|What is the best way to prune a potato bush tree to get a nice dense round ball?|
|Solanum Rantonnetii 'Grandiflorum', the Blue Potato Bush, is probably what you're growing. It's sometimes classified as Lycianthes rantonnei, or Paraguay Nightshade. It's generally grown as a shrub but can be staked into a tree form. It's evergreen in mild winters, deciduous in colder areas. The bright green oval leaves are about 4" long and the plants is covered with violet blue, yellow-centered, one inch wide flowers throughout the warm months.
You can help the plant keep its shape by pruning lightly in the winter or early spring to encourage new flowering wood. Simply cut each twig or shoot back about one-third.