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Caustic Vine
Cynanchum viminale
SA tree number
English: Caustic Vine, Caustic Bush, Caustic Creeper, Pencil Milk Bush, Succulent Milkweed
Afrikaans: Melktou, Melktoubos, Melktoubossie, Spantou, Spantoumelkbos, Spantou-melkbos, Wolfsmelk, Melkbos
Xhosa: umBelebele
Zulu: Ingotshwa
S. Sotho: Ntlalamela
Tswana: Nneta
Venda: Mutungu
Khoi: K’au
Historical scientific names: Asclepias aphylla, Asclepias nuda, Cynanchum aphyllum, Sarcostemma andongonense, Sarcostemma aphyllum, Sarcostemma aphyllum, Sarcostemma nudum, Sarcostemma tetrapterum, Sarcostemma viminale
Misapplied scientific names: Euphorbia viminalis, Stapelia viminalis
Cynanchum viminale, commonly known as Caustic Vine or Melktou, is a distinctive indigenous succulent scrambler or climber. It is easily identified by its unique, leafless, slender, cylindrical, grey-green stems that produce a milky latex when broken. This fascinating plant forms a tangled mass of stems that can scramble over rocks or climb through shrubs, adding an unusual textural element to dry, rocky environments.
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Distinguishing features:
Mostly leafless, with slender, cylindrical, jointed stems.
Stems are grey-green, often with darker mottling or reddish tips, and exude milky latex when broken.
Flowers are small, star-shaped, greenish-yellow to creamy white, borne in clusters along the stems.
Fruits are slender, paired follicles (seed pods).
Forms a tangled mass of stems, scrambling or climbing.
Drought-tolerant and often found in arid or semi-arid regions.
Overall appearance:
The Caustic Vine (Cynanchum viminale) is a truly unique indigenous succulent scrambler or climber that looks quite different from most plants. It's mostly leafless, instead forming a tangled mass of slender, cylindrical, grey-green stems that resemble pencils or thin ropes. When broken, these stems ooze a milky white sap. During summer, it produces small, star-shaped, greenish-yellow flowers along its stems, adding a subtle touch of interest.
Height:
Can scramble over or climb up to 1 to 3 metres, often forming dense bushes if unsupported.
Width:
Can spread 1 to 3 metres wide, forming a sprawling, tangled mass.
Leaves:
Leaves are usually absent or very small, scale-like, and fall off quickly (deciduous), so the plant appears leafless for most of its life. Photosynthesis primarily occurs in the green stems.
Flowers:
The flowers are small (about 1 cm in diameter), star-shaped with five spreading lobes, and typically greenish-yellow to creamy white, sometimes with a purplish tinge in the centre. They are borne in small clusters (umbels) along the nodes of the stems. They are often somewhat fragrant, especially in the evening. Cynanchum viminale flowers are hermaphroditic.
Flowering season:
Flowers mainly in summer (around November to March), but can flower sporadically throughout the year in favourable conditions.
Fruits:
The fruit consists of a pair of slender, tapering, smooth follicles (seed pods), typically 5-10 cm long, that are borne at an acute angle to each other, resembling "horns" or "spindles." They are green when young and turn brownish as they mature and dry.
Seeds:
Each follicle contains numerous flat, oval, brown seeds, each with a tuft of long, silky, white hairs (a coma) at one end, which aids in wind dispersal.
Stems:
The most distinctive feature of the plant. The stems are succulent, cylindrical, jointed, and very slender, typically 3-10 mm in diameter. They are usually grey-green to bluish-green, often with darker mottling or reddish tips, especially when exposed to strong sun. When broken, they exude a milky white latex (sap). They are numerous, forming a tangled, scrambling or climbing mass.
Thorns:
None.
Roots:
Possesses a fibrous root system, sometimes with slightly thickened or tuberous roots that help store water, enabling it to survive in arid conditions.
Soil type:
Prefers well-drained, sandy, stony, or rocky soils. It is highly tolerant of poor, infertile soils but absolutely requires excellent drainage. It is very sensitive to waterlogging.
