Landscaping elements give you a great opportunity to use salvaged or recycled materials. Lumber made of recycled plastic or recycled composite lumber makes very durable landscape edging. Broken concrete can be used to create attractive paths or retaining walls, as can tumbled glass.

Use slavaged or recycled-content for hardscapes (planter beds, patios, edging, walls, walk-ways and driveways). You can also use FSC-certified (Forest Stewardship Council) sustainably harvested wood, though this can cost a bit more.
 
For landscaping and hardscaping, composites and recycled plastics are generally more durable than wood since they don't rot, crack, splinter, or require wood treatments like staining.
 
The Forest Stewardship Council website has many resources on green building suppliers and the FSC-certified program:
Visit them at: www.fsc.org