Indolebutyric acid (IBA) is a natural chemical that stimulates plant roots to grow on cuttings. In fact, Jeff Schalau of the Arizona Cooperative Extension says that IBA is excellent for propagating plants and "has had some success in root regeneration in transplanted trees and some evidence indicates that it may redirect resources to the roots by suppressing crown growth." Commercial root stimulators contain a synthetic version of IBA. Willow wood is also a source of IBA, which you'll need to make a homemade starter solution for your cuttings.
Harvest two cups of willow branches and remove all leaves. Willow wood of any species contains IBA.
Cut the limbs into 3-inch sections.
Boil ½ gallon of water.
Put your willow wood pieces in a large metal bowl or large glass container. It needs to accommodate all the twigs and ½ gallon of water. Add the boiling water and allow the wood to soak overnight.
Strain the solution through a sieve or colander. Discard the solids.
Collect the plant cuttings you'd like to root. Put one of their ends in the willow water. When they develop roots, discard the starter solution they were in.