http://www.colostate.edu/Depts/CoopExt/4DMG/Garden/beware.htmA good example is vitamin B1 (thiamine), often sold to "prevent transplant shock" and "stimulate new root growth" when planting trees, shrubs, roses and other plants. A study in the 1930's provided the basis for such claims. Pea roots cut off from the plant were placed in a culture medium in the laboratory.
The researchers knew that thiamine was normally found in roots, so they put thiamine in the culture medium and found that root growth did occur. Vitamin B1 is manufactured in plant leaves and sent to the roots, but if roots are cut off and placed in a petri plate, vitamin B1 stimulates growth of the roots when it saturates the culture medium
The bottom line: While root stimulator products are not necessary for transplant success, if you do use one, make sure it contains a rooting hormone and fertilizer rather than just vitamin B1. The vitamin B1 is for marketing purposes rather than actual effect.