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The presence of the wreaths is logical, I guess--many people keep them up well into the New Year, and even the heartiest of the "live" wreaths has to dry up sometime.
I've always wondered about those trees, though: Do they sit in people's yards and gangways for days before someone summons the energy and will to drag them to the trash, or do people really keep formerly living trees up in their living rooms this long?
Granted, I saw a fully decorated white Christmas tree peeking out of the second-story window of an apartment building just last week, but that tree was clearly artificial. It may have been up well past the reasonable time to take it down, but it posed no potential hazard beyond annoyance. I've been there--once left my artificial three-footer up until around Valentine's Day.
But leaving up a real tree for that long? That's just asking for disaster.