If you're growing something outside for the summer, like vegetables or flowering annuals, you definitely should add fertilizer during the season, starting about 4-6 weeks after you plant. The amount of fertilizer in Miracle Gro and other standard potting mixes is very small. In outside containers, water and rain will wash the fertilizer out of the mix. It might be an adequate amount for slow growing house plants indoors, but needs to be supplemented for fast growing plants outdoors.

We use Osmocote or generic equivalent in our planters at time of make-up. But then, throughout the growing season, we augment with water-soluble in our water tank, to boost nutrient levels and to help with our alkaline water. So, I select one or the other of either "Acid Special" which is a very strong acid-forming fertilizer, or the somewhat more mellow "Alkaline Water" types of water-soluble fertilizers. So yes, this can be done.

+oM