Short answer
Yes, pencil cactus (Euphorbia tirucalli) is commonly grown as an indoor plant, but it asks for a lot of light and has sap that is a real safety concern. It does best in a bright window, and it is a poor fit for homes where children or pets could chew on or break the stems.
Not actually a cactus
Despite the name, pencil cactus is a succulent in the Euphorbia genus rather than a true cactus. Other common names include pencil tree and milk bush. The “milk” refers to its sap. Some sources also use “pencil cactus” for certain Rhipsalis plants, which are unrelated jungle cacti. If you want the trailing type, see rice cactus care.
Light
Light is the main challenge. Care guides generally recommend a south or west-facing window with at least 4 to 6 hours of direct sun daily, and some suggest more. Without enough light, stems stretch, lean toward the window and become weak. Rotating the pot every so often helps it grow evenly. If you cannot provide enough natural light, a grow light can supplement it. Keep the plant from pressing against hot glass.
Watering
It is a succulent, so let the soil dry out before watering again, then water thoroughly and let the pot drain. Roughly every couple of weeks in summer and less often in winter is one grower’s experience, but your schedule will depend on pot size, light and temperature. Overwatering is the typical way to lose this plant. Use a gritty cactus mix and a pot with drainage holes. In winter, when growth slows, water only enough to prevent the stems from wrinkling.
How big will it get?
Quite large over time. Nursery plants are often sold in the range of one to a few feet, and indoor plants can reach several feet in a bright room with enough headroom. In the ground in warm climates, it can become a tree many feet tall. Heavy, top-heavy plants can tip, so use a sturdy pot as it grows.
Can I put it outside?
In warm weather, many people move it outdoors for the summer, but only gradually. Moving it straight from indoors into strong sun can burn the stems, so introduce it over a week or two. It is only reliably hardy in the warmest climates (USDA zone 11 is one source’s guidance), so bring it in before temperatures drop. Cold can cause sudden browning of branches.
Toxic sap: pets, children and handling
This is the most important section for anyone considering the plant. Pencil cactus leaks a milky latex when cut or broken. The sap can irritate skin and eyes and is considered toxic if swallowed, and it is listed as toxic to pets by the ASPCA. Gardeners report different reactions, from none to significant irritation, so do not assume you will be unaffected.
- Keep it where children and pets cannot reach it, or choose a different plant if that is not possible.
- Wear gloves and eye protection when pruning, repotting or taking cuttings.
- If sap contacts skin, wash with soap and water. If it gets in the eyes or mouth, rinse and seek medical advice promptly.
- If a pet chews on it, contact your vet or an animal poison hotline.
- Sap can stain floors and furniture, so protect surfaces.
Households wanting a safer option can browse popular cactus house plants, keeping in mind that spines bring their own hazards.
Pruning
Prune in spring or early summer to control size or shape, using clean tools and protective gear. Do not remove too much at once; around a third at most is a common rule. New growth typically emerges below the cut. Avoid cutting off most of the base, which leaves the plant top-heavy.
Common problems
- Stretching or leaning: too little light.
- Dropping branches or mushy bases: often overwatering or root problems.
- Browning after cold: caused by cold drafts or low temperatures.
- Sunburn: from sudden exposure to intense sun.
- Pests: mealybugs are the most common; treat with insecticidal soap or similar, as directed on the label.