There are a few things that you shouldn't use Miracle Grow on. One is tomatoes. Tomatoes are very sensitive to the chemicals in Miracle Grow and can be seriously harmed by them. Another is any plant that is already stressed or weak. The chemicals in Miracle Grow can harm these plants and make them even weaker. Finally, don't use Miracle Grow on seedlings that are less than a month old. The chemicals can damage the delicate roots of these seedlings and cause them to die.