My hardy containerized succulents live outside in a custom cacti and succulent mix marketed by a local specialty grower. The exact same mix I use for the less hardy ones that need to be brought indoors for winter. And I would guess I receive a significantly larger dose of winter rains in my climate than you would in the Bay area :-) There have been NO issues with rot. Nor do they dry out excessively, even in summer (which is very dry here).

I doubt you need to overthink this.

Sorry to contradict an esteemed forumite, but a google search will reveal that aeoniums do have slightly different soil needs than the average succulent, and do like a moister soil.

Several growers recommend combining 2/3 succulent mix with 1/3 potting soil.

One example:

"To give our Aeoniums a little more moisture, we like to plant them in a less porous soil blend than the one we plant our other succulents in. We blend a little bit of regular potting soil with our favorite succulent soil to create a soil blend that retains just a little more moisture.
Two parts succulent soil to one part potting soil is what we like to go for. It’ll drain a little slower than regular succulent soil and keep the Aeoniums moist, but not too moist."

https://succulentcity.com/easiest-way-to-take-care-of-aeonium-arboreum/

This guy (who seems to know a thing or two about aeoniums) actually recommends straight potting soil:

"Aeoniums perform better in standard potting soils rather than super well draining and nutrient deficient cactus soils. Remember these plants do not like to dry out."

https://davesgarden.com/guides/articles/view/1058

Hmmm......I guess 30 years of growing these plants - including the so called moisture-loving "jungle cacti" (Schlumbergia, epiphyllums, rhipsalis, etc) - in the very same potting media with great success accounts for nothing :-) Those that want more moisture get watered more frequently. How hard is that to grasp??

@gardengal48 (PNW Z8/9) I think it is easy to explain the difference in your experience versus the quoted articles. First, my own experience was that a gritty mix does very well with Aeonium in the Winter. They get enough extra water from the rain to thrive during their growing season, and the soil prevents excess water.

During the Summer is where I have had problems. These plants go dormant and you can get a delicate situation where too little water makes them wilt and die, and too much water makes them rot. Finding the optimal soil for that time of year, that allows for the maximum amount of neglect, is what I am trying to find. I think your results are explained by the fact that you are a diligent expert gardener, and you hand water. I think my results are explained by the fact that I am less diligent and can sometimes leave plants without water for two weeks, and some plants are on a drip system with either too much or too little water coming in.

So gritty mix leaves less margin for error, and you make up for that with your diligent hand watering. I am trying to figure out if there is a soil that will allow me to neglect the plants a bit in the Summer and not have them roll over on me.

The photo below was my prize Aeonium display. I killed them all by letting them wilt in the Summer, all in gritty mix. A lot of that was lack of experience, but to be honest it also felt like the plants were never very happy in that soil, in the Summer.

I've been growing Aeoniums for decades in the gritty mix and have never had a single plant die or even give the slightest hint it might like a change to a different medium or even my care regimen tweaked. I treat them pretty much the same as I treat my trees. If I have any doubt whether or not it's too soon to water, I withhold water.

I really enjoy starting Aeoniums in TINY pots and giving them away to people I encounter in my day to day travels. I gave away at least 50 in the last year, maybe 75.

in a Chapstick cap ^^^

I use cut-offs of lumber to keep them from blowing over/ spilling. In July, I made a block like this but only drilled 3/16" holes in it. Then I took a bunch of single cuttings and inserted the stems into the holes. They have been in my unheated garage since, and haven't been watered. If I go out in the garage right now and lift one of the cuttings from its hole ....... might as well do it instead of describing it.

Note the roots growing on the 2 over-turned plants. All the cuttings are rooted, and if you look close you'll see more roots on the way, even though they haven't been watered for more than 4 months and have been in the block of wood, w/o soil, since I took the cuttings. All I would need to do is press them into the top of the soil and start watering. After a month they would be well-rooted and ready for a new home.

The plant is nearly impossible to kill by withholding water, and in the right medium, quite content to be watered well before they're completely dry.

This ^^^^ is a plant I was harvesting propagules from. Note what occurred after the rosettes were taken for cuttings. ALL those little buds are new branches, each with a rosette that can be planted in a bottle cap or other small plastic cap

Al