Please identify this large cactus... It is located in Quartzsite AZ and is over 20 years old. It is 18 feet tall and about 8 inches in diameter. It has 2 trunks and is split in 2 trunks at the bottom. It has many arms and heavily fluted. Can I cut off an arm and start new plants.

Looks like cereus repandus or peruvianus.

Cereus for sure, not sure about repandus (btw peruvianus is a synonym of repandus so the same plant). It gives me more of a hildmannianus vibe, but generally when it comes to Cereus plants in the US, there is a good chance it is of unclear ID or potentially a hybrid formed in cultivation.

Yes you can start new plants from those cuttings. Let the ends callous over (dry out) and then stick them in the ground or a pot with soil. Then keep them dry as much as possible till you have some evidence of root development.

@Mcvansoest
Yep, I know they are synonyms thats actually why I put "or peruvianus".

@Thijs, with the fact that I grow tropicals that need water one or twice a day, until now it is still a big challenge for me to 'not to water' my succulents as much.

Reading the statement "keep them dry as much as possible" make me feel like watching a horror movie 😀. My question is, can the cuttings grow new roots without any moisture in the media?

Even though I have grown succulents for years now, whenever I get a new plant, or repot an existing plant, I struggle a lot on watering issue.

Because one of the rule to keep tropical plants is 'make sure to give a thorough first watering after repot", then we can stop watering until the soil and plant is ready for next water.

Aeonium2003 said:@Mcvansoest
Yep, I know they are synonyms thats actually why I put "or peruvianus".

But why would you suggest a synonym if you know it should be repandus? That just spreads misinformation.

Kaktus said:
Reading the statement "keep them dry as much as possible" make me feel like watching a horror movie 😀. My question is, can the cuttings grow new roots without any moisture in the media?

Cactus pieces can lay around for months before even being potted. So dry potting soil isn't a big deal and in fact, its luxury.

DaisyI said:

But why would you suggest a synonym if you know it should be repandus? That just spreads misinformation.

How does that spread misinformation? Giving a synonym will only give someone more information. That's not misinformation.

I don't see any misinformation here, that's an old name that got lumped with the currently accepted one. I don't think anyone's disputing the "correct" name, right? And you have your instructions for starting those cuttings, so I'd say we've more or less answered the original poster's questions.

Fun fact: those two names were both published by Linnaeus in 1753, so they are primordial.

This plant should be relatively easy to start from cuttings (I put something very similar in the public garden). To add to the previous instructions... You don't want the cuttings (especially fresh cuttings) sitting in wet soil if and when it rains over the winter. So if you can't provide overhead protection during this time, use a smaller (shallower) pot and a rockier mix. Cold and wet is kind of a bad combination when you're trying to root cuttings.

My advice would be to water deeply after a month or two and from there on out wait for the soil to go completely dry at depth (not just the surface) before watering again. When in doubt, wait to water. When it's cold, wait to water.

Do not bury much stem (buried succulence invites rot) beyond what is needed for stability.

You might want to cut across the bottom of those pieces (using a sharp, clean instrument) in order to minimize the surface area of the cut. So cut straight across at the narrowest point. The two cuttings at the right might benefit from this kind of cleaning up.

And be patient because it does take a few years for a cutting to achieve tree-like stature.

I read that it is best to retain the collar at the base of the arm when taking cuttings because thats the region where roots originate. Therefore when I cut the arms I was careful to cut deep enough to keep the collar. I f I cut them straight I lose the collar region. Any advise on where roots originate..

@Shermt
Roots can occur from:
1. the areoles
2. the vascular bundle in the calloused area
3. corked areas on the stem.

These particular cacti are not that particular about sending roots like Evan lists, as long as they are properly dried out/calloused over so as to limit the chances of some kind of rot developing even if the cactus does get wet from time to time before developing roots (but better to avoid that) it should have no problems rooting.
I have started a good number these from cuttings, even from ones that sat around unrooted for 6 months plus (like Daisyl already suggested). Not sure how things are in Quartzsite, but right now it is on the cold and wet side over here in the Tempe/Phoenix area so my advice would be to keep the cuttings in a dry place till it all dries out and warms up a bit before putting any of them in soil. The wait should not harm them (it may actually help them).