The lyric from “Deck the Halls” is “Deck the halls with boughs of holly,” so the stored answer is holly.
The song’s familiar English words are paired with a much older Welsh melody, “Nos Galan,” whose title refers to New Year’s Eve. The tune was published in the 18th century, while the English lyrics appeared in the 19th century. “Deck the Halls” is therefore an adaptation rather than a wholly new Christmas composition.
Holly is an evergreen plant traditionally associated with winter celebrations. Its green leaves and red berries made it a natural decoration during the darker months, and the lyric specifically evokes using holly branches to decorate a hall. Cedar, laurel, and ivy are also plants used in greenery or wreaths, but they are not the canonical word in this line.
The song’s repeated “fa-la-la” refrain also reflects its folk and seasonal character. Although it is now strongly identified with Christmas, its Welsh predecessor was connected to the turn of the year.