The hand tree is an imaginary creature that appears in Uma's dreams.
The hand tree only appears in the "Uma Dreams!" episode. When Uma starts dreaming, she sees the tree in the dark and is stricken with fear. It moves slowly and freaks her out. Later, Uma finds a magical chain that turns her nightmare into a brightly-lit sunrise scene. The hand tree turns out to be well-meaning and almost sentient, and it dances around with Uma.
It is a creepy, moving plant made from a bunch of sprawled-out hands. Its trunk is made from a pair of arms. The fingers are all bent to give it a scary look. When Uma turns the lights on, yellowish leaves can be seen on the hands.