By incorporating organic matter like humus, compost or peat into native soil, good things start to happen. Organic matter helps loosen the soil for better water penetration and aeration leading to good root development. That more than likely was due to no soil preparation. Remember that soil improvement is a continual process. Organic matter is equally important in cases of more sandy soil.
Sand is made up of the largest particles allowing for quick drainage and leaching of nutrients.
By adding organic matter, the water holding capacity improves, and the nutrients needed by plants can be retained. Plant the Panola at the same depth it is growing in the container. Planting too deeply will most likely prove fatal. Temperatures may still be real warm when planting, so keep them watered and apply a layer of protective mulch. Deadhead to keep the flowers coming.