has it gone soft? Looks like it's collapsing wrinkling... too much water, pot too big?

There's no such thing as a pot being too big, commercial growers plant small plants in big pots all of the time. If this cactus is rotting it is for some other reason. And since it is in an indoor environment, a coarse soil-less potting medium and a clay pot it might actually be instead needing to be watered again!

Unfortunately we here looking from afar cannot tell if your plant is puckering due to dryness or collapsing with rot. Or neither. If it might be relevant to your situation I think when I had a small one of these (Echinocactus, if that is what we are talking about) for some years inside ordinary rooms I seldom watered it during the winter. But this was well within a comparatively cool (50s and 60s F) house in a dull climate, with no supplemental lighting placed directly over the plant.

I disagree with your statement that there's no such thing as a too big a pot. Commercial growers know what they are doing. Growers on here who overpot don't know what they are doing, that is why they have the problems they continually ask us to help them solve! The pot size isn't the 'cause' of the problem, watering probably was, or the soil it was growing in, or indeed other factors....but, the oversized pot didn't help one bit. Just saying : /

Gill UK

I agree that there is no such thing as too big of a container. Caveat: as long as the substrate is on point for the specific planting.

I am a commercial grower, small scale.

More important by far than container size is soil condition. The soil for this plant looks 1) far too fine and water retentive- thereby drowning the root structure or 2) far too likely to become hydrophobic and deny the root structure adequate moisture when it does receive watering.

If this was my beloved plant, I would

1) pull it out of the pot and rinse off all the old substrate from the roots with running water (I use the 'flat' setting on my hose end sprayer).

2) inspect for rot and cut away any found

3a) if rot is found and cutting is necessary, cut away all rot with sterile instrument then dust with a fungicide or ground cinnamon and let the plant callous over in a brightly light and dry area for 3 weeks

3b) if no sign of rot, allow to dry for 1-3 hours (or blot roots with paper towel), replant in coarse or rapidly draining mix (many suggestions available on this site), place planting in bright to partial sun, withhold water for 3 weeks.

4) water from below for the following month (weeks 4-8) until actively growing (respective to zone and weather conditions) and perhaps fine mist once or twice a week to prevent desiccation (probably unnecessary but we just love to love on our plants).

5) after last frost, place the planting outside in morning sun for 1 week. Then transition to some afternoon sun for 2 weeks. And finally, place in full sun if possible.

6) these types of barrel shaped cacti rarely require supplemental watering. Instead, be wary of too much rain in conjunction with humidity. Pull them under cover during rainy weeks unless your substrate is extremely fast draining.

I recently discovered root rot on my very large Mammillaria mystax, which I rescued from a neighbor and am very attached to. They had planted in it garden soil. I cut away the rot, dusted the cut surfaces with ground cinnamon and left it in a bright location for 6 weeks (just to be sure) and it bloomed before I repotted it 3 weeks ago. It's under a 50% shade cloth (all day sun) in my greenhouse and I still haven't watered it. I'll probably wait until March to give it any water at all.

Good luck with your cactus! They are so incredibly rewarding, particularly when they bloom and double in size over a good season with excellent weather.

The myth is that the plant gets too wet because the soil area in the oversize pot is bigger than its roots. If this were true plants in the ground would all rot and die.

It all really has to to with the perched water table. In containers, depending on soil structure, water perches at 10-50+% above the container bottom. This is a zone in which roots die after watering because it is saturated with water ("perched"). In the ground, there is rarely a perched water table because the soil beneath wicks away moisture, almost eliminating the PWT. We can correct for this problem in containers by providing a coarse and well drained soil structure (the gritty mix is an example), or by installing a wick at the drainage hole.

As an aside, with my very very favored plants, I will water thoroughly until it courses from the drainage hole or water/ soak from below, wait until drainage slows, then... It's hard to verbalize...

