The underlying problem, if not excess shade, is root competition. The tree was there first and its needs come first. Its roots have most likely explored every cubic inch of soil. To establish any new plants in that root zone, you will need to manage them as if they're in intensive care. Perhaps set up a drip system for each newbie.

As Babka mentioned, you can use pots and bury them into the soil. But you have to remember to actually turn the pot a couple of times during the season to break the roots that are entering via the drainage holes. I currently have a few pots that are buried. Along with turning the pot, you would need to water more frequently. You may be interested in reading my post. It's about the Fragrant Blue hosta pot that I had buried and never turned for a couple of years. http://forums.gardenweb.com/discussions/3918002/im-a-bad-hosta-momma-fragrant-blue?n=20

Reversion is where the plant's color/growth characteristics revert back to parentage material and new growth will not have any color or not exhibit the color qualities of the clone you purchased. Some plants when young may do this but later show better variegation and color while others the reversion needs to be cut out or the whole plant can end up without variegation. Variegated specimens overall have more direct sun sensitivity than solid color specimens. Even a variety that I think will handle full sun I will keep protected from midday on in the first year if the plant was delivered to you with leaves on. Once it goes through a winter and has a chance to emerge in the spring in the more sunnier environment it will tolerate the sun better. I think you would still be best protecting your Radiant from direct afternoon sun. If it gets good morning sun it should still color up well. Up potting from a one gallon to a 15 gallon is a big step and the plant may do better in increments. You could up pot to a 3 gallon pot and keep the 3 inside the bigger 15 and later up pot to a 5 or 7 gallon and then maybe to a 15. Just suggestions that I have found work for me.

Tom, I have always enjoyed manual labor and getting dirty. :) Very good point Embothrium. My whole yard is made from "fill sand". It is straight sand with no soil or organics. Looking at the local topography it's probably at least 15-20' deep. Under that may be some clay and organics as the Anclote River is near by. The sand most likely was over burden from phosphate mining here FL but could have just as easily come from sand mining. So I assume the sand is going to easily drain. The soil I use to replace the sand is also fast draining but of course contains organic material and probably packs looser than the sand. You are absolutely right that I am forcing some larger plants to deal with different types of soil as their root systems grow. My experience is most plants don't do well starting out in sand, so I let them start in better soil and make the move to sand after they are larger. Obviously not the best solution but a compromise that works most of the time. If you have any suggestions of a better way, I would appreciate hearing them. Cause digging out wet heavy sand is hard work. :) Anna