Sounds like it's either getting too much water or getting too cold when it does get water. Take it out of the soil, Cut off Any soft or rotten parts. Put it in a sunny window and leave it there for a few months. Do not plant it, just lay the pad (or lean it) on the window ledge. Eventually it will form a callus on the cut edge. If you're lucky, it may even start growing roots. Eventually you can put it back in a pot with New, WELL DRAINING soil and DO NOT Water it until you see new growth (unless it had some roots when you planted and then wait until spring). In the spring (after the last frost) put your cactus outside if you can.
Good luck.

If it's not clear, soft & mushy equals rot. One needs to cut it all the way back to the point of firm, healthy looking tissue.

If you have any cinnamon (like used in baking) at home, try sprinkling some on the cut end where the rot had been. It acts as an antifungal & MAY stave off further rot. Good luck!

Living in central of Sweden sun is quite rare now in wintertime, just a few hours between 9am and about 2pm.

I have cut off the soft parts and put it on a plate for now. I could put it in the window but the sun is very sparse and it's also a bit colder on the ledge (outdoor temp is below 0 degrees Celsius) so would it be better keeping it on my desk?

Lots of thanks, Viktor

Here is a link that might be useful: {{gwi:730504}}

Viktor,

In times of little sun, little to no water for the Opuntia is best - it will shrivel a little, but when there's more sun (in the Spring) you can begin watering again and the plant will plump out and begin growing again. However, as the other posters have noted, you must remove all non-healthy tissue for the plant to survive. Opuntias are difficult to kill, so it's hoping your plant will make a full recovery.

Here in Calif they can take frost and lower temperatures,
we grow ours in well drained coarse sand, pumice, only 10% oranic mix. They like low temps. in the winter and no water at all they store water in their plump leaves, they need to go dormant, then they will flower in the spring. The only way to kill an opuntia in my experience is to over water the plant, do what Pirate Girl tells you then put it under a reading light at least 6 hrs a day. That will keep the temps warm for it as well. It will grow under the same conditions that you like to live in. it doesn't need a massage, or soup for lunch, do not fertilize the plant, Yours sounds like it derasticlly needs more light. Yes and do cut off all of the mushy stuff, keep only a firm leaf, even a piece and restart it with that it is the wrong time of year for this, but do what you can. The main problem is that you are growing a plant that comes from the S. Western U.S and Mexico. No. of the Equator, The New World, and it doesn't like it and it is really protesting. Grow only plants that like your climate, you really need special growing conditions where you live and experience. You need to provide that for growing New World plants. This plant will not objecct to the cold but will object to water, and lack of light. It's the water that is doing the damage and I am suspicious of the soil. Most TV has heat on the receiver, so do the water heaters on top, but they MUST have light. Use a 60-100 light bulb, or bedside reading light, put it under that 6 hrs a day, this light bulb will also provide the heat. Or use a snake or lizard heat pad and set the plant on top of that, put a light in an aquarium, that light will work as well, or use hot a hot water bottle or water bottle with hot water for the heat. Just be creative.
I need to ask you this do you know anyone where you live growing these out side in open run. ThAT long new growth is caused by etiolation, the bleaching or loss of chlorophyl caused by insufficient light and resulting in elongated internodes. Put it near the window it's won't mind how cold it gets in the house, unless you are wearing a fur coat, and if so put one on the plant as well. You may want to try and start it from those leaves. Don't take them all off just enogh to get a new start, it if works then do them all. Don't water until you see new roots, put it on a heating pad to achieve this. Norma

First of all, thanks for all the replies!

I got the cactus in Greece ("caught it" in the wild) some 10-11 years ago and it has been happy since then, I moved a couple of months ago and that's when it started looking sad. Before moving it has always wintered in a basement. The soil is cactus soil and is the same type that it has always had. I have improvised a heating pad with an external hard-drive, the temperature is about 35 degrees Celsius (97 fahrenheit). I have also put a reading light on it now about 1.5ft away with a 60W bulb.

I am not sure whether to keep what is left of the round pad, it is not mushy but has gone a dark green color which doesn't look healthy. I have put cinnamon on the cut like Pirate_girl said. The long growths are looking healthy.

I'm not sure what "growing in open run" but if it means outdoors then no, it is usually below -5 degrees Celsius (25 degrees F) and down to 0 degrees Fahrenheit on cold nights in the winter.

