Did this plant get acclimatized to that area before you planted it there? Imo it looks sun bleached.

Did this happen quickly or slowly? You live in my area and it’s been really hot lately, except for the past couple days. I’m wondering if your plant cooked.

I'd remove from planting and check roots. If good roots, replant, place in shaded area and water sparingly until new growth appears.

I echo what everyone else has said. I would guess too much water. Like Howard said, I'd dig it out and check the roots. Clean off any rotted/dead roots and plant it in dry mix. I'd hold off watering for a few days (or maybe 5ish), and then water very lightly.

@kara Yes, the plant cooked. It was thriving mid-summer and then within a month became like it is now. Is this yet another succulent I can only grow in the shade?

@aloebot No, it has been getting water every one to three weeks, and it is in gritty mix so the roots are not drowning.

I‘ve had something similar happen twice. Last year to a Gymnocalycium and a couple weeks ago it happened to one of my Rebutia rauschii. All of a sudden they were burnt. Even tho they had been in the same spot all summer long. I think what happens is in September the sun changes position in the sky, and some plants that were once protected become exposed. It also doesn’t help that September was HOT.

You may want to try growing Euphorbia esculenta. It’s a Medusa type of Euphorbia, but the branches are thicker. I know Ruth Bancroft Gardens in Walnut Creek has this particular Euphorbia all over the place.

@Kara but is there a solution to the plant I have now that is burned?

I would definitely uproot to see what’s really going on with the whole plant. In my experience when the plant’s tissue gets white like that those areas of the plant are gone. Check to see the the roots are good and the central caudex is ok. If they are ok repot in a pot and keep warm. Baby it till it hopefully bounces back and grow new branches.

So should this species be grown in the shade?

Llifle says this about E. caput-medusae:

“It can tolerate moderate shade, and a plant that has been growing in shade should slowly be hardened off before placing it in full sun as the plant will severely scorched if moved to suddenly from shade into sun.”

I would take this as it would do best in morning sun/afternoon shade or even really good bright filtered light and a couple hrs of direct sun.

It can take full sun if hardened off, but when I read it can tolerate moderate shade that too me means it wouldn’t mind shade in the hottest parts of the day. It’s up to you, you could try hardening it off next year.

I have these growing in direct sun and in partial shade (afternoon). Most of the plants in direct sun have some redness, while the ones in partial shade are mostly green. Any that I have moved from full to partial sun have reverted to green with one exception. I do have full sun plants that have remained mostly green as well. I guess it depends on the clone and its resistance to the sun. I'm not sure what happened to yours. I have been lucky in that no matter how much neglect mine have received, they have not sunburned. My zone is generally dry, while I consider yours to be generally humid (when I lived in S.F. one year I had about 20 sunny days and it rained for 20+ consecutive days). I think E. caput medusae is not fond of humid environments. This is counter intuitive as I believe it is native to beach cliffs and scrub.

I don't think this plant is going to come back. I think it fried in the sun, and I think it was lack of watering during a particularly hot period that somehow combined to fry it.

Is this a Euphorbia that tolerates a moister soil - as long as it is fast draining? Or will a moister soil in Winter rains rot it out? Many Euphorbias just cannot handle rain + soil that retains much water.