You can prune the main stem of Euphorbia trigona. You should make the trim carefully to protect yourself and the plant, and with the expectation that the plant will have a different appearance afterwards. When you make a cut to the stem, the stem will not regrow from that point or grow in the same direction.  As long as the plant is healthy, it should send out one or more shoots from a point below the cut in any direction. Euphorbia trigona may send out a burst of branches from the area of the cut that gives the plant a fuller top or it may grow just one or two new branches.

Make sure that your plant is in a healthy condition and getting the best care before you cut for best results. Spring and early summer are the best time to prune a vigorous plant. Pruning at a period of strong growth gives the Euphorbia the best chance to recover from the stress easily. Be careful not to remove too many branches at one time because that could shock the plant. As a general rule, no more than 1/3 of the plant should be removed during  pruning. Here is the procedure to follow.

• Protect your eyes, skin and clothes from the white latex-like sap. This sap can cause skin and eye irritation which can sometimes be severe. It will also stain your clothes and become black when it dries. Also, because of this sap, these plants are not good for composting. Be aware that the latex can also stain your pruners.
• Remove any dead, damaged or diseased branches. This will both tidy up the plant and promote new growth.
• Determine the shape and height you would like for your plant. Then use pruners to cut off the stem to the desired height.
• After each cut, spray the cut on the remaining plant with water. That will help to reduce the flow of sap. Once you are done, protect this wound from getting wet to discourage infection.
• Carefully throw out the parts of the plant you removed or use them for propagation.(See How do I Propagate Euphorbia trigona?)

Take your time. After pruning, your plant will need some time to recover so give it the best possible care and don't subject it to other changes in routine or location.

Courtesy of NYBG Plant Information Service