The Roboform Christmas trees were used by the Roboform to wreak havoc on Earth during the two alien invasions that took place on Christmas in 2006 and 2007.
These remote-controlled trees could spin at extremely high speeds and rip apart anything in their path with its razor-sharp leaves. They could also play songs such as "Jingle Bells". One of them nearly destroyed Jackie Tyler's flat shortly before the Sycorax invasion of Christmas 2006. It was neutralised by the newly regenerated Tenth Doctor. (TV: The Christmas Invasion)
Another feature the trees had was to send out remote-controlled bombs disguised as baubles. These baubles were used by the Tenth Doctor to flood the web containing the Racnoss children with the River Thames. Like many other robots, they could be stopped using a sonic screwdriver. (TV: The Runaway Bride)
A tree was later used by a Changeling under the influence of the Roboforms. (WC: Monster File: Christmas)