"Trees" is by Sergeant Joyce Kilmer, and the Whomping Willow Tree belongs J.K.Rowling. I don't know why she wants it, but it's hers.

I think that I shall never see
A thing as violent as the Whomping Willow Tree.

A tree that in a week, at least,
Kills enough animals to make a feast.

A trees that kills in a day,
Crows, wrens, larks, and jays.

A tree that will not consent to bear
A nest of robins in its hair.

Minions of beast were by this tree slain;
This tree kills, day or not, sun or rain!

Begonias are planted by people like me,
But only a fool would plant a Whomping Willow Tree.

And here's an extra line that I made up:

However, Dumbledore planted the tree, too,
Leaving us to think that his brains were made of goo.