A huge genus of more than 2,000 species, Euphorbia ranges from small succulents to trees that reach heights of 30 feet or more. Some species, including snow on the mountain (Euphorbia marginata), are strictly grown as annuals. Many are perennial, including wood spruge (Euphorbia amygdaloides), hardy to U.S. Department of Agriculture plant hardiness zones 7 through 9, and cypress spurge (Euphorbia cyparissias), which grows in in USDA zone 3 through 9. Propagate euphorbia by taking softwood cuttings in spring or summer.
Euphorbia requires a free-draining potting mix such as a ratio of three-quarters commercial potting soil for cactus and succulents mixed with one-quarter coarse sand. Fill a 3-inch container to about 1 to 1 1/2 inch from the rim, then top the potting mix with a layer of coarse sand or fine gravel. The gritty covering protects the top of the cutting from rot. Water the potting mixture until it is just barely moist.
Cut a 2- to 3-inch stem from a healthy, actively-growing Euphorbia. The stem should be young and tender but not too small, because immature tissue is more likely to rot. However, woody, mature growth takes much longer to root, if at all. Avoid making the stem longer than 3 inches because the Euphorbia may bend over and collapse.
Many types of Euphorbia are succulent plants, including annual pencilbush (Euphorbia tiricalli) or candelillia (Euphorbia antisyphilitica), hardy in USDA zones 8 through 11. If the stems and leaves are thick and fleshy, set the cut stem aside in a warm, dry place until the cut forms a callus that prevents moisture from entering the stem. Formation of a callus generally takes at least 48 hours and may require as long as a week.
Remove the leaves from the lower half to two-thirds of the stem, leaving at least two or three leaves at the top of the cutting. Insert the stem carefully through the gritty top covering and into the potting mix. If the stem doesn't slide easily, make a small hole with a pencil or other long, thin object. Plant the stem with the leaves slightly above the surface of the potting mix, just deep enough that the stem stands upright. If the stem is planted too deeply, the base may rot before roots develop. Euphorbia requires no rooting hormone.
Euphorbia often roots in one to three weeks, depending on room temperature. If the room is chilly, place the pot on top of a warm appliance such as a refrigerator or freezer. Partial shade or indirect light is best. Avoid intense bright light that may scorch the cuttings and do not cover the pot with plastic because Euphorbia requires excellent air circulation to prevent rot. Water only enough to keep the potting mix from becoming completely dry.
Allow the young plant to mature throughout the winter and plant it outdoors after all danger of frost has passed the following spring. Although most Euphorbias are relatively slow growers, you can move the plant to a larger container if it outgrows its pot. Use a gritty, well-draining potting mixture and a pot with a hole in the bottom.
Many types of Euphorbia, including pencil tree and crown-of-thorns (Euphorbia milii) -- hardy in USDA zones 9 through 11 -- produce a milky sap that may cause minor skin irritation, including redness and blistering. Sap that comes in contact with the eyes may result in a burning sensation and sensitivity to light. Euphorbia is not believed to be highly toxic when ingested, but it may cause gastrointestinal discomfort, including nausea, vomiting and diarrhea.