Natal plum

Identification and origin

Carissa macrocarpa belongs to the Apocynaceae, the family of oleander and periwinkle. It is native to a broad band of eastern and southern Africa: Kenya, DR Congo, Zambia, Zimbabwe, Mozambique, KwaZulu-Natal, the Cape provinces and the northern provinces of South Africa. There it grows in coastal thickets and forests and even on sand dunes. Its English names, Natal plum and big num-num, and its Zulu name, amatungulu, recall this homeland. It is still sometimes found under its former name, Carissa grandiflora. Planted in many warm regions, it has become naturalized in Florida, Texas, Hawaii and the Caribbean.

It is a dense, spreading, heavily branched shrub with a fairly fast growth rate. In cultivation it most often measures 6–10 ft tall, but it can become a small tree of 13–18 ft in good conditions. Dwarf cultivars, such as ‘Nana’, ‘Boxwood Beauty’ or the groundcover ‘Emerald Blanket’, stay much lower, sometimes under 18 in.

The stems carry sturdy Y-shaped forked thorns 1–2 in long: wear gloves when handling it. The opposite, oval, leathery, glossy leaves, dark green above and paler beneath, measure 1–2 in and often end in a small point. A cut beads up with a white, milky latex, as in most of the Apocynaceae.

The white, star-shaped, five-lobed flowers reach 1–2 in across and give off a sweet fragrance close to orange blossom or jasmine. They appear in flushes almost year-round in mild climates, with a peak from spring into summer. Some plants carry only functionally male flowers.

The fruit is an oval berry 1–2 in long, green and rich in latex when young, then magenta red and finally deep crimson when ripe. Its strawberry-colored flesh holds a few small, flat seeds that are edible too. Flowers and ripe fruit often grow side by side on the same branch.

Light

The Natal plum grows best in full sun, but it tolerates part shade or light shade. The more light it gets, the more abundant the flowering and fruiting. It stands up remarkably well to wind and salt spray, which makes it a shrub of choice for seaside gardens.

Watering

Once established, it handles drought very well and, planted in the ground in a warm climate, generally does without supplemental watering. For the first few months after planting, water when the soil starts to dry on the surface to help the roots spread. In a pot, the soil dries out much faster: check it regularly with your finger and water when the top layer is dry, letting the excess drain away. It cannot tolerate waterlogging: soil that stays soggy makes the roots rot.

Potting mix

It prefers sandy, perfectly drained soil, but it also adapts to loam or clay soils, acidic or alkaline, as long as water does not stand. Good garden soil enriched with compost suits it. On very alkaline soil, micronutrient deficiencies can show up and turn the foliage yellow; a micronutrient supplement corrects them. It tolerates salty soil well.

Temperature

The Natal plum can be grown in the ground in USDA zones 9b to 11b. A well-established plant withstands temperatures down to about 25 °F, but young plants need protection as soon as the temperature drops toward 28 °F. It tolerates a light frost if it is planted in a sheltered spot. In colder regions it is grown in a container, brought in to a bright, frost-free space for the winter, and moved back outside as soon as mild weather returns.

Repotting

In a container, choose a pot with drainage holes and a very fast-draining mix based on potting soil and sand or mineral materials. Repot in spring when the roots fill the whole pot, protecting your hands from the thorns. Dwarf forms are particularly well suited to container growing and to the art of bonsai. In the ground, it makes an excellent defensive hedge: for a hedge, space the plants about 3.5 ft apart.

Pruning

It takes pruning well and can be shaped into a hedge or a ball. Prune after the main flowering, removing no more than the upper third of the growth at a time. For a hedge, keep the top narrower than the base so that light reaches the lower branches. On dwarf cultivars, promptly remove any vigorous shoots that stick out: left in place, they distort the compact silhouette. Wear protective clothing and gloves to shield yourself from the thorns.

Propagation

The seeds germinate in about two weeks, but the seedlings grow slowly at first and produce highly variable plants; they can, however, bear fruit as early as two years old. Vegetative propagation is therefore preferred to keep a variety’s traits: air layering, ground layering or budding. Cuttings root poorly, unless you cut halfway through the tip of a young stem and leave it in place for a few weeks before taking it; planted in sand afterward, it roots in about a month.

Common problems

• Root rot: the main cause of decline, due to poorly drained soil or overwatering.
• Scale insects: notably Florida red scale; inspect the undersides of the leaves and the stems.
• Spider mites, thrips and whiteflies: especially on young plants and in dry air.
• Leaf spots: caused by various fungi, a parasitic alga or bacteria; some develop into anthracnose.
• Cankers, galls and twig dieback: remove affected parts by cutting back into healthy wood.
• Yellow foliage on alkaline soil: a micronutrient deficiency.

The same symptom can have several causes: before taking action, check the drainage, the soil moisture and the light exposure.

Toxicity

The Natal plum is not toxic to dogs, cats or horses. Its ripe fruit, deep red and slightly soft, is eaten raw, whole with its seeds, or made into jams and jellies. The milky latex is generally considered harmless; swallowed in large quantities, however, it can cause mild digestive upset, such as vomiting or diarrhea. Do not eat the fruit while it is still green and rich in latex, and watch out for the thorns, which easily injure the skin.