Whether it's visible or not, vegetables usually have dirt, dust and other residue on them. Here's an easy and thorough way to wash all kinds of leafy greens:

Grab a bowl large enough to easily hold the greens and fill it with lukewarm water. Submerge the greens in the water and shake them back and forth to dislodge or dissolve the dirt. Then just lift the greens out of the water so the dirt stays behind. Pour out the water and repeat with clean water until the water stays clear when you pull out the greens.