Euphorbia knuthii

Euphorbia knuthii is a small, spiny succulent in the spurge family. Its native range runs from Mozambique to South Africa, including Swaziland. It grows in dry shrubland and desert regions. Yet its local sites include black soils that turn seasonally wet. Its thick root feeds into a twisting caudex, or swollen stem. This sits partly below ground and branches from its top. Some plants also form underground rhizomes.

The branches start upright, then scramble as they lengthen. Wild stems measure about 0.1–0.5 feet [0.03–0.15 metres]. Cultivated stems can grow much longer than this range. They have three or four angles and raised bumps. Their pale green skin bears lengthwise grey-green stripes. Strong light can bring out red tones. Small paired spines turn from brown to grey with age. The tiny, narrow leaves soon fall. Severe drought can also make the plant shed its stems.

Flowers appear from mid to late summer. Small green, cup-shaped involucres hold the flowers between the stem bumps. Each cup has five nectar glands, coloured yellowish or green. The smooth fruit forms a capsule with three chambers. It holds three brown seeds and ripens about a month later. Ripe capsules burst and throw their seeds away. Some seeds travel more than 10 feet [3 metres].

This species suits pots and collections of plants with swollen roots. Growers often expose the roots above the soil for display. Seed-grown plants tend to form larger roots with fewer branches. Cuttings produce thinner roots that branch more. Stems grow fast in warm months, but roots enlarge over years. LLIFLE rates it easy to grow and tolerant of neglect. It survives lower light but looks best with heat and sun. The source gives USDA zones 9–11. It also reports survival at 21°F [−6°C], with some snow.

Water during active growth, keeping the soil reasonably moist. Never leave water standing around the roots. Keep it almost dry through winter, with bright light. Summer feeding can use a potassium-rich succulent fertilizer with trace elements. Propagate it from fresh seed or stem cuttings. Stored seed may lose its ability to sprout by the next year. Let cut surfaces dry and heal before planting. Fresh cuts can rot when planted too soon.