You figured it out what open run is. Keep it dry, give it light any way you can. and perhaps put it on a heating pad, keeping the temps below 90F It does like cold but needs light, the top of water heater, the top of warm surfaces, PC receiver, no water, this will help make good roots. This plant is a native to the new world. You need to give them these conditons, it can't get up and find it. Norma

Cutting away the softened parts is not enough, all of the discolored tissue must be removed.

If the soil is the thing sold as cactus soil ("SÃ¥- och kaktusjord") in Sweden it means peat with some lime and sand. Even if a cactus can survive in it for years it is risky to use, as it retains too much water, becomes too compact with time and fungi thrive in it.

Cuttings do not need to lie in the sun, I always keep them in a shaded spot until they have rooted. It can take a long time in winter, and if it is a big cutting I let it heal until February before rooting it, instead of trying to force it. Actually the whole plant does not need much light now, the 60 W bulb will do more bad than good, as the heat encourages the plant to grow at the wrong time of the year.

The thin shoot is extremely etioliated and should normally be cut off and discarded, but it should be possible to root it as an emergency rescue. The other offset looks better but still not good. An ordinary incandescent or fluorescent lamp will not provide enough light, it needs to be placed in a sunny window, or outside (during the growing season, that is). It should not be watered from early autumn to mid-spring.

I am not sure whose advice to follow but I have separated the two shoots from the pad, it wasn't healthy. So what I have to do now is just leave them lying somewhere without soil or water until they start growing roots by themselves and then plant them?

Isn't the light a good thing if I want them to start growing roots?

The larger cutting is already quite "calloused" and dried up where I cut it, this is a good thing right?

Lots of thanks, Viktor

They should be left until the wounds have healed (which takes weeks or even months), then put in dry soil. Sounds strange, but they don't need water to put out roots. Sometimes I add a few drops of water to the bottom/rim of the pot, but it is easy to overdose, too much moisture and the cutting rots.
Actually I think the absence of light is more important for root formation, Opuntia can put out roots from any areole that is below soil level. The cutting as a whole does not need much light to form roots, it contains enough material and energy to do that anyway. A little extra light does not hurt, maybe it can have some positive effects, but it is important not to put a cutting in strong light/hot air, it dehydrates too fast then. An incandescent lamp produces more heat than light, hence my concerns. Bottom heat promotes root formation, and it might be more effective to place the pot in a window and put the bulb beneath it ...

Okay I have put them on a shaded shelf on top of a router to provide bottom heat. It's about finger-temperature.
Are they going to start rooting before I plant them again or just wait till the wounds have dried up? What type of soil do you recommend?

Sometimes cuttings do grow new roots before they are potted, you could try covering the lower end with a black cloth for a few weeks and see what happens. The wounds should not just dry, they should preferably form a hard scar tissue.
When rooting I usually use cat litter, the reddish, fired, non-clumping type (marketed by COOP and Willy:s in Sweden), but coarse sand works too. Pumice is useful as well, but useful forms are difficult to find. Opuntia can be rooted directly in a proper soil, which I make by mixing mineral garden soil/loam (20% by volume perhaps) with cat litter, pumice, Leca + some fertilizer.

My two cuttings have been lying on a shelf for about a month now and are both hard and dried up where the cut was made. The larger cutting is looking pretty much as it did when I cut it but the smaller one has gone a bit floppy and isn't as solid as the large one.

When I plant them should I just stick them in the soil? How deep should I put them and should the soil be completely dry?

1 Yes.
2 Depends on size, about 2 cm should do. Some of the areoles should be below soil level, it is often easier to get roots to grow from them than from the cut area.
3 Yes, to start with. The cuttings are still vulnerable to mold. Wait for a week and give the soil a light spray, repeat when the soil has dried. If they are in a warm place they can be sprayed more often. The exact procedure is best learnt by trial-and-error, I'm afraid.

The idea is to make them grow roots close to the surface while keeping the cut end dry for a while. It is usually easy to see whether roots are formed or not if the uppermost soil is carefully moved first. It will take a couple of weeks for the average Opuntia.

This usually works for me.

Victor,

Follow TJ's advice. Aside from living near you (I think) the advice is excellent. Your pieces should be rooted in early Spring and growing well in Summer.

TJ,

I just knew you were a Mister!