Steph, I really don't know why that particular Miracle-Gro product cannot be used on indoor containers or house plants. Send Miracle-Gro an E-mail and ask.  They do show another slow release fertilizer called "Miracle-Gro® Shake 'n Feed® Continuous Release Plant Food for Containers" which can be used both indoors and outdoors.  However, I noticed that it DOES NOT contain any trace minerals, so you would have to add some STEM or other similar product, in addition to the 19-6-12 fertilizer.  I have never been much of a fan for any of the Miracle-Grow products.  I would much rather use Osmocote brand slow release fertilizers.  Osmocote is a very high quality professional slow release fertilizer. Osmocote should be found most anywhere. Whatever you use be sure that you also supply trace minerals to your tree. Take care. - Millet