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I have never rooted a cactus from a cutting. I have a Mammillaria microhelia that has become too tall and floppy. I would like to rejuvenate the plant by taking a top cut and rooting it. I would also enjoy the learning experience. From all the references to propagation throughout this forum I infer that it is not overly difficult. I have searched this forum for hours and not found a detailed step-by-step for rooting top cuts. Perhaps I have missed it. If so point me to it. Some concerns that I have are: how to deal with the mammillaria tubercles with spines around the bottom edge of the cutting, how to know when the callus is fully formed, how deep to plant the cutting, how to water the cutting? I am ready for a learning experience.

Yes, it's not difficult.

The tubercles around the bottom edge of the cutting you cut off - bevel the edge of the cut (best to first clip the spines, fingernail clippers work well). The reason or this is skin of the cactus will shrink less than the interior as it dries, causing the bottom to cup, by cutting the corner back at the start you avoid this. Here's a site that shows it, though I think they trim the corner more than necessary: https://cactus-info.com/propagation

For your second question, how to know when it's callused enough, just use your best judgement - as long as it's thoroughly dried you should be okay. Large cuts need to callus longer, but M. microhelia is not a large cactus.

Precise potting depth isn't important - not very deep, just so it will stand up. Clipping the spines on all the part that will be buried is not a bad idea. As the link above says, pot it in dry medium - I recommend pure grit for the initial rooting, but many people do fine rooting in regular cactus mix (best to sterilize it).

Don't water until roots begin to form - though a little light misting may help encourage it if it's slow to root. Water shallowly at first, gradually more deeply as the roots extend farther.

Thanks Spence. Great info. What is the difference between grit and sand? I had been planning to use off the shelf cactus mix by Espoma. I could microwave it. I see what you mean by the dramatic bevel on the cutting shown in the link. I think I will make a less dramatic bevel. What is the best way to suspend the cutting while it is drying? Now I just need to wait until the plant stops flowering, which it has been doing since early February.

Grit means coarse, gritty sand - "sharp sand" - to fine gravel. Fine sand should not be used.

You can just lay it on its side to dry. Turn it frequently - if it lays in one position too long, it may send roots out the side that's on the bottom, and cylindrical cacti will start to bend to get their top pointing upward.

I finally gathered the courage to take a 2½ inch cutting off the top off my M microhelia. I found that manicure scissors worked well to cut off tubercules around the base of the cutting. I also made a small bevel. I made no further cuts on the mother plant. I was surprised how fast the wounds on the mother plant and the cutting shriveled and shrank. (I love alliteration.) These photos were taken about 30 hours after cutting. Do you have any idea how the mother plant will respond to the loss? I was thinking of tossing it.

Many ball shaped cacti will grow pups all around the top of the plant at the areoles. Let it grow and you may be able to cut off a bunch of new babies next year.

Agreed - and those pups, taken off at a joint, will root more easily, since the joint contains meristem cells which can quickly develop into any type of growth including roots.

I don't like that cupped top on it, though, it can hold water and lead to rot. I forgot to mention that you should cut the top of the mother plant at an angle so it can drain. It will be okay if it's kept indoors and you never water from above, but it would take just a second to re-cut it.

The cutting looks good, I think you beveled it just enough. Yes, the shrinkage is quite dramatic - now you see why it's best to bevel the edges of the cut1

I agree with Spence on the cup shape-didn't even think of that till now, but it is so obvious in hindsight.

Well, if I toss the mother plant there is no need to worry about the cup. On the other hand the shrunken wound is approx 1/3 the area of the original cut. You know ᴨr². I doubt it will get much smaller, but if I bevel it, the wound will stay large, don't you think? It is a windowsill plant and won't collect water, but I have limited space and am seriously considering tossing the mother plant. I am just curious how the mother will react to the cut. Undecided.

Beveling the cut has certain advantages. For example, it might help to stop water from collecting on top.

loyall wrote: ↑Fri Apr 29, 2022 1:52 pm Well, if I toss the mother plant there is no need to worry about the cup. On the other hand the shrunken wound is approx 1/3 the area of the original cut. You know ᴨr². I doubt it will get much smaller, but if I bevel it, the wound will stay large, don't you think? It is a windowsill plant and won't collect water, but I have limited space and am seriously considering tossing the mother plant. I am just curious how the mother will react to the cut. Undecided.

I would at least keep it until you see if your cutting takes. It probably will, but still good to have the backup.

You wouldn't need to cut it clear around, just slice some off one side.

I potted the M microhelia top cut today in cactus soil mix. Here it is. You can see this year's bud scars.

Hope it roots well for you. That soil looks a bit too organic in my opinion, but if you control the watering, it shouldn't be a problem.

Yep. Don't give any water until roots start to grow. You can just pull it up and look - but if there's resistance, don't pull it up, it's rooted down. Water very shallowly when it first roots, gradually give more as the roots go deeper. Caution is the key, better to under-water than over-water.

I did it twice, with Echinopsis sp. and Cleistocactus straussi. I cut the bottom part to make it conical and then placed it in the empty pot in vertical position hanging in the air (paper towel helps very much). When the callus formed and new roots started to appear, I just potted it as any other plant. It worked perfectly.