When is it generally too late to add root stimulator to growing trees during the year?

have planted a row of Tsuga Canadensis (canadian hemlocks) last spring and would like to ensure that they will grow up strong, have thrown some starter fertilizer 24-27-7? around the trunk today at the end of June in zone 6b, however I am afraid that if I pour some root stimulator now it will be too much.

anyone with sure knowledge when is the best time to do this and when is it too late?

also when do roots usually grow the most (I am thinking winter and early spring)

Thanks for your answers!

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