Cape plumbago

Identification and origin

Plumbago auriculata belongs to the family Plumbaginaceae. It is native to southeastern Africa, from the Cape provinces and KwaZulu-Natal to northern South Africa and Mozambique. It was long sold under the name Plumbago capensis.

It is an evergreen shrub with a spreading, rounded, almost scrambling habit: its thin, flexible stems cascade, and with some help the plant behaves like a semi-climber. In the ground, in mild climates, it reaches 6–10 ft tall and 8–10 ft wide, and more against a support. In a container it grows 12 in – 3 ft a year and stays compact with pruning. It suckers and readily resprouts from the base.

The light green, oblong to spoon-shaped leaves are up to 3 in long; their base forms two small ear-like lobes, hence the epithet auriculata. The flowers, clustered at the tips of the current season’s shoots, have a long tube that opens into five spreading lobes, like a phlox, about 1 in across. Pale blue is the classic color, but white and pinkish forms exist. Butterflies visit them eagerly. Where there is no frost, bloom can last nearly all year; elsewhere it runs from summer into fall. Small brown capsules covered in sticky glands follow.

The most widely grown cultivars are ‘Alba’, with white flowers, and ‘Imperial Blue’, a deeper blue. Don’t confuse it with Ceratostigma plumbaginoides, a hardy groundcover in the same family with darker flowers. The name Plumbago comes from the Latin plumbum, lead: Pliny the Elder felt the plant’s hue recalled that metal.

Light

Full sun gives the most vigorous growth and the heaviest bloom. The plant accepts part shade, at the cost of fewer flowers. Choose a spot sheltered from strong winds, such as the foot of a south-, east-, or west-facing wall.

If you bring it indoors for the winter and keep it in leaf, give it the brightest, sunniest room in the house.

Watering

During active growth, Cape plumbago does best in soil that stays evenly moist. In a container, water generously as soon as the surface of the mix starts to dry, until water runs from the drainage holes, then empty the saucer. Check the root ball with your fingers rather than following a fixed routine.

Once well established in the ground, it tolerates drought fairly well; water it regularly until it roots in. In winter, whether the plant is in leaf or dormant, cut back sharply: the mix should stay barely moist.

Potting mix

It needs fertile, organically rich, and above all well-drained soil. It adapts to clay, sandy, or loamy soils, from acidic to slightly alkaline. In very alkaline soil, the leaves can yellow from a mineral deficiency.

In a container, use a peat-free, loam-based mix in a pot with drainage holes. During the growing season, a balanced liquid fertilizer once a month in a pot, or one or two applications over the season in the ground, keeps growth and bloom going.

Temperature

Cape plumbago tolerates cool weather as long as it does not freeze, but frost kills it back. It is grown in the ground in USDA zones 9a to 11b. In the cooler parts of that range, a freeze often kills the top growth; the plant resprouts from the base in spring and sometimes doesn’t bloom again until late summer.

Elsewhere, grow it in a container. Bring it in before the first fall frost and keep it above about 41 °F. You have two options: keep it in leaf in a bright, sunny room, or cut it back and store it dormant in a cool, dry, dark, frost-free place such as a basement or garage. Move it back outside in spring once the danger of frost has passed.

Repotting

Repot or top-dress the plant in spring as growth resumes: to top-dress, scrape off the upper layer of mix and replace it with fresh mix. If you want to train it as a climber, put a trellis or stakes in place at that time, since its stems do not cling on their own.

In summer, the pot can stay on a sunny patio or be sunk to the rim in a garden bed, then lifted before frost. In mild climates it can be planted in the ground at any time of year; in a mass planting, space plants 4–6 ft apart so each develops its natural fountain shape.

Pruning

Cape plumbago blooms on the current season’s growth. In late winter, cut the stems back hard to encourage new shoots that will carry the season’s flowers. Under glass, some restrictive pruning is sometimes needed to keep it in bounds.

Overlong or leggy stems can be shortened at any time. Remove frost-damaged growth in spring, once new shoots show what is still alive. It tolerates being sheared as a hedge, but each shearing removes most of the flowers. For a climbing form, tie the long, arching stems to their support.

Propagation

Sow seed in spring at 55–64 °F. It is easy, but young plants usually don’t bloom until their second year.

Cuttings give a flowering plant sooner: take semi-hardwood cuttings in midsummer and root them with bottom heat. You can also divide clumps or detach the suckers that arise from the roots. Wear gloves, because the sap is irritating.

Common problems

• Yellow leaves: very alkaline soil causing a mineral deficiency.
• Curled leaves and stunted shoots: chilli thrips.
• Spider mites: mostly indoors or under glass.
• Whiteflies and mealybugs: watch for them on plants overwintered indoors.
• Cottony cushion scale, with cottony masses on the stems: a nuisance but rarely serious.
• Leggy stems, few flowers: not enough sun, or no late-winter pruning.
• Brown, dry growth after a cold snap: frost damage; wait until spring to prune.

Diseases are rare and of little concern. Still, a single symptom can have several causes: check exposure, watering, soil, and the undersides of the leaves before acting.

Toxicity

Cape plumbago irritates the skin. All parts contain plumbagin, a quinone that can cause redness, severe irritation, and blistering after contact. Wear gloves and long sleeves when handling it, and wash your hands afterward.

For dogs and other animals, the plant irritates on contact and is harmful if eaten, except for the ripe fruit. Keep it out of reach of pets that chew plants and, in case of ingestion or a significant skin reaction, contact a veterinarian, a doctor, or a poison control center.