Mop Top Robinia Trees (Robina psudoacacia inermis) are a popular and hardy tree used widely in landscaping. The original 'ball on a stick' tree these plants are grafted to create the popular look with a compact dense head of foliage on a single trunk.
The head will spread to around 3 -4 m depending on conditions. Often uses as street trees they can also be grown in pots or containers. Consider using a root barrier if planting near buildings, paths or infrastructure.