Euphorbia stems keep rotting in succession

Question

I purchased an Euphorbia from IKEA last October. It was very robust in the beginning, but after being repotted multiple times it lost a lot of its vigour and some parts of died. I'm looking to understand what I did wrong so that I can prevent this in the future.

I didn't water this plant for the first three months I had it, from October to January. Here's a rundown of its history (pictures included below):

• October 11: I moved the plant from the pot it came in, sloppily placing it in new soil that wasn't suited for it (low drainage).
• November 20: I tried to improve the job by using a better mix (cactus mix + perlite) and repositioning the stems to make them stable.
• December 6: the smallest stem started rotting. The first symptom was a black spot, then the whole stem became soft and squishy. When I cut it and removed the root, it was much softer than the roots of the healthy stems. Only 4 stems were left.
• January 7: for the first time since I purchased it, I watered the plant.
• January 10: yet another arm rotted. This time I removed it very carefully in order to avoid damaging the other ones. It was again very soft.
• January 21: the plant seemed to have survived, so I watered the plant sparsely.
• February 12: I watered the plant again, a bit more profusely (I had recently learned that Euphorbias need water more frequently than Cactaceae).
• February 14 (today): another arm rotted, I removed it without impacting the other two, and also noticed that it had been sitting very loose in the soil, hardly any rooting had happened.

The first two times I repotted the plant I was a bit careless and probably hurt the stems or the roots it by letting the plants hit each other or hang badly.

The leading theories I have about what could've caused the rot are:

• Overwatering: while the plant has good drainage, two of the stems rotted right after being watered profusely, while when I watered the plant lightly another time, nothing happened.
• Bad handling: only explains the first two stems rotting, but the plant took a beating when I repotted it initially.
• Bad soil: the mix that the plant now sits in is very dense in perlite compared to normal soil (looks like a 3:1 ratio or higher). Maybe this is unsuited for the plant?

Picture time! Here's how the plant looked when I had just purchased it:

Here's how it looked after the repotting on November 20.

Here's the first stem that rotted and what it looked like when I inspected it (I later on found out you should never touch Euphorbias with bare hands 😅):

The arm that rotted on January 10 looked like this:

The third arm to rot, today:

Answer 1

It looks to me as if, originally, it was planted just a bit too deep, and when you 'sloppily' repotted it, and subsequently, you planted it at the same or even a fractionally deeper level. In both 'arms' that have rotted, its apparent that some of the ridged growths were below soil level, and that's where the rot started, probably caused by damp soil sitting against the base of the stems.

When plants are presented for sale, they have often just been repotted, or, more usually, the soil at the top of the pot has been topped up with new to make the plant look more appealing or to give more support to splaying stems, but this often means the soil level is higher than it should be, even if that's only by a couple of millimetres. With many plants, it doesn't matter too much, but with a succulent, it does. It needed to be potted a bit lower, and if you needed to support the topgrowth at the base to prevent it splaying out, a layer of fine shingle or small pebbles would have done the job without retaining water against the plant stems. The trouble is, the poor thing's been disturbed so many times, it may not survive at all, and may not respond well to being turfed out of its pot yet again and repotted, but you can try if you want.

This answer assumes you're obeying the usual rule of never leaving the plant sitting in water in an outer pot for longer than 30 minutes max.