Cultivating Dumbcane

The optimum method of propagation is air layering. With a sharp, clean knife make a shallow cut around the stem below a stem ring, at the point which you would like to cut the plant. Make your cut well through the outer layers, into the cambium layer. Dust the open area with a rooting hormone such as Hormonex or Rootone. Secure a piece of clear plastic around the stem close to the bottom of the cut. Use a twist tie to keep it closed. Fill the plastic with moist sphagnum moss and seal the top.

In 3 to 8 weeks when the roots become visible, sever the stem just below the plastic, remove it, and plant your newly formed root ball into a new pot. Water it into the new soil, and you are finished. Shorten the stem that remains in the original pot to a height at which you want new branching to begin. Be careful, it will be easy to overwater both the new plant and the old root system in the first few months.
It is possible to just cut the top of the plant off at the desired height, dust both sides of the cut with rooting hormone, and simply replant it into new clean soil. The cutting will root, and the parent plant will branch at that point. This is a slower and slightly more risky method, but is usually quite successful.

You can also cut discarded portions of the stem into segments containing 3-4 rings. Place the stem cuttings on their sides with an 'eye' upwards onto the surface of sterile potting soil. (It's a good idea to create a mini-greenhouse with a frame of cut wire coat hangers and clear plastic over the pot to avoid drying.) Keep the cuttings at a minimum temperature of 72 degrees, and you will be rewarded with several new plants.
Keep in mind that when you are doing any type of plant propagation that cleanliness is extremely important. Always use a clean sharp knife and sterile potting soil for the best results!