Forget metal or plastic edging; grass always finds a way to get over or under it. The English method is to edge by creating a gutter around beds.
Use a flat spade. Cut straight down about six inches or more. Use either stakes and string or a board for a straight line. Use a hose for a rounded contour.
Remove the grass and soil from the new gutter. It will keep the grass in check: it can't grow off a cliff.
Periodically clean the gutters to remove seedlings or groundcovers that have crept in. Repurpose these plants if you need to. Thyme and sedums are easily transplanted.
This edging is free and almost foolproof if you maintain it. It creates a pleasant "picture frame" around borders or beds.
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