Don't know for sure but with so few leaves and no flowers, I'd question maybe a milii type Euphoriba.

I agree w/ Biker that w/ so few leaves & no blooms it's tough to say. If as he suggests, it's a E. milii type, it should still be in leaf, all of mine still are & I'm in NYC (where it's been unusually warm, in the 40s & 50s in Dec.)

Sorry, can't see the bloom in the pic at all.

Does this thing bleed white when you cut/tear it? If so, pls. be cautioned about Euphorbia sap, if you know some succulents you may have heard Euphorbia sap can be a skin irritant (depending on the type of Euph), so pls. read up on that if you don't already know.

I suspect it's rootbound & if it were mine, I'd turn it out to see what's going on down below.

(My cynical side suspects someone dumped this on you as they didn't wish to bother w/ a repot of such a a large & spiny thing as this, you'll need gloves & lots of newspaper.)

@pirate I am usually pretty cyincal myself, but I don't think that is the case here. They are an all-volunteer staff of a 1+ acre succulent and cactus garden, with plenty of spikey plants including a cactus green house, so they don't seem like the type to dump a plant, especially for that reason.

The milli type euphorbia is an interesting guess, I'll have to research that angle tonight. Thanks for the suggestion!

I am no expert, but I also vote for Milii. You have an interesting plant and if it ever flowers it will look spectactular.

I need to post another picture of the flowers. They are in the third pic but very hard to see. Much, much smaller flowers than a typical milli flower. I sent an email to the president of the Succulent Society where I got it to see if I can get an ID. I will update here when I hear back.

Yes, definitely a milii type. I wonder if they can hybridize easily.

I have a Euphorbia genoudiana that has the same shape foliage and the blooms are smaller then the milii. My genoudiana is dormant now but my milii isn't.

Chem..I too have an un-identified Euphorbia, purchased as a baby. It was sold as Euphorbia 'spurge.'

It resembles yours to a degree. I notice some of the thorns on yours look to be rounded..safer than other thorny succulents.

Here's a pic of my Euphorbia, taken in summer. It still has leaves but many dropped as the days shortened, plus, I let soil dry a little too much.

BTW, it has one trunk, yet some flowers are pink, others yellow. It's not a very clear pic, but will do.

Imagine my Euphorb minus leaves. Do you think ours match? Toni

Toni,

If you're talking about that centrally placed plant in your pic (w/ the yellow blooms), that IS a E. milii, the blooms pretty much clinch it (as the other plant on the right w/ the red blooms, also E. milii). Can't see from here, but assume the leaves are solid green not variegated.

I can't see the flowers on your plant but do they look anything like these?
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Yes! Very similar, except mine seem just a little more brownish in color, but they could just be old blooms that have changed color a bit.

Do you have the name of that plant?