Is it acceptable to use Green Light root stimulator on a tricolored beech in early spring or at some other time?

Don't be in too big a hurry to feed your tree, if you prepared the soil properly you need to give it time to settle in and put out new roots. After all you do not give a patient in intensive care a 3 course dinner. A light feed the following Spring after planting will suffice. Takeing care of watering is more important.