The pencil cactus (Euphorbia tirucalli) isn't a true cactus, but a sun-loving, spineless succulent plant native to tropical areas of India and Africa. Pencil cactus plant is named for the thin, somewhat brittle, pencil-like stems. Once established, pencil cactus thrives with little water and tolerates extended periods of neglect, but doesn't survive cold temperatures. Also known as milk plant, pencil cactus isn't difficult to propagate by taking cuttings in spring.
Fill a small planting container with a commercial potting soil for cactus and succulents. If you choose to make your own potting soil, use crushed rock, sand, pumice or perlite mixed with a very small amount of compost. Improve drainage by placing a thin layer of fine gravel on top of the potting soil. Use a planting container with bottom drainage, as succulents quickly rot in soggy, poorly drained soil.
Wipe a razor blade with rubbing alcohol, then use the blade to cut a 4- to 5-inch length from a healthy, vigorous stem. Remove the leaves from the lower part of the stem, leaving 2 to 4 inches of bare stem.
Dip the bottom inch of the bare stem in powdered rooting hormone, then set the stem aside to dry for in a warm spot for about a week. The stem forms a callus that prevents rotting.
Plant the callused pencil cactus stem in the prepared potting mixture. Water the soil well, then place the container in a warm, bright spot. Keep the soil lightly moist while the stem is rooting. Use distilled water if your tap water is chlorinated, as chlorine may burn the roots. Always allow the soil to drain completely after watering, and never allow the pot to stand in water.
Transplant the pencil plant cutting into a larger pot after the cutting roots, which usually requires several weeks. Water the new pencil plant immediately after repotting. After that time, water only when the soil feels dry. Keep the pencil plant in a warm, sunny spot.