why do prickly pear cactus turn purple

This has been going on for a while (about a year), one of my cacti has turned purple. I've just replanted it after its pot fell and broke (no, it did not spend the year on the ground). What is the cause of this and what should I do about it? I put the cacti out on the open balcony in the summer and put them on the windowsill for the rest of the year, so the usual temperature they are exposed to is 20-35C (as low as 10C during the night). I water them twice a month during the summer and once a month otherwise with tap water, about 100 ml per plant. The height of the cactus is about 6. 5 cm. The cactus in question spends most of its time very close to my other cacti (~10 cm), and some of them don't look too hot, pictures below.


Cactus in question about to be replanted, click to open full image
Section of the offspring, the flesh is green with a white center and has no smell, click to open full image Neighbouring plants showing possibly related symptoms (The other side of the one on the left is perfectly green), click to open full image The worst thing is, I can't decide which cactus this actually is - I'm inclining towards possibly a Euphorbia of some description, or more likely Orbea tubiflora maybe, but I really am not sure. Even so, there are some things you can try - first, can you get hold of some sterilised potting compost rather than just garden soil?


It needs a sandy or gritty free draining potting medium as soon as possible, and a pot with drainage holes in the bottom if that one doesn't have them - in the meantime, and ongoing, stop watering, you're watering way too much. I don't know what part of the world you're in, but most cactus can go weeks without any water at all, particularly during the winter months, when they should be watered very sparingly occasionally just to prevent shrivelling. I wouldn't suggest you water it at all at this stage for at least a month, and then see how its doing - even when you do start watering, just a little maybe every two weeks, or less often if its winter where you are.


The purple streaks may mean its already too late and a fungal infection or rot has already set in and the plant will just die, but if you want to try and save it, follow the advice given above. In the meantime, I will spend more time researching what plant it could actually be. if its one of the Orbeas, they are at real risk of rot if watered too much from autumn through winter, though they need more water once spring arrives, at which time, rot should not occur.

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