The so-called pencil cactus is actually a Euphorbia tirucalli, thus making it a succulent as opposed to a true cactus. There are many thousands of Euphorbia, and some of them are popular landscape plants in arid, hot regions. Like all Euphorbias, the pencil cactus has milky white stem latex that is a local irritant and can cause rather strong discomfort. Care should be taken to avoid direct contact with the latex.
Light: Full sun. Perfect for a sunny window.
Water: Water during the summer and spring, making sure drainage is immaculate. Reduce water in the winter to monthly.
Temperature: Prefers average summer temps (65ºF - 70ºF). In winter, cool to 50ºF.
Soil: A well-drained succulent mix, with an ideal pH around 6.0 (slightly acidic).
Fertilizer: Feed with a controlled-release fertilizer in the beginning of the season or weekly with a weak liquid solution. Use a balanced 20-20-20 fertilizer at 1/4 strength on mature plants, and a fertilizer with less nitrogen on young plants.
Pencil cactus can be readily propagated from cuttings. After taking a cutting, dip it in fresh water to stop the flow of latex, then allow to dry for a week and form a callous before potting in moist sand or a cactus mix. During trimming and propagation, wear protective clothing to avoid getting the latex on your skin or in your eyes.
Repot as needed, preferably during the warm season. To repot a succulent, make sure the soil is dry before repotting, then gently remove the pot. Knock away the old soil from the roots, making sure to remove any rotted or dead roots in the process. Treat any cuts with a fungicide. Place the plant in its new pot and backfill with potting soil, spreading the roots out as you repot.
Leave the plant dry for a week or so, then begin to water lightly to reduce the risk of root rot.
There are thousands of species of Euphorbia. As far as houseplants go, most people concentrate on the E. tirucalli, the globular Euphorbia and the Crown of Thorns, or Euphorbia millii, which each have a completely different growth habit and hardly look related at all.
Given bright light and warmth, the pencil cactus can easily grow into a 6-foot specimen plant. They have a rangy, open growth habit, and are more valued for their novelty than the sheer beauty of their foliage. Pencil cactus can be trimmed back if they become too large, but always wear protective clothing to prevent irritation from the sap. As with all succulents, it’s better to let it dry out rather than risk too much moisture and the ensuing rot.