I have a prickly pear cactus at home for about 8 months, and I put it near the living room window. Over the last 2 months its color has changed to a strange yellow, the spines are still normal but the body looks different. I've watered it about 3 times this month because I thought it was lack of water. What could be happening?
Cut back on watering urgently - a yellowed cactus with frequent watering is almost always from too much water, which rots the roots. Let it dry out a lot between waterings (like, only when the soil is completely dry) and put it in a pot with drainage holes and well-draining soil. Prickly pear can handle dryness, but it can't handle constant moisture. Also check if your window faces too far north or if there's cold air drafts - weak light or too much cold can also make the color look off, but that's less likely if it's a living room window.
Stop watering immediately - excess water on a cactus is basically a slow death sentence! What's probably happening is root rot, and with 3 waterings in a month you've created the perfect environment for it; prickly pear cacti can handle weeks without water, so let the soil dry out completely before watering again. The other possibility is serious lack of light (even near a window, if it's an interior room without direct sun, that won't cut it), which also causes that uncomfortable yellowing.
The yellowing can be more than just overwatering, even though that really is the most common culprit. But before you panic, let me break this down: if you've watered 3 times in a month and the cactus is near a window (plenty of light), there are two likely scenarios here. One is straight-up root rot, which starts exactly with that yellowing color change. The other is nutrient deficiency or weird photoperiod issues - sometimes a window with intense direct sunlight in summer, followed by low light when the seasons change, causes this effect. A prickly pear is more resilient than most cacti, so it's not exactly dying slowly, but you need to act fast.
First, stick your finger deep in the soil - if it's very wet or smells moldy, water is the problem and you need to stop watering almost completely. Let 2 to 3 weeks pass without watering at all, not a drop. Then, when you touch the soil and it feels completely dry (really dry), only then water again, and just enough to dampen it. Second, check if the pot has drainage holes - a lot don't, and water gets trapped. If it doesn't, consider switching pots. If the soil is too compacted or has regular potting soil instead of cactus-specific substrate, that's another problem: regular plant soil holds too much water. A good mix for prickly pear is coarse sand, perlite, and some peat moss or cactus soil.
If the soil is actually dry and water isn't the problem, then you could be looking at nutrient deficiency or light stress. Try moving the pot a bit (maybe not right in the hottest part of the window during the day) and when you water again in 3 weeks, add a very diluted cactus fertilizer - but just once. Wait another month or two before fertilizing again. The cactus can recover, but it's going to take time.
Did you check if the soil is actually waterlogged right now, or did you just water it without thinking? Those guys up there are right that too much water is the culprit, but that amount of watering (3 times a month) only becomes a real problem if the soil isn't draining properly. Prickly pear handles being too dry way better than being too wet, so the thing is to stop watering completely for a while - let it dry out for about two months before you start again, and when you do water it, use very little. If the roots have already started to rot, sometimes you can save the cactus by cutting off the healthy part and replanting it in fresh, dry soil.
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