Question
I have 3 large barrel cacti which have been living contentedly in my flat for 2 years.  Recently I noticed that they have yellowed around the base.  This yellowing has spread and killed one and is starting to take hold of the second.  The third has the initial signs at the base.  

It does not appear to be from overwatering, they have yellowed and shriveled like deflated basketballs but are not mushy.  I have repotted them all after noticing that they had very little soil in their pots (I did this as soon as I noticed the yellowing, so this wasn't the cause).  I think that perhaps I may have not watered them enough.  

I'm hoping not to lose the second two - can you advise whether this may have been caused by lack of watering and if there is anything I can do to save the 2 remaining cacti?  Advice on how often and how much to water would be much appreciated.

Many thanks,

Jo

Answer
Dear Jo,

It does sound like desiccation and having pot bound plants..  Under watering can kill cactus as easily as over watering, tho we are usually guilty of the latter. When the cactus is pot bound even if you do water, the water can't penetrate the roots. So it sounds like you might have both problems.  In addition, when a plant gets potbound most of the nutrients in the soil are gone. To try to reverse the desiccation it was a good idea to repot and that when you did, you loosened the roots and removed the dead ones.  I would repot in pots an inch or two larger than the ones the plants were in. Then, you'd think you could just saturate them but you can't.
It is much better to water only a little bit at first and gradually increase the watering over many weeks. This is largely due to the fact that desiccated plants will have lost many of their healthy roots and will only be able to take up a little water at a time. As the roots grow, it can take in more water. If too much water is given and the roots can't take it all in, the plant will start to rot from sitting in damp soil.

I can't tell you exactly how much to water because I don't know your climate or the size of the plants.  Hopefully you repotted in well draining potting soil that will let new roots grow and also not hold too much water.  When I bring my cacti out of their imposed dormancy, I water a little every 2-3 days because I know the fine roots have died from no water over the winter, and gradually give more sunlight so I'd suggest something similar. Temperature has a big effect on the rate that desiccation occurs. The hotter it is, the faster it dries up. Typically, when temperatures are high, cacti want water, light, and will do their growing under these conditions. When the water runs out (or the heat gets too high), then the plant will shut down and sit dormant until conditions improve. If conditions don't improve eventually, it will die. In my case the winters are cold and dark and I keep my plants in an unheated room. I know that as soon as it starts to get warmer and lighter I need to get to work.  This is also when I at least look at my plants roots and repot if needed.  Despite the controlled dormancy I've lost a few plants because I didn't realize how pot bound they'd been.  But let's not get into how dumb I can be. You put yours into dormancy unintentionally by underwatering and poor soil/room so desiccation started.

It's really important to repot your plants every year or two and also when you see them taking over the pot.  You need to check the roots to see if they're healthy, make sure there is sufficient soil.

Temperature has a big effect on the rate that desiccation occurs. The hotter it is, the faster it dries up. Typically, when temperatures are high, cacti want water, light, and will do their growing under these conditions. When the water runs out (or the heat gets too high), then the plant will shut down and sit dormant until conditions improve. If conditions don't improve eventually, it will die.

As for watering the more light and heat, the more water, keeping in mind that a healthy cactus can do well for a long time without much water especially when it is cooler and darker.

Make sense?

Hope this helps.

If I've confused you let me know and good luck
Maureen