I have to admit, I didn't think this project up myself. I borrowed it from another art teacher - That Artist Woman. She has some interesting art projects on her site.
Although I got the idea from her, I changed things up a little to save time. Instead of dying the yarn, I bought thick yarn that was already green (various shades of green).
I also read them the book "Weaving A Rainbow" by George Ella Lyon to introduce the concept of weaving.
My 4th grade students loved it and their trees turned out fantastic.