This type of tropical shade allows almost any plant to be happy. For this reason, growing grass to near the trunk is a big waste of an excellent environment. Much better to have a large bed of fancy tropicals under your Royal. Young , it tends to grow as wide as tall causing an appearance that it will invade the air space of driveways, roofs and garages. However, as it matures, it picks up its drooping branches to create a high umbrella.
Some careful light pruning during maturation can help make a better formed tree.
Take most of the branches that grow beneath the main crown. These lower branches will just add more shade and are not needed. If your tree is a little thin for branches, skip this pruning. Royal Poinciana is tolerant of some salt drift, not too fussy as to soil and has no pests. Light fertilization is recommended.