It is not a cactus. It's Euphorbia tirucalli, and it's white sap is a serious hazard. Be very careful when cutting or handling this plant. Any sap in the eye could mean temporary blindness. It will produce rashes on skin. Use gloves and eye protection to keep sap off your skin and away from your eyes.

I don't grow it, so I'll leave it to others to give you better advice about what to do, but my guess is that cutting it down significantly will be the quickest way to grow it back to a dense shrub.

Brad

Hi. I have the red version of this cactus...ok...euphorbia, the 'Sticks on Fire' variety. It is so easy to root it is unreal. It sound like you need to take TOP tip cuttings,not cut it to the ground let it grow from the root. It wont, that will kill it.

You need to take as much of the top off that is not damaged, and root it in a good cactus type soil, or a another good potting soil with a lot of perlite in it to drain fast. Are you in south Fla? Mine did fine during our cold snaps,down to the upper 30's so far.

Tonight it is the upper 40's,so no freeze. I don't know about these 'dents' in the main stem. I don't know if that is rot, or damage from rotted stems falling off?

If the stem is still firm and doesn't rot when you cut the tips off, leave it alone and let it resprout where it will.

It should do it. I hack mine back with lopping shears like 3 times a year, it sprouts out all over the place on the main trunk. Just depends if those dents are from frost damage or just from the stems falling off after the freeze killed the stems. The plain green one, which I assume you have will root and respout just as well as the 'Sticks on Fire' one.

Thanks, Brad. I've had them in my yard for years and I'm careful to keep the sap off of me. I've never had problems when it's gotten on my hands and arms, but I know some people do, and I definitely keep it away from my eyes! :)

floridabear, thanks for the great advice. I think you're right about cutting off and rerooting the healthy parts. It's so ugly right now --- tall, gangly, without stems --- that I don't think I have the patience to wait and see if it will regrow stems from where the old ones dropped off. :D

The green form of E. tirucallii is typically much cold hardier and more vigorous than the orange/red 'Sticks of Fire' cultivar. If you aren't in Florida or south Texas, I might add that this species roots much more readily with heat, and this time of year in California, it will only reluctantly root from cuttings without the benefit of bottom heat or being kept dry and out of the rains inside a greenhouse. Cuttings will root best with 75F and up temperatures during the day, and typically won't do much at all if temps are still in the 50's and 60'sF, at least in my experience.

If your plant is the straight green species, it will probably resprout from the base of the plant once it warms up. The Sticks on Fire cultivar, having less chlorophyll producing parts to give it the energy to regenerate new growth, is slower and weaker to do so unless you live somewhere dryer and with early spring warmth to kick it back into gear.