The Humble Split Rock: Pleiospilos nelii

pleiospilos nelii

Written byJen Greene

Animal lover, plant enthusiast, and addicted to the sunshine and warmth in San Diego.

April 20, 2021

Originally printed in Volume 56, Number 04 of Espinas Y Flores – the San Diego Cactus and Succulent Society Newsletter for April 2021. 

For many of us, the experience of first spotting a split rock is the same: walking through the nursery or garden center, and you see a bizarre little green lump resembling a stone. Entranced, you check the label, and see the care requirements: “bright light, water when dry”.

Sounds easy enough.

Unfortunately, the initial delight over a living, growing, pet rock fades, as the plant either melts or shrivels away into nothing. The grower who waters diligently as soon as the top layer of soil dries out will wonder why their living stones always melt; the grower who is too afraid to water more than a spritz will see theirs desiccate into a crisp.

Even those who get the care dialed in on the first try typically find themselves bored of their little green lumps, and quickly move on to more exotic or rare species of succulents.

For me, their charm has never faded, and I still keep them years after taking home my first. With the right level of benign neglect, they can be rewarding succulents that reward you with large, showy blooms year after year.

pleiospilos nelii

Origins

Pleiospilos nelii originates in South Africa, in the Little Karoo region, straddling the border between the East and West Cape Provinces. They are members of the family Aizoaceae, making them related to lithops, aloinopsis, as well as the common ice plant used for landscaping. Their natural climate is semi-arid and the plants are commonly found in shale or sandstone flats in stony soil. Their shape and color very closely mimic the stones in the environment around them, with sand commonly blown into the crevices between leaf bodies. In situ, they are very challenging to spot, almost impossible if they are not blooming.

Basic Care for the nelii

Fortunately for us, the natural environment for the Pleiospilos nelii is extremely close to our environment here in San Diego. The most notable difference, and what we have to work around to grow our nelii into little lumpy clumps, is that our wet season arrives during cold winter and spring months, while in the Little Karoo, the wet season is through the hot summer months.

In an appropriately fast-draining soil mix (I like to add about 25 – 50% pumice to a good succulent soil), you can keep your nelii outdoors year-round with little to no protection from our cold weather rains.

Pleiospilos nelii

This is dependent on your microclimate, naturally: at the coast, where you may get weeks of gloomy marine layer and higher humidity, you’ll probably want to keep your nelii sheltered from rainfall. Meanwhile, further inland, more consistent sunny skies allow the plant to dry out and process water more effectively, which prevents rot. In the high desert, the temperature extremes might be a bit much!

During the hottest weeks of the year, Pleiospilos nelii goes dormant to preserve water, and needs very little (if any) moisture. Kept outdoors, in cold winter months and into spring, our natural rainfall is more than enough water for your nelii.  

Light is also a key aspect of care for Pleiospilos nelii: namely, they prefer plenty of it!

Your nelii should be in an area that gets multiple hours of direct sunlight a day, ideally until around noon or 1 pm, with some shade protecting it for the hottest portion of the day. If bringing a nursery plant home, be sure to slowly move it to areas of increasing sun exposure over the course of several days or even weeks. Immediately placing a shade grown or greenhouse grown Pleiospilos into an area with several hours of direct sun will likely scorch the plant and potentially kill it.

I keep several Pleiospilos in-ground on a west-facing hill, where the ground never develops frost, the soil is rocky and well-draining enough that they don’t stay soggy even with days of rain or hail. So far, they have done well for me in-ground here in south Escondido, with all the plants blooming and developing seed pods as of March. I am not alone in having thriving Pleiospilos in-ground and encourage you to try it for yourself if you have an appropriate sheltered slope!

split rock plants

Where New Growers Most Often Go Wrong

The biggest mistake a new enthusiastic makes as they start collecting Pleiospilos and/or Lithops and other mesembs is to treat them with the same routine as their other succulents. A strict routine of weekly, every other week, or monthly watering is the surest way to kill your mesemb!

These little plants are all cyclical growers, with distinct periods of rest. They’ve adapted well to their small, seasonal amounts of rainfall, and if this type of seasonal, cyclical growth isn’t respected, they will slowly die. The sturdy Pleiospilos nelii tolerates over and under watering exceptionally well, but even the adaptable split rock will succumb eventually if it is forced to adhere to a watering schedule.  

Cyclical Growth Routine: Nitty Gritty Details

In broad strokes, your neliii should follow a general yearly cycle of growth and dormancy.

In spring (March & April), you should offer little supplemental water unless we are having a severe drought year – our typical spring rains will be plenty. When the weather warms up in May, June and July, your Pleiospilos will appreciate the occasional soak if there is no additional rainfall. If we receive normal rainfall quantities, your Pleiospilos won’t need water again until late June or July, if at all.

