ageratum, you don't say where you looked up the information, but unless it's specifically geared to the Southeast, the information usually has to be adapted to our region which means more shade will be needed, and the size is underestimated. In other words, sounds like the saleslady was telling you the truth.

Writing from her experience in Chesapeake, Virginia, Pamela Harper says 'Mary Fleming' is well adapted to our heat. If the roots are pot-bound, Harper recommends washing them clean of the potting medium. She plants her Rhododendrons a little high in soil mixed with ground pine bark and adds a little leaf compost if the site is apt to become dry. She mulches with either pine straw or ground pine bark because the plants dislike hardwood mulch.

This is an old thread, but for what it's worth, when I worked in a Rhododendron production nursery just about ten miles north of Philadelphia the nursery owner had this on his property in the shade of a large maple or oak (forget which), and it was about 3' (high) X 4' (wide), and had the most gorgeous flowers in the spring! The more sun the flower received, the pinker it was and thus the more sun the plant received the more strongly one perceived the flowers to be peach, rather than yellow. In the winter its leaves were burgundy colored where the sun shone on them, but not so intensely as PJM's leaves are. Based upon my memory of that one plant, it can sprawl a little, but it's neither very "tall" nor particularly upright. It's general growth pattern is more horizontal than vertical.

In flower it's a lovely Rhodie (especially for the Northeast where yellow Rhododenrons are often impossible to grow)!!