Because holly remains green even in the depths of December, it has been credited with many mysterious powers.  In England, if prickly holly is brought into the house before the smooth-leafed kind, folklore holds that for the next year:

The prickly variety of holly is sometimes called "he" holly, and the smooth "she" holly.  Holly's other powers include warding off severe weather and subduing stubborn beasts.  In some places it is thought unlucky to bring holly into the house before Christmas Eve.