In an area of lower flow, they can attach to sand well enough to stay put. Keep in mind that usually they are still attached to the greater mat of zoas as well that is still on the rock or plug, which also helps keep them from being blown away. I've had them grow off of rocks and onto the sand, which does make them very easy to frag. They never seemed to grow too far off into the sand for me, but perhaps I just frag those before they spread very far.