather.
However, constructing a brick raised bed is a one way street, as the bricks will need to be mortared together and firmly planted on soil or substrate to provide structural strength. As such, brick raised beds cannot be disassembled and moved like rock or timber beds, so careful placement is a must.