The Plumbago auriculata, commonly known as Cape Plumbago, is a fast-growing ornamental shrub celebrated for its delicate sky-blue flowers. This semi-woody perennial produces blooms nearly year-round in warm climates. The ultimate size of this plant is variable, depending on the environment and the management it receives. Understanding its natural growth habit is necessary for predicting its size in any specific location.
Maximum Potential Size and Growth Habit
In its native South African habitat and in warm, frost-free regions (USDA Zones 9-11), Cape Plumbago acts as an evergreen perennial and exhibits its most expansive growth. If left unchecked, it can develop into a substantial shrub, commonly reaching heights between 6 and 15 feet. Its weak, arching stems contribute to a wide spread, often extending 6 to 10 feet horizontally.
The plant’s growth habit is more vine-like than a dense shrub, often sprawling or cascading over walls and fences. It can be trained on a trellis or support structure to function as a vine. The rapid growth rate allows it to reach its full potential relatively quickly, sometimes within a single year under ideal conditions.
In colder climates (below Zone 8), the plant is cultivated differently, which significantly restricts its size. Where winter temperatures consistently drop below freezing, Plumbago is grown as an annual, remaining much smaller before dying back. When grown in containers and overwintered indoors, restricted root space and seasonal dormancy typically limit its size to a compact 1 to 4 feet in height and spread.
Controlling Plumbago Size Through Pruning
Because Cape Plumbago is a vigorous grower, pruning is often necessary to maintain a manageable size and attractive shape. Pruning prevents the plant from becoming leggy or overgrown and encourages new growth, where the flowers appear. Gardeners use two main types of pruning: maintenance and hard pruning.
Maintenance pruning involves light clipping and deadheading throughout the active growing season. Deadheading spent flower clusters redirects the plant’s energy from seed production back into flowering, promoting a continuous bloom cycle. This lighter pruning also includes cutting back errant stems that detract from the desired shape, keeping the plant tidy.
Hard pruning provides the most significant size control and is a rejuvenation technique performed in late winter or early spring before new growth begins. This aggressive cutback is necessary to manage the plant’s natural tendency to sprawl and become woody over time. A common guideline is the “one-third rule,” where up to one-third of the plant’s overall size is removed, or in cases of severe overgrowth, the plant can be cut back by as much as one-half.
Removing the oldest branches and cutting back the remaining stems encourages vigorous, healthy shoots from the base, which will produce the season’s flowers. For plants that are overwintered as dormant container specimens, a hard cutback is typically performed in the late autumn or late winter before storage. This technique allows gardeners to successfully keep a plant with a 15-foot potential at a compact 3 to 6-foot size.
Varietal Differences in Plumbago
While Plumbago auriculata is the most common species, the maximum size a plant reaches is also determined by its inherent genetics, as different cultivars and species have distinct growth potentials. Standard P. auriculata is the variety that achieves the largest dimensions, but intentionally bred cultivars offer naturally smaller alternatives. For example, the cultivar ‘Imperial Blue’ is a popular choice specifically because it is more compact, typically maturing at a height of 3 to 4 feet with a spread of 4 to 5 feet, regardless of intensive pruning.
Other species within the genus demonstrate a different profile entirely. Plumbago indica, sometimes called Scarlet Plumbago, features red to rose-pink flowers and exhibits a more lax, scrambling growth habit than its blue-flowered cousin. This species generally remains smaller, reaching an ultimate height of about 3 to 5 feet and a spread of 1.5 to 3 feet.
The plant commonly called “Dwarf Plumbago,” Ceratostigma plumbaginoides, is in a different genus altogether. This species is an herbaceous perennial that forms a groundcover, topping out at a mere 6 to 18 inches tall, highlighting the vast difference in size potential across plants sharing the common name of plumbago. These genetic differences provide gardeners with options to select a variety that naturally aligns with their desired landscape size, minimizing the need for constant, aggressive pruning.