It seems like dehydrating and drying up from the tips. Above heat vent? Could you point out in the photo where is the location?

I saw an african violet nearby, which requires a bit of humidity....

that's what i was thinking too but the cactus in the small pot on the counter grows all the time and gets air from the vent too. I have pushed the table all the way to the wall to try and block it and am going to order one of those things that redirects the air out instead of up. I water all my cacti once a week or every 10 days. I was worried i was watering it too much but it doesnt seem like rot so I dont know.

Let's start off with what mix is it in? How long have you had it? Did it always have those marks or is it something new? What hardiness zone are you in?

Watering once a week or every 10 days could be a little much in the winter. Especially if the mix is not thoroughly drying out between waterings. I would water once a month during the winter. Most of my cactus I keep completely dry from the end of Oct-Feb. This depends on the temps of course. My cactus are kept outside where it's cooler than your warm home too.

The big Opuntia below will grow new pads all spring/summer and even keep going into the fall, but once the temps drop and the light changes the Opuntia will put a stop on all the new baby pads. Here's a pic of a baby pad that stopped growing.

One of my fav things is to flick these dried up pads off;). It's the little things;).

I'm curious if that's why yours shriveled.

Honestly your cactus doesn't look too bad. Despite the couple pads that are shriveled. Cactus don't stay looking perfect. They will get corking and marks. I really don't know why those two pads are shriveled. Maybe a fungus idk;).

Your cactus doesn't look thirsty. The other pads look hydrated.

I will warn you if this cactus lives inside the new growth will be long and lanky. Opuntia need lots of light to grow those squatty roundish pads.

Above the heating vent will actually keep the cactus aerated. It needs good circulation. Do you know what type it is? Actual species and is it cold hardy? Cactus like this one only need to be lightly watered about once a month in the winter and not too much more in the spring-summer. They are desert plants. The plant needs the brightest sunniest window possible.

It is in a very gritty looking mixture. I have only had it for a few months. Those marks are new and keep getting worse. I wish my cactus looked as nice as yours! Ok i can try to withhold water for awhile and see what happens. I live in zone 6 I believe? It's what came up when I googled it. I live in Spokane, Washington. I dont know if I should try to give it back to the greenhouse I got it from or keep trying, but I really don't want to kill a 17 year old cactus! This is the sunniest window I have and it is a western facing window if that helps.

The tag says Opuntia macrocentra

From what I've read your Opuntia can handle your zone as long as it is bone dry. If you have somewhere to put it I would definitely put it outside. Do you have a porch or somewhere it will be protected a little? It would do much better outside. Opuntias are outside plants imho;). Be careful of humidity. Once outside no more water till spring;). Plus the cold and dryness will bring on it's lovely stress color. Purple;).

Here's some Info;) on your friend.

Opuntia macrocentra is usually an outdoor cactus. I am sending you a link so that you can read about it. We grow some of these in southwest Idaho, zone 6 - 7, and they outdoors strictly. The plants go through a very severe dehydration cycle outdoors to protect themselves from the cold, and then plump themselves out again in the spring. Sometimes they turn very purple from the cold. Large yellow flowers and form quite large clumps.

That was why I asked the variety. It will never do well inside unless you have sun all day long, and seldom water it during the winter. They don't look good during the winter indoors or out. Too humid in your area to put it in the ground and try to overwinter it, and the heavy snows would most likely kill it.

I would suggest getting a more house tolerant, tropical like cactus, that can take the indoor conditions, such as the polka dot cactus or teddy bear types. You would still need to put them in that bright window, but they are more forgiving than the one you have. You might look at them at Home Depot or some place like that or see if a local greenhouse carries them. Good luck.

forgot to give you that link: https://en.wikipedia.org/wiki/Opuntia_macrocentra

Another lady had said she has hers under a tree and they winter fine. I'm just scared to kill it. I have a tiny front porch and tiny back porch. I also have a small tree in the back yard that is surrounded by a corner of a wooden fence. Should I try putting it in one of those spots? Or do you think that would be for sure death for the cactus? Currently we have 5 inches of snow and it's only approx 30 degrees. I'm also scared to shock it since it's been inside this whole time. And if it goes outside would I leave it in it's clay pot?

Clay pots tend to crack during winter, but if kept away from snow that could freeze and thaw constantly, they could last. If either of your porches has roof that would help. If my plant, I would transition it to colder location slowly but maybe I am too careful? Do you have garage? That could be place to start with - acclimatizing it to colder temps. After a while there (maybe a week or 2), I would put it on porch. Do not take my word for it - that is just what I would try.

Kara gave you some good tips.

Ok. So I'm thinking let it dry out for a couple weeks from my last watering. Then transition to the garage. Then to the front porch. It does have a roof. It is small but no snow would get on it. It wouldn't really be protected from the wind or anything else though I don't know if that matters? Thank u! And no i don't think you're too careful! I always feel like I am lol. And yes Kara did :) thank u!

Your plan sounds perfect. Lastly I keep beating a dead horse, but keep dry until the weather warms up. This can be hard for people, but don't worry it'll be able to survive the cold much easier when dry and dehydrated.

Ok I will try that plan :) Thank u! And ok no water! Lol. How do I know each year when to start and stop water?

Where I live I usually stop watering in October. Then begin whenever it warms up. Which could be as early as February. Yours will be different of course because you live more up north. So it's hard for me too tell you when to start and stop. Maybe Rina could help a little better than I could in this department because she lives in zone 5a.

I would start watering when the nights don't get any lower than 50F.

I was checking out the average 2017 temps for your city Spokane and from what I can see it looks like you should start to water around the last week of May and stop the last week of August. This will vary of course. But like I said the magic number for me is 50F so no water if nights are reaching lower than that magic number;).

Don't forget the zone you live in there in Spokane. If you keep the plant in a pot, you will need to lightly water it --- like Kara from the Bay area said. But don't get carried away with that. It is a true desert plant. It also has to go through a cold period, several weeks below freezing, which will trigger the blooming cycle in the late spring to early summer. Some winters here in the Boise Valley, these cacti will freeze solid, but that does not injure it. Another reason not to water during the winter. I would also see if there are other folks in the Spokane area that have experience with growing cacti indoors and out. Check and see if there is a cactus and succulent society there or possibly a botanical garden that could steer you into what to do in your area. There are certain cacti that are native close to your area.

I never water hardy succulents during winter; whatever water they get from rain is enough in fall or early winter. And when snow starts, there is, again, plenty of moisture. I do not grow any opuntias (too prickly for me :), just one Austrocylindropuntia subulata and that is not hardy. If I had a hardy succulents under the roof where they doesn't get any snow, I would most likely put a handful of snow on it once or twice - just to get little moisture. Don't forget that snow is good insulator, and potted hardy plants survive well if placed directly on ground in a spot where they get some snow. Mine are covered right now with a more than a foot of snow...

ETA: I googled O. macrocentra+ snow and there are many photos of it (and some other opuntias) here: opuntia macrocentra under snow

That is a beautiful cactus! Thank u guys for all the info. So if i have it where it can get snow it wont hurt it when it thaws and refreezes? Won't it become too wet? And I did email a cactus society that I found too. I will see if they respond.