Flexible and strong, willow edging is a traditional English garden border whose form is naturally in line with its function. The willow branches have even been known to take root and sprout, creating a living, organic boundary.
Martha Stewart Living, March 1999
Buy willow sticks at a local landscape supplier (they're usually sold in bundles about 7 feet high and a hundred sticks to a bundle), or cut your own. Start from one end of the area to be edged; insert branches of equal length into the ground 4 inches apart, leaning sideways at 45-degree angles. Bury enough of the branch in the ground so that they'll be secured, 1 to 2 feet.
Repeat from the other end, with the willows angled the opposite way. Weave the branches together so that they hold one another in place. Tie the branches with waxed or linen twine where they intersect at the top.