Many greens can withstand being cut with a knife. However, cut greens, if prepped too far in advance, are prone to browning along the edges as the knife cuts straight through the cell structure, exposing the outer edges to oxidization. For this reason, it is best to gently tear delicate greens by hand. When tearing, be sure to not squeeze or crush the greens, as this too can damage the cell structure and cause browning.