For a couple years now there has been a newsworthy controversy over whether one of the most beloved symbols of Christmas should be called a Christmas tree. Of course it should. The tree is a symbol of Christmas, a time in the year that is supposed to be meaningful to a significant number of people. The Christmas tree should not be forced to represent a generic holiday.
Since other groups also enjoy a holiday around the end of the year, greetings during this time could be more generic. If we know the significant event being celebrated by the person we greet, then we could use the appropriate specific greeting. However, if as is more often the case we aren't sure who celebrates what, then a greeting of "happy holiday" should be acceptable.
If we need a plant to represent this time of year for everyone, I would suggest a single red rose. We could call it the "freedom flower". It would symbolize freedom from the ways we try to fill the void in our lives which are the cause of all conflict from deadly wars to the petty squabble over whether to call a Christmas tree a Christmas tree. While giving the flower we could greet each other with simply, "Freedom".