How to Clean Leeks: Leeks can trap a lot of dirt between their layers, so it's important to clean them thoroughly. To do this, trim off the root end and dark green tops, keeping only the white and light green parts. Slice the leeks into thin rounds. Place the sliced leeks in a large bowl of cold water and swish them around with your hands. Let them sit for a few minutes so the dirt can sink to the bottom. Drain and rinse once more. Pat dry before using.