Pencil cactus is a fitting common name for Euphorbia tirucalli, even though the plant would be useless for writing and is not really a cactus.
A single plant looks like many slender, green pencils, each stuck on the end or growing off the side of the one before it. A couple of small, elongated leaves perch inconspicuously and briefly at the end of the "pencils," relegating photosynthesis to the succulent, green stems.
Lack of thorns is one indication that this plant is no cactus. Even more telling is the milky sap that oozes from broken or cut stems. That sap and the plant's flowers — not very showy and rarely appearing indoors — put pencil cactus in the spurge family, along with more familiar houseplants such as poinsettia and crown-of-thorns.
On the positive side, the sap has been used in its native Africa as folk medicine, and to repel mosquitoes and kill rats. It's also a potential source of latex rubber and oil — 10 to 50 barrels of oil per acre by one reckoning.
On the negative side, the sap has been implicated as a potential carcinogen and, if it gets in the eyes, is said to cause temporary blindness. At the very least, it can be toxic — especially for dogs and cats — and can irritate skin, as does the sap of many spurge family plants.
Making new plants
All that's necessary to get a pencil cactus started is to snap a few stems, each 2 or 3 inches long, from an existing plant. (Again: Avoid touching the sap!) My pencil cactus cuttings came from a living fence I happened upon during a recent visit to Florida.
There was no need to keep those cuttings moist until I returned home because this plant, like all succulents, roots best if its cut ends are allowed to callous over in dry air before being put in soil.
So it wasn't until I brought my cuttings home that I stuck them into pots of soil, watered them and then waited each time until the soil was thoroughly dry before watering again.
Growing this pencil
Where winter temperatures don't drop below freezing, pencil cactus can grow outdoors as high as 30 feet. There, the dense tangle of stems and a sap that virtually every animal avoids make the plant an ideal living fence.
It also makes a nice houseplant (keeping in mind the cautions about the sap). As a succulent, the plant loves light but otherwise tolerates the threats facing most houseplants: dry air and forgetful watering.
If in doubt about whether or not to water this plant, don't. It won't die from under-watering. Taper off or completely avoid watering in winter. Extra perlite added to any potting mix further ensures that the mix drains well and stays on the dry side.
One variety that's particularly attractive indoors or out is "Sticks on Fire." Its "pencils" are reddish yellow, the red becoming more prominent in cooler weather.