Are you certain? Do you really know what was in that Miracle Gro?
But the real point was that growing superior pot, or whatever other plant you might choose to grow, is unlikely to be any more difficult, than growing most any other superior plant, or raising superior animals for food. With commercially grown stuff, including that grown by family farmers, much of the "difficulty" is out of sight to the layman, and sometimes even to the grower, if he purchases commercially produced supplies. The same would be the case if pot were legal, there'd be whole above board support industry, for those who chose to grown their own, and a more or less sophisticated horticulture industry to grow it for those who don't.
The legal market might come up with an easier way to prevent fertilization of female plants than what the black market has ... but that would still have to be done, which makes growing good marijuana still significantly more difficult than simply sticking plants in the ground.