Real Christmas trees are lovely but certain species like fir and pine have oils that are mildly toxic to cats if they eat or lick it. Also, pine needles are sharp and can be harmful to feline tummies if eaten.
Mostly, it’s the decoration that’s an issue. Tinsel is terribly tempting but when it’s chewed and swallowed, it’s extremely dangerous. This is why animal charities recommend avoiding tinsel altogether.
Fairly lights are pretty but be careful of kitties, rabbits and other pets that like to chew wires. Keep the lights unplugged when you’re out of the room.
Finally, don’t hang chocolate rings, soldiers or coins as chocolate is toxic to cats, even in relatively small doses.
