The tradition of kissing under the mistletoe goes back to the same Roman festival as holly, Saturnalia.In those days it was said that kissing under a sprig of mistletoe aided fertility.
A lot of northern European cultures considered mistletoe to have magical properties and the collection of mistletoe by long bearded Druids is still a popularly portrayed image.
Well, those are the three most popular greeneries at Christmas (not counting the big tree of course). What other herbs (plants) are synonymous with the festivities in December? Feel free to send many more.