Wall of Thorns (PHB, p.187):
'The spell does not enable plants to uproot themselves and move about, but it does allow them to move there branches, stems, and leaves.'
Personally, I would say that using the 'entangle' option listed in the spell, that the targets would be wrapped in thorny vines, suffering mild discomfort, but not actually taking damage unless the targets resisted the entanglement. That's a really good question though. There's alot of different ways you could take this one.