Grown for its purplish-blue, rounded blooms, the "Royal Robe" potato bush (Solanum rantonnetii "Royal Robe") adds color from spring through late summer or fall. Individual blooms reach 1 inch in diameter and sport bright yellow centers. This large evergreen shrub or small tree grows in U.S. Department of Agriculture plant hardiness zones 9 through 11 where it requires full sun and fast-draining soils. Light pruning in late winter, when the plant is dormant, and heavy shaping in the summer after blooming will ensure that the "Royal Robe" potato bush retains a healthy, pleasing shape.
Pour 1 part bleach and 4 parts water into a bowl. Submerge the blades of the pruning shears and loppers in the solution and leave them for five minutes. Blot the blades dry with a cloth.
Cut out any dead branches as soon as you see them. Use pruning shears to cut through branches with a diameter of 3/4 inch or less and loppers for diameters 1 1/2 inches or less. Make each cut at a 20-degree angle, 1/4 inch above the swollen ring of tissue, known as the branch bark collar, surrounding the branch's base.
Remove broken and damaged limbs when you notice them. Make each cut 1/4 inch above an outward facing lateral branch, leaf, growth bud or limb base located at least 1 inch below the injury.
Cut back each limb in the canopy by one-third once the "Royal Robe" potato bush finishes blooming. Make each cut at a 20-degree angle, 1/4 inch above an outward facing lateral branch, leaf, bud or base.
Cut out any limbs that cross or rub against another in late winter or early spring while the plant remains dormant. Remove any vertical, weak shoots, known as water sprouts, from the bush's canopy. Cut off any limbs or shoots growing from the bottom one-third of the "Royal Robe" potato bush's trunk to train it into a tree shape, if desired. Pinch back any vigorous or overly long limbs, cutting each one back to 1/4 inch above the second set of leaves.