How long exactly do succulents need to dry their roots?

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A succulent enthusiast showed me photos of a succulent whose roots had just been trimmed and asked me how long it needed to dry its roots. After looking at the photos of the root system, I told him to let it dry for 1 day, then pot it, and then wait one more day before watering.

Later, another succulent enthusiast showed me photos of a succulent whose roots had just been trimmed and asked me how long this one needed to dry its roots. After looking at the photos, I told him to dry the roots for 3 days, then pot it, and then wait one more day before watering.

I answer several questions every day about how long to dry roots, so I didn't realize that these two questions came from the same succulent enthusiast. As a result, the consequences were serious. This succulent enthusiast was enraged by my "extremely irresponsible" behavior, and pasted both photos to me, asking why one needed to dry for one day while the other needed to dry for three days? You have to give me an explanation. You say dry for however many days you like, can we still happily grow flowers together?

So I think it's necessary to talk openly about the matter of root-drying time.

First, why do we need to dry roots? This is easy to answer: for fear of black rot.

Then why does black rot happen if you don't dry the roots? This is also easy to answer. Because when you cut the succulent's root system, a wound appears at the cut surface. The thinner the root, the smaller this wound; the thicker the root, the larger this wound. Regardless of wound size, after a root is cut, it has a structural feature: from outside to inside, the cut surface is divided into two parts, the cortex and the vascular cylinder (of course, scientifically speaking, the structure of roots is much more complex than what I'm describing). This structure is like a water pipe. The cortex cells can absorb water and then channel the absorbed water to the vascular cylinder, and then the plant can transport water through the vascular cylinder to all parts of the plant body.

When we cut the root system, it's like cutting a water pipe, and the vascular cylinder at the core of the pipe is exposed. This place is like a high-speed channel leading to every part of the succulent's body. Once fungi enter here, black rot will undoubtedly occur, and the speed of black rot will be very fast. This is what we often call vascular-level black rot. Nothing can be seen from the outside, yet the leaves have already been dropping all over the ground.

So we must dry the roots, giving the succulent enough time to repair these wounds one by one before potting and planting.

Some succulent enthusiasts notice during the root-drying process that as long as the succulent is exposed to sunlight to dry, no matter how large the wound is, it will quickly become dry in no time. Even a thumb-thick head that has just been cut off, as long as it is sunned, the stem tissue can quickly dry and close up. So they think this means the wound has been dried, and they can't wait to pot it.

But don't celebrate too early. If you pot it at this time, you have already planted the seeds for black rot to occur.

Because the wound that looks dried to us is actually just the cell tissue exposed on the surface that has been sunned to death and lost water. The succulent's wound itself has not undergone very rigorous repair. This is like a squad of soldiers guarding a key traffic route. With one snip of the scissors, a large group of soldiers is severely injured and begins to bleed. So we start baking these soldiers in the sun. The sunning causes these soldiers to die, and of course they stop bleeding. But this does not mean the remaining soldiers have formed a defensive formation and are standing ready. It merely means the intersection has been blocked with the corpses of dead soldiers. Such a defense is quite fragile against the enemy.

The succulent may undergo complete black rot in the coming days. Although the probability of this complete black rot occurring cannot be considered very high, it is still very worth taking seriously. Even if complete black rot does not occur, varying degrees of rot will appear at those wound locations, which will seriously hinder the sprouting of new roots, ultimately leading to many problems such as the succulent not taking to the pot for a long time and the original root system gradually necrosing.

So true wound repair is not drying, but rather forming a thick layer of callus tissue on the wound surface through cell division, thoroughly isolating the environments inside and outside the wound. Only then is the wound repair complete. The formation of callus tissue mainly relies on a method of amitosis. Amitosis is somewhat different from ordinary cell division. The DNA material in the cell is not copied very precisely during amitosis. The cells of the healing tissue just keep roughly dividing: the nucleus simply splits in two, the cytoplasm also splits in two, and then a new cell wall is established, and the division is complete. This very rough method of cell division is well suited for repairing wounds, because amitosis does not require precise DNA replication, so it consumes very little energy and can rapidly generate new cells. These new cells have no special function either; they are just used to wrap up the injured plant body.

But things are not that simple either, because only the succulent's epidermal cells are good at this kind of rapid amitosis. The internal vascular tissue is not good at doing this. Therefore, all repair work proceeds from the periphery toward the center. Everyone knows the formula for the area of a circle, right? If the radius of the wound increases just a little, the area of the wound grows geometrically. And the callus tissue must divide out enough cells to fill the entire cut surface.

If a thin little white root breaks off, the succulent may need only 10 minutes to repair this wound. A root system with a diameter of 1 millimeter may cost them 1 hour. What about a 2 millimeter root system? Perhaps 1 day is not even enough.

So the thicker the root system, the longer we need to dry it, waiting for the succulent's healing tissue to form completely. When the healing tissue has formed completely, that wound is no longer a wound. It will have the same defensive capacity as uninjured tissue and will no longer fear fungal invasion.

Of course, measuring the diameter of the wound and then calculating the area and finally deciding the root-drying time is definitely not a reliable method. I have a very simple method for estimating root-drying time: dry for 1 day for every 1 millimeter of wound diameter. Generally speaking, simple root trimming will not damage the thick main roots, so depending on the thickness of the cut root system, drying for 1-3 days is enough. If you have beheaded the succulent, this calculation method is equally suitable. A 1.5 cm beheaded seedling can simply be dried for fifteen days to let its wound heal, and after 15 days it can be cuttings.

The principle of watering after potting is the same as that of drying roots. During the potting process, we often bruise some roots. These are all very small injuries, and they will repair these wounds quickly, so waiting until the next day to water is a good habit, because by the second day these small wounds have mostly healed.

Developing the good habit of drying roots will be very helpful to you. Some cases of black rot that occur a few months later are very likely related to your not drying the roots well at the beginning. At those wounds, the callus tissue and fungi are constantly engaged in a tug-of-war. If the formation of callus tissue gains the upper hand, then nothing happens. If one day a few months later the callus tissue is defeated, heh heh, black rot occurs mercilessly. You may not even know that you planted this hidden trouble months ago when you were in a hurry to pot.

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