Cacti are known for their amazing resilience and ability to put up with a range of conditions. Whether it’s the challenging temperatures or the dry, sandy soils of the desert, they tend to make their living with whatever resources they get. But, can a cactus grow in rocks?
Given that they are very hardy, cacti can tackle difficult conditions and survive. So, can their hardiness make them survive on just rocks?
Let’s find out!
Table of Contents
Cacti Are Epiphytic
Since cacti tend to be epiphytic plants, they always need a medium to grow on. They derive nutrients and moisture from this medium for sustenance. Thus, the soil is an excellent medium to grow cacti on. On the other hand, rocks are not optimal for growing plants.
Thus, almost all cacti cannot grow solely on rocks for multiple reasons.
Why Cacti Do Not Thrive On Rocks
First off, rocks tend to be more consolidated than soil, and are not effective nourishing substrates for cacti to thrive on. Since rocks are consolidated, they also do not let the cacti anchor their roots inside.
All plants require nutrition, and rocks are rich in nutrients. But since there is an absence of weathered material the cacti can directly borrow nutrients from, they remain devoid of them.
The third reason is that cacti need water. Now don’t get us wrong, cacti indeed are drought-resistant plants, but they do require a little water to thrive. Rocks are unable to provide that under most conditions, since they do not hold any moisture.
Cacti also love excellent drainage, but they still need the water to hang around long enough for them to absorb as much as they need.
Now, most cacti won’t be in favor of surviving on just rock. But there are exceptions everywhere.
A study found out that the cactus Pachycereus Pringlei can live off of rocks, with the help of a friendly bacteria.
The Grand Pachycereus pringlei
Pachycereus pringlei, also known as the Elephant cactus or the Mexican Giant Cardon cactus is known to be a robust form of cacti in both stature and nature. It can grow to heights of 60 feet with a diameter of up to 39 inches. It is one of the largest, tallest growing and most resilient cacti in the world.
As a result of their hardiness and their symbiotic relationship with a special bacteria, they display unique behavior, growing on just rocks as discovered by Yoav Bashan et al in Baja California Sur in Mexico.
Very Resilient – All Thanks To Bacteria
The towering Pachycereus Pringleii may look like it has demanding needs, but it’s actually quite the opposite. In the arid regions of Baja California Sur, Mexico, this cactus is found growing on bare rocks, with no soil around.
To understand this behavior, we must have a deeper understanding of what the Pachycereus Pringleii need in order to survive.
How They Do It
Cacti love a good combination of NPK (nitrogen, phosphorus, potassium). In the presence of these minerals, they tend to thrive better. But, as already mentioned, rocks possess these minerals but in a bound form. They are not readily accessible to the cacti.
This is where the endophytic bacteria come into action. These bacteria viz. Acinetobacter, Pseudomonas sp., Bacillus sp., Klebsiella, and Staphylococcus (as found as used in the study) live in the roots of the plants and do not negatively affect the health of the plant in any way.
Symbiotic Relationship
The cactus provides water and carbon to said bacteria, which in turn produces acids that dissolve the rock. This allows the roots of the cactus to drill inside the rock and also derive nutrients from the weathered material.
This also gives impetus to other vegetation to grow in the neighboring areas. Even in Baja California Sur, smaller cacti and succulents were seen growing on rocks around the Cardon cