Grass is not a complementary look to any flower bed. To keep your flower bed free of grass invasion, create a trench along the border of your flower bed not only to keep grass out, but keep your flower bed looking sharp and clean. The only cost involved is a simple edging tool or spade, and some manual labor.
Remove all grass and weeds from your flower bed, making sure all the roots are removed. This is important to keep pesky grass from showing up again.
Create the shape you want to around your flower bed by using your garden hose to draw the outline. Complement your flower bed with curvy, kidney-shaped or straight lines--whichever you prefer. A garden hose is not necessary; you can do this freehand if you like.
Dig vertically 6 inches into the ground using a half-moon edging tool or a square spade, following the line of your hose. Take caution to avoid puncturing the hose.
Create angled cuts at 45 degrees from the inside of the flower bed to meet the vertical cut in the ground, creating a trench that should be about 4 inches wide.
Remove the excess soil from the trench and reinstate it back into the flower bed, provided it is free of any weeds or grass.
Recut your trench each year in the spring to maintain its clean edge. Trim the bordering grass by hand if possible.