Pencil Cactus

How to care for Pencil Cactus

Light

Euphorbia tirucalli thrives in the brightest spot available indoors, ideally a south- or west-facing windowsill that receives several hours of direct sun each day. Insufficient light causes the stems to etiolate and lose their compact, bushy form. A position close to a sunny window is strongly recommended, and the plant can be moved outdoors to a sheltered, sunny spot during the summer months if temperatures remain reliably above 15 degrees C.

Watering

This species is highly drought-tolerant and far more likely to be harmed by overwatering than by underwatering. During spring and summer, water moderately once the growing medium has dried out completely, then allow it to dry again before repeating. In autumn and winter, reduce watering to once every three to five weeks or even less. Always ensure surplus water drains freely from the pot, as waterlogged roots rot rapidly.

Soil and potting

A gritty, very free-draining compost is essential. A proprietary cactus and succulent compost is suitable, or a standard peat-free potting compost mixed with at least thirty to fifty per cent horticultural grit or perlite. Good drainage is the single most important factor in preventing root rot. Avoid any moisture-retentive mixes formulated for tropical foliage plants.

Feeding

Feed once a month during the active growing season from April to September with a balanced liquid fertiliser diluted to half the recommended strength, or use a specific cactus fertiliser low in nitrogen. Do not feed during autumn and winter when the plant is resting. Overfeeding encourages soft, weak growth and makes the plant more susceptible to pest damage.

Humidity and temperature

Euphorbia tirucalli prefers warm, dry conditions and tolerates the low humidity typical of centrally heated homes without difficulty. It grows best between 18 and 28 degrees C and should not be exposed to temperatures below 10 degrees C. Keep it away from cold draughts, unheated conservatories in winter, and condensation-prone windowsills. It cannot tolerate frost.

Repotting

Repot every two to three years in spring, moving the plant into a pot one size larger when roots begin to emerge from the drainage holes. Use a heavy, stable container such as terracotta to offset the plant's top-heavy habit. Wear gloves and eye protection during repotting to avoid contact with the latex, and handle the plant with care to prevent stem breakage.

Pruning

Pruning is rarely necessary except to limit size or remove damaged stems. If you do prune, wear chemical-resistant gloves, safety glasses, and long sleeves, as the white latex is caustic and can cause severe skin and eye irritation. Allow cut stems to callous in fresh air for several minutes after pruning. Prune in spring or early summer using clean, sharp secateurs, and wash tools thoroughly after use.

Is Pencil Cactus toxic?

Common problems

How to propagate

Stem cuttings

Take a stem cutting of five to ten centimetres in spring or summer, wearing gloves and eye protection. Allow the cut end to dry and callous for at least 24 to 48 hours until the latex has dried, then insert the cutting into dry cactus compost and do not water for the first week. Rooting typically occurs within four to eight weeks at temperatures above 20 degrees C.

Division

Large, multi-stemmed plants with separate root sections can be divided at repotting time. Carefully separate the root ball and pot each section individually into well-draining compost, again wearing appropriate protection due to the latex.

Where Pencil Cactus comes from

Native to eastern Africa, southern Africa, tropical Africa, Arabian Peninsula, Indian subcontinent.

Story and origins

The genus name Euphorbia honours Euphorbus, the Greek physician to the Numidian king Juba II of Mauretania, who reportedly used a related plant medicinally around the first century BCE. The species epithet 'tirucalli' is derived from 'tirucalli', a vernacular name for the plant used in the Malabar region of southern India, recorded by early European botanists. The species was formally described by Carl Linnaeus in 1753 in his foundational work 'Species Plantarum'.

Euphorbia tirucalli is native to tropical and subtropical Africa, particularly East Africa, and has also long been naturalised across the Indian subcontinent, Southeast Asia, and parts of South America, largely through human cultivation. It has been used for centuries in traditional medicine across Africa and Asia, with various preparations applied externally to treat skin conditions and warts, though such use is hazardous given the toxicity of the latex. European botanical explorers documented the plant during the colonial era, and it entered cultivation in Europe as a curiosity and ornamental during the eighteenth and nineteenth centuries. In modern times, interest in the plant has been renewed by research into its hydrocarbons as a renewable energy source, and its bold, sculptural form has made it a fashionable subject in contemporary minimalist interior design.

Varieties to know

Euphorbia tirucalli 'Sticks on Fire'

The most widely cultivated ornamental form, with stems that flush vivid orange, red, and yellow, especially in bright light or cooler temperatures. Sometimes sold as 'Rosea' or 'Fire Sticks'.

Euphorbia tirucalli 'Rosea'

Closely related to or synonymous with 'Sticks on Fire' in many commercial contexts; produces pink to coral stem colouration under high light and cooler conditions.

Fun facts

• The latex of Euphorbia tirucalli has been studied as a potential biofuel feedstock, as it contains significant quantities of hydrocarbons that can be processed similarly to crude oil.
• In parts of East Africa, the plant is used as a living fence or hedge because its dense, caustic stems deter livestock and intruders effectively.
• Nobel Prize-winning chemist Melvin Calvin promoted Euphorbia species including E. tirucalli as potential renewable hydrocarbon crops in the 1970s energy crisis era.
• Despite resembling a cactus, Euphorbia tirucalli is not related to true cacti at all; it is a member of the spurge family and its cactus-like form is an example of convergent evolution.
• The stems of Euphorbia tirucalli carry out photosynthesis directly, as the leaves are tiny, scale-like, and quickly shed, making the green stems the primary photosynthetic organs of the plant.

Pencil Cactus FAQ