All creative decorators take inspiration where they can find it, and your garden shed may be the source of your greatest artistic triumph -- centerpiece-wise anyway. Those ceramic or clay pots, artfully mossy and filled with fresh daisies, could strike just the right note of quirkiness or eco-friendliness. Include a ceramic frog, a bird feather or flowering tree branches and you have an introduction to spring (and new beginnings) that's a showstopper.
Want something more formal? Try constructing a fresh or artificial topiary using ivy and a simple metal frame. This approach is more whimsical than elegant, but your wedding should reflect your sensibilities, and if you spend more time in the garden than you do in the nail salon, a refreshingly earthy centerpiece may be just what you're looking for.