Cacti can be capricious plants, expecially because of their roots. It can happen, in fact, that despite all the cares we dedicate to one of our succulent plants, it stops growing, stops producing thorns and flowers and, in the space of a few weeks (or months, in some cases), begins to deteriorate, deflating and turning yellow. At the origin of this phenomenon, not always a pathology exists, as a bacterial attack that can cause the rot. In the same way, the cause may not necessarily be due to a parasitic attack. If you look closely at the plant, for example, you might not find any traces of spider mite or mealybug, the two main pests of succulents.
With experience I have learned that when a plant, even apparently healthy (i.e. not affected by parasites or bacteria) and grown in the best conditions (light, air, watering, soil, etc..) begins to deteriorate deflating and yellowing despite watering, it is always better to remove it from the pot and check the health of the root system. More often than we might think, the problem can hide just there, below the collar.
In this article, we will see everything we can do to save a cactus or a succulent plant in evident difficulty or when, after having flared it, we realize that it has lost all or part of its roots.
Many plants can suffer a sudden stop in growth, perhaps caused by sudden changes in temperature, and some can remain “stuck” for a whole year. Others may suffer from repotting and stall for several weeks. But if I notice that the stem does not swell after a few heavy waterings and instead begins to shrink and shrivel, I remove the plant from the pot without thinking twice. In nine cases out of ten, this is the real cause, and it’s hidden in the soil. It’s invisible until the plant has been taken out of the pot. The root system could be rotten or otherwise compromised, or infested with root mealybugs (small white insects that are easy to spot to the naked eye).
Fortunately, thanks to the “frugal” and as natural as possible type of cultivation I’ve adopted for years, I’ve rarely seen strong attacks of root rot. Even lesser, always related to my experience with plants, are the cases of root mealybug. In fact, the two or three samples I’ve had to treat were on plants that I’d recently purchased and had not yet repotted.
In all these cases – rot and parasites – it’s necessary to act radically, eliminating all the compromised roots, shortening the healthy ones and treating the plants with specific products according to the case, literally by soaking the cactus in water and its fungicide or acaricide. Next, you let the plant dry out well in an airy, shady place, keeping the roots in the air for at least a couple of weeks, and then repot in suitable potting soil.
But cacti, as I said, can be fickle. Sometimes, after repotting the plant, you can find that the potting soil is not infested with mealybugs or other parasites (such as the terrible nematodes, or eelworms) at all and, much less, the root system is affected by rot. In other words: the stem is not attacked by any pest and, except for being deflated, does not present any problem; at the same time, no root rot is taking place, neither bug has attacked the plant’s roots.
However, if we give the loaf of soil a shake, the roots shatter between our fingers, break into small pieces, and fall off along with the potting soil. The result? After the good news, all that’s left is the stem and a few root butts. It can happen. I haven’t identified the exact cause yet, but it happens. Simply, the root system has dried up (this can happen, for example, after the prolonged drought of the winter months) and the plant couldn’t generate new roots. The sap has dried up, and the old roots have dried out. Among other things, this problem can also be caused by asphyxiated and old soil: it’s important to remember this. Anyway, it’s no big deal, and it’s better to find a root system that does not exist any more than with a root rot that has reached the collar of the plant. Sure, it’s not the best, but there’s a solution.
How to proceed
Cacti, and succulents in general, are very robust plants and can withstand long periods of drought, as well as being able to cope with the adversity of various kinds. If they lose their roots – unless they are Melocactus with the cephalium already formed – they can survive and recreate the entire root system without too much difficulty. To speed up this process, and help the plant to recover, some simple tricks can be useful.
First of all, the end of the stem needs to be cleaned, removing all the old soil and dry roots. You can use a brush or an old toothbrush and clean it hard, even risking to get the collar of the plant: if the roots are dry, they are simply useless and, indeed, can be an obstacle to the release of new rootlets. We can readily notice if some roots, especially the thicker and fleshier ones, are still healthy: viable roots are elastic, and the sap inside them has a light tint. These roots should obviously be preserved as much as possible, at the most they can be shortened a little by cutting off the tip.
Once we have finished the root cleaning, even if with very little in hand, it’s opportune to leave the plant in an airy and shady place. Here the plant can stay even several weeks without any risk. On the contrary, I’ve noticed that cacti that are left with the stem in the air, often produce new roots faster, probably stimulated by the environmental humidity.
Once two or three weeks have passed, if we want, we can repot the plant in a new vessel, possibly of a size slightly larger than the diameter of the stem, since the root system is not yet formed. To help the rooting there’re also specific products on sale in nurseries (they are rooting hormones), but I have never used them because if the plant is not sick you just need patience.
What substrate to use
To help the production of new roots is very useful the pumice of small and medium grain size (from 2 to 6 mm), which retains moisture releasing it little by little, thus helping to maintain a certain level of humidity and at the same time avoiding water stagnation. It’s possible to repot the plant in pumice only or fill the pot with soil up to half, completing with a layer of pumice later.
Fine sand, which retains moisture, is also useful. You can fill the pot with pumice and create a superficial layer (no more than half a centimetre) of sand. Silica sand (the kind used for aquariums) or ordinary river sand found at building material retailers is excellent for this purpose.
The repotted plant, for obvious reasons, cannot be treated like all the others. It will need to be in a bright place, but not in direct sunlight, and should be protected from the rain. In fact, more than real watering, for the first two or three months is very useful to have frequent misting (especially directly on the soil), and light watering. Within a couple of months, if the plant is healthy, new roots can form, and the stem begins to regain vigour. In some cases, more patience is needed (I’ve had plants that took a whole season to recover), but with the help of pumice, rooting is practically assured.
When the plant has fully recovered, and the stem is back to the right colour and thickness, we can reposition it in full light and water it regularly. Generally, if I repot in pumice only, I wait at least a couple of years before repotting in real soil, to give time to the specimen to reform all the root system.
Here you can see a video from the Youtube channel of Il Fiore tra le spine in which I show how I proceed with plants that have lost their roots.
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