Pencil cactus

Identification and origin

Euphorbia tirucalli belongs to the family Euphorbiaceae. It is native to Madagascar and long ago became naturalized in the semi-arid regions of tropical and southern Africa, as well as along old trade routes from the Arabian Peninsula to India and Indonesia.

It is a spineless succulent shrub or small tree, often mistaken for a cactus. It develops a thick trunk and brown, woody main branches, from which grow young green branchlets, cylindrical and pencil-thick, clustered at the branch tips. Its tiny leaves (up to 1 in long) drop very early: the plant looks leafless almost all the time. All parts contain an abundant white latex.

In the wild it can reach 19.5–29.5 ft tall. Indoors it stays 2–6 ft tall and 12 in – 3 ft wide, and grows slowly. The tiny, yellowish flowers are clustered at the branch tips, hidden by small bracts. Plants are either male or female; the fruits are three-part capsules about 0 in across, pale green tinged with pink.

In the strongest sun it can take on gold to red tones. The cultivars ‘Sticks of Fire’ and ‘Flame’ emphasize this trait: their orange-red to pink branches fade toward yellow in summer and color up intensely in winter.

The epithet tirucalli comes from Malayalam, a language of southern India, and refers to a “good medicinal plant.”

Light

Put it in the sunniest spot in your home, ideally in front of a south-facing window, with six to eight hours of direct sun a day. Give the pot a turn every week so all sides get the same light. Without long hours of direct sun, the branches stretch and grow weak.

If the window isn’t enough, add grow lights, run fourteen to sixteen hours a day. To move it outdoors for the summer, acclimate it gradually over a week to ten days, and shield it from harsh late-afternoon sun.

Watering

Pencil cactus hates soggy soil but doesn’t tolerate long dry spells either. Water with tepid water until the whole mix is moist, let it drain completely, and empty the saucer. Wait until the mix is just dry before watering again.

Cut back on watering in fall as the days shorten. In winter the plant is resting: water only enough to keep the branches from wrinkling slightly. It doesn’t need humidity: dry indoor air, below 50% relative humidity, suits it, and misting is not recommended.

Potting mix

The mix should resemble the loose, free-draining soil of its native habitat, with little organic matter. Combine equal parts potting soil without added fertilizer and coarse sand or perlite, with a little leaf mold if the mix is very gritty. A commercial succulent mix works, as long as it contains no fertilizer.

Choose an unglazed terra-cotta pot with a drainage hole. Fertilize sparingly: only during active growth, from spring to late summer, with a balanced organic houseplant fertilizer at half strength every four weeks.

Temperature

Ordinary household temperatures suit it: 64–75 °F during the day and 61–64 °F at night. In winter, a rest at about 55 °F, still in full light, helps it adjust to short days. Avoid temperatures below 50 °F.

Keep it away from heating and air-conditioning vents and from cold windowpanes in winter: cold turns the branches brown. Outdoors it is hardy only in USDA zones 11a to 12b, and it sometimes survives in the warmest parts of zone 10.

Repotting

While it is young and growing steadily, repot it in spring about every other year, moving up one pot size: a pot that is too large stays wet and damages the roots.

Moisten the root ball before unpotting and handle it gently: the roots are fragile. Don’t bury the stem any deeper than before, or it will rot. Wear gloves and eye protection.

Pruning

Pruning keeps the height in check and balances the branching. Prune in spring, or in early summer on a vigorous plant, and never remove more than a third of the plant. With a sterilized tool, take out dead or diseased wood, then cut just above a node.

A cut branch does not regrow from the cut: one or more new shoots emerge lower down. Mist the wound with water to slow the flow of latex, which stains clothes and tools and turns black as it dries.

Propagation

Seed is possible, but cuttings are easier. Never root it in water: the cutting would rot. In late spring or early summer, take a young branch about 5 in long. Dip the cut end in water and mist the wound on the parent plant.

Let the cutting dry in a warm, dry spot until a hard callus forms, then plant it in a sterile, barely moist mix of sand and peat. Keep it warm, in indirect light, and water very sparingly; if nights drop below 61 °F, a heat mat helps. Roots form in about two months: a gentle tug tells you when.

Common problems

• Dropping branches: stress, often caused by root problems; check light, drainage, and watering.
• Soft, brown stem base: rot, from a stem planted too deep or a mix kept too wet.
• Branches that suddenly turn brown: cold exposure; cut out the dead parts back to green tissue.
• Dry, drooping branches: a heating or air-conditioning vent, a cold window, high humidity, too little light, very cold water, or overfertilizing.
• Sunburn: a sudden move into direct sun.
• Pests: mealybugs, spider mites, aphids, nematodes; fungal and bacterial diseases can also appear.

Brown, bark-like tissue spreading up the base of the trunk, on the other hand, is normal. A single symptom can have several causes: correct one thing at a time.

Toxicity

Toxic to dogs, cats, horses, and people. The white latex, found in all parts, contains irritating diterpene esters. On skin it causes mild to severe dermatitis; in the eye it can damage the cornea, temporarily or permanently, and cause temporary blindness. If eaten, it irritates the mouth and stomach and sometimes causes vomiting and blistering of the lips and the lining of the mouth.

Handle it with gloves and eye protection, keep it out of reach of children and pets, and contact a doctor, a veterinarian, or a poison control center right away in case of ingestion or eye contact.