Aspect:
Thrives in full sun to partial shade. It is highly drought-tolerant once established due to its succulent stems and reduced leaves. It is sensitive to severe frost; temperatures below 0°C can cause damage, and prolonged freezing will kill it. It prefers dry, warm conditions and is often found on rocky outcrops or slopes.
Ediblity:
Historically, the stems were chewed by some indigenous communities for their watery sap as a thirst quencher in arid areas. However, caution is advised as it contains milky latex, and some related species are toxic.
Toxicity:
The milky latex (sap) is known to be a skin irritant and can cause blistering, especially on sensitive skin. If it comes into contact with eyes, it can cause severe irritation. It is generally considered mildly toxic if ingested due to the latex, and it's best to avoid consumption.
Uses:
Highly valued as an ornamental plant for rockeries, arid gardens, succulent gardens, and as a container plant, especially for its unique, leafless form.
Can be used to scramble over walls or through other drought-tolerant plants.
Traditionally, the stems were chewed to quench thirst in arid areas (with caution due to latex).
The stems were also used to make rudimentary ropes or bindings.
Ecology:
The small, star-shaped flowers attract various small insects, including flies and beetles, which are responsible for pollination. The seeds, with their silky hairs, are efficiently dispersed by wind, allowing the plant to colonise new dry, rocky habitats. As a succulent, it plays an important role in arid ecosystems by providing biomass and some moisture, especially during dry spells.
Distribution and habitat:
Cynanchum viminale is indigenous to South Africa and has a very wide distribution, primarily in the drier, warmer parts of the country. It is found in the Western Cape, Northern Cape, Eastern Cape, KwaZulu-Natal, Free State, North West, Gauteng, Mpumalanga, and Limpopo provinces, extending into Namibia, Botswana, and other parts of tropical Africa. It typically grows in arid and semi-arid regions, on rocky outcrops, stony slopes, sandy flats, and in bushveld, often scrambling over other shrubs or rocks.
Derivation of name:
The genus name Cynanchum is derived from the Greek words 'kyon' (dog) and 'anchein' (to strangle), possibly referring to some species being poisonous to dogs or having a constricting habit.
The species epithet viminale is Latin for 'composed of long, slender twigs' or 'osier-like', referring to its characteristic slender, flexible, and often leafless stems.
The common name "Melktou" is Afrikaans for 'milk rope', referring to the milky latex exuded by the rope-like stems.
"Caustic Vine" and "Pencil Milk Bush" similarly describes its pencil-like stems and milky sap.
Its reclassification from Sarcostemma viminale to Cynanchum viminale reflects recent genetic studies.
Historical aspects:
This plant has long been known and used by indigenous people in arid regions for its sap, which could provide some hydration. Its unique succulent, leafless form has also made it a subject of botanical interest and a popular plant in succulent collections worldwide. Its presence across vast dry landscapes of Southern Africa highlights its remarkable adaptation to harsh conditions.
Interesting facts:
This plant is practically a living pencil case! Its stems are long, thin, and grey-green, looking exactly like a bunch of pencils growing in the ground.
If you snap one of its stems (which you shouldn't do without gloves!), it oozes a milky white sap – that's where the "milk" in "Melktou" comes from.
It's a true survivor, thriving in super dry and rocky places where most plants would give up. It doesn't even bother with leaves for most of the year!
When its seed pods finally burst open, they release hundreds of tiny seeds, each with a fluffy white parachute to help it fly away on the wind!
Growing tips:
Propagation: Easily propagated from stem cuttings. Allow cuttings to dry for a few days to a week to form a callus before planting in well-drained succulent mix. Can also be grown from seed, but germination can be slow.
Light: Requires full sun to bright partial shade for best growth.
Watering: Extremely drought-tolerant. Water sparingly, only when the soil is completely dry, especially in winter. Overwatering is the most common cause of failure.
Soil: Absolutely requires very well-drained, gritty, sandy, or rocky soil. A succulent or cactus mix is ideal.
Container: Excellent for containers where drainage can be controlled.
Maintenance: Very low maintenance. Protect from severe frost, especially when young. Avoid handling the milky sap without gloves.
This plant is a fantastic alternative for:
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