The Royal Robe potato bush ( Lycianthes rantonnetii "Royal Robe") is an evergreen plant that boasts colorful purple flowers throughout summer and early autumn. They are hardy in U.S. Department of Agriculture plant hardiness zones 12 to 15, but can be grown in a container and brought indoors in areas that experience freezing winter temperatures.
The Royal Robe potato bush has lush, deep green foliage and grows up to 8 feet tall and 8 feet wide. The sweet-smelling blooms have a shape similar to those of a morning glory. They are a deep shade of purple, and each flower has a yellow center. The blooms appear in summer and continue to open until early autumn. The flowers are followed by 1-inch long red, potato-like fruits. All parts of the plant, including the fruits, are very toxic.
A site that receives exposure to full sun most of the day is ideal for the best foliage color and flower production. The plants prefer loamy, well-drained soil and aren't particular about pH levels. The Royal Robe potato bush makes a good border plant and also does well planted next to a wall or fence. They also thrive in a container, though the plant will be smaller than if it were planted directly into the garden.
Maintenance and Care
The Royal Robe potato bush doesn't require a lot of attention to do well. Regular rainfall is often enough to keep the plant healthy, though supplemental moisture is necessary if the top inch of soil feels dry. A layer of mulch under the plants prevents the soil from drying out as quickly, and also eliminates the need for regular weeding. A monthly application of balanced liquid fertilizer during the growing season encourages rapid growth and keeps the plant healthy. Dead or unattractive foliage can be snipped as often as necessary.
Pests and Disease
This bush is susceptible to a host of insects and diseases. Thrips and aphids are the most common invaders. Both insects are very small, but thrips have wings while aphids are limited to crawling. Common diseases include powdery mildew, blight and stem rot. Symptoms of insect infestation and disease are similar and may include wilting, distorted foliage, dropped leaves, as well as spotted or discolored foliage. Contact your local extension office for help with diagnosis and location-specific treatment advice.