Miracle gro potting mix is ok, but it would be better if you amended it with some of the coarser orchid mix. But that's an expensive plan for 7 trees. Alternatively, add a little more perlite (miracle gro will be almost all peat) or some composted pine bark. (small pieces). Composted bark can usually be found as "soil conditioner" at big box stores. But numerous products should be available in houston where you have native pine trees.

Even easier might be to find a nice nursery or nursery supply store that sells bark based container mixes. Pro-Mix makes a bark mix.

I also like to use coir to lighten the mix. That might not be that easy to find either.

Willis

Try looking in a pet shop for coir. Frequently you'll find it in shops that carry a lot of reptile supplies. It's often sold as bedding in a 'brick' that you add water to so it expands. Try Petsmart or Petco.