I don't use superthrive directly on the new roots. I use a root stimulator. Using superthrive every time you water is a great way to add vitamins and minerals. Use along with your normal fertilizer regimen.
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Cheryl
“Respect does not come from the work you do, it comes from the way you do your work.”
Cheryl, SuperThrive does contain a rooting hormone, as well.
If you're trying to stimulate root growth in a plant with none, I mix about a teaspoon in a gallon of tepid water, and soak the entire plant for a couple of hours.