At it's most specific, plant antlers are antlers that seem to be made out of plant matter. They may appear as branches, perhaps flowering in spring. Other instances may seem more like dried twisting vines, or saplings springing upward. These are not limited to deer, elk, or other cervines, of course. Other species may have similar growths: Please be aware that 'messy' antlers are not necessarily plant antlers. There should be some obvious suggestion of growth and life to them.
Other animals with horns, tusks or other growths that seems to be made out of growing plant matter may also be tagged with this, despite that fact that horns, antlers and tusks are all quite different.