As summer hits its peak in August, it may feel like you absolutely must water your succulents or our hot weather will kill them, but the opposite is true for the Pleiospilos. Ensure they are protected with shade for the hottest part of the day and let them stay dry: they’re usually dormant at this time. If you’re really worried, use the squish test to see if they need water.

I typically test if my Pleiospilos are thirsty or not by squishing the leaves. The outer-most leaves can get quite squishy without any harm to the plant; in the wild, the nelii may have absorbed these leaves long before the summer (and rain) arrived. Instead, I base my decision to water on the inner leaves. If they are firm and resist a finger push or squeeze, the plant is fine and does not need more water. When I feel a bit of give in the inner leaves, it’s time for a good soak.

When September rolls around and the weather starts to come down from the hottest extremes, your Pleiospilos and other mesembs will all be waking and ready to drink up. They can and should be watered regularly, but not necessarily often, during this time, up until the days start getting quite short – usually around late October or early November. It’s at this time that many lithops and most of the Pleiospilos bloom, and they’ll appreciate the moisture up until the blooms are spent and they begin to divide.

By December, nearly all the mesembs should be dry again ahead of our coldest months. Only the sturdy nelii needs water at this time, and only a little: without it, they won’t bloom! When we have our typical warm spell(s) in December, I’ll usually take advantage to give my Pleiospilos a bit of a drink while there’s enough warmth for the soil to dry out again.

blooming split rock

By January and February, ideally, all of your mesembs should be quite dry. Pleiospilos will spend the winter cool months developing new leaf bodies, slowly absorbing the old ones, much like lithops and Conophytum. While the nelii will bloom at the end of winter, its cousins typically bloom at the beginning, making this one way to tell a Pleiospilos nelii apart from a spoiled Pleiospilos bolusii (they often look nearly identical).

Keeping your Split Rocks dry at this time also helps them weather the occasional freezing night: kept dry, they can tolerate night time drops down to below freezing, although you’ll want to protect them from actually getting frozen.

The Best Part: Blooms

A happy Pleiospilos nelii will send out multiple blooms at once, often staggered somewhat so that they are not all blooming in the same stage at one time. Flowers open in the afternoon with exposure to sunlight, and close by the time the sun goes down.  Compared to the size of the plant, the flower can seem absurdly large, which is part of their charm. I’ve read that the blossoms have a distinctly coconut smell, but have never experienced this myself.

Colors range from pale orange-yellow through to deep orange to pink-tipped blooms, while the “royal flush” cultivar can have neon-pink flowers.

Long Term Growth Notes

A healthy, well-grown nelii will rarely have more than two leaf pairs – this year’s new pair, and last year’s old pair. In cultivation, we commonly ‘spoil’ our plants and they do not need to absorb the nutrients from the outer leaves to create new ones. The result of new leaf bodies developing without the old ones being absorbed will result in a phenomenon known as “stacking”, when there are 3 or more pairs of leaves visible on the plant. In and of itself, “stacking” will not kill your plant, but it is a sign that it is being too well cared for and should have longer dry periods before being watered again. A nelii being watered enough to ‘stack’ is also getting enough water to put it at risk of root rot, and caution should be taken to acclimate the plant to a more appropriate watering regime.

Older plants eventually will develop clusters, with multiple heads visible that will also bloom. While you can divide your nelii when it reaches this point, leaving them as a cluster will often lead to larger and more profuse quantities of blooms. They seem to prefer the crowding and will thrive this way.

As mesembs that grow in near-desert conditions with very little water, fertilization requirements are low to non-existent. If you really feel you must, a dilute cactus/succulent fertilizer could be applied once a year during the spring, but it is generally unnecessary unless your soil mix is very high in inorganic medium.

The “royal flush” cultivar is a pretty, purple variant on the green of Pleiospilos nelii, but they can be less robust than their green cousins. For the best color, they should be given afternoon shade, as full afternoon sun and heat tends to turn their purple shade into more of a deep brown.

Pleiospilos nelii royal flush

In Conclusion

For the novice grower, Pleiospilos nelii is a forgiving mesemb that works well as an introduction to this cryptic and hands-off group of plants. Their adaptability to a relatively large range of growing conditions makes them potential candidates for in-ground rock gardens in the right microclimates, or even for windowsill culture in climates less suitable than those found in San Diego.

Nearly every succulent enthusiast has at least one Pleiospilos in their collection, and the humble nelii has earned a spot (or ten) on my bench for years to come. I hope it has a treasured spot in your collection as well!

Like to see content like this?  I invite you to join the San Diego Cactus and Succulent Society! We happily accept members from all over the globe. You can join online and find more information at the website: https://www.sdcss.net/ 

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