This is a great post, because it gets people talking and learning about how to grow a tree from scratch.

What type of tree are you attempting to root? Are you doing the air layering technique, or removing a cutting to root?

Trees that can be rooted from cuttings will often make roots easily if you put them in a vase with water, and keep them out of sunlight. Not a tree, but with ginger rhizomes, I often just rinse them ever week in clean water, and keep them in a plastic bag until they form buds. I believe darkness, clean water, humidity, and oxygen are all essential for healthy new roots. Rot and disease are main killers.

Honey is another odd one I have heard of. It's sticky and antibacterial, so that helps protect the cut end from disease and drying out.

Another favorite natural rooting hormone is willow. You can use willow cuttings purchased from a florist or look for an old willow tree to pluck a few stems off. Cut the stems up and let them sit in water for a day or two. I believe the theory is that because willow trees create tons of long feeder roots when sitting in water, it must have a chemical that can help other trees to have the same rooting ability to the water. Don't use dead stems, obviously. Green healthy ones.

I grow willow bonsai, and the pruned willow stems I usually save and add them to the water I use on my trees. Maybe it gives the roots a natural boost.