I'll hold the container with both my hands, begin at hip level, and then lift quickly to head level. Lower slowly, repeat. This causes a centripetal force that yanks out excess water from the bottom while pulling in oxygen from the top. I do it until the water shed dwindles to nothing. I look ridiculous doing this. But my favorites don't. It's a good shoulder workout. :-p

Emb....I beg to differ, err no! If the soil is sodden and the water is contained within a pot due to restricted drainage and retentive mixes.....that is why it rots. Plants in ground have a far greater area for excess water to be dissipated, that is why they don't rot.

Dkstenia, I have also done that very same procedure to many times to mention! Your perched water also explains this well. As I said a large pot combined with the wrong mix is doubly wrong, and I believe you agree in principal.

Gill UK

On the one occasion I watered, it was a very small amount-- the smaller end of a jigger! I was and still am so worried about overwatering that I just supplied a very small amount. I checked for rot last night and didn't see anything at all. The soil was bone dry.

You're in zone 8, I'm in 8b (central Texas). I wonder whereabouts are you? It's, rule of thumb, better to provide full watering rather than 'sips'.

Today, it's sunny and a high of 57F. I haven't watered any of the Mammillaria at my home in over a month. I probably won't water those until March. Remember that these types of cacti (the heavy barrel shaped) can go extremely long periods of time between watering. By long, I mean a year in my zone in a similarly sized pot and soil (a bit water retentive) before showing drought stress. As long as you provide bright light in a western or southern window (or your brightest window), you can likely and safely forget about it. Just rotate the pot once every other week to prevent leaning (which is probably what it's doing now- which is not a great concern).

One of the problems with that type of soil and withholding water for so many months is that the substrate itself may become hydrophobic and when you do take up watering again, water may drain slowly and even simply sit up top for quite a while.

If you don't want to change the soil:

What I would do in spring after the cacti start producing flower buds: I drench the soil with water that has a surfactant to break that hydrophobicity (a soap like Dr. Bronner's or detergent like Dawn (no bleaching agents)) and a very weak concentration of a high phosphorous and low nitrogen fertilizer (1/10 of the package recommended dose and I add a tsp of soap per gallon of water). This will aid in bloom development and prepare the soil for active growing conditions. The soap will not effect the plant, just the soil. Keep the pot under some cover to prevent rain access but in a spot that allows lots of light.

Mostly, I would suggest repotting in a coarse mix. Home Depot and Lowe's carries a cacti and succulent potting mix for about $5. It will do the job. There are far better soil options, but they are a bit more time consuming and expensive to locate and gather materials. Since you stated that this is your first experience with a cactus, I will assume you are a beginner and that these other options may not yet apply to your interests and life bandwidth. Though, you can google shop for 'gritty mix' and determine if the price is viable for you. Push come to shove, you can convince me to ship you some mix for shipping cost- I appreciate your new interest and concern ;). And information on a grittier mix will be available for you on this site as good reading when you are ready. I'm also keen to resources for getting the ingredients for a gritty mix.

Should you choose to repot, put on some protective gloves and hose off all the old soil from the roots. Don't be afraid to blast off the peat and take the roots to 'naked'. Allow the roots to dry for a day in a protected area (dry and frost free, preferably with good air flow). Repot in the mix of your choice. Do not water until you see signs of growth (or until March).

Zone 8 is quite warm. I want to suggest taking your cactus outside on nicer days, but then we run the risk of forgetting on cold nights and I don't know how your Mammillaria will fare in freezing temps.

I'm in New Orleans, specifically. Just got back from a local garden shop, and the staff there compared my cactus to a few of theirs. It doesn't feel soft nor look severe, so they seemed confident in telling me it's just the shape of the cactus.

My soil mix is MiracleGro's cacti and succulent potting mix cut with about 40% perlite. I have the same mix for a couple other plants and it's seemed to drain well.

Living in an apartment complex, it's hard for me to find a good, safe outdoor spot with good light, so on nice days, I've just been opening the window where they sit for the time being.

Sounds like I'll hold off on watering for a while, and when I do water, I'll refrain from the small "sips". If the soil does become hydrophobic, I'll take up your advice on how to remedy. Thank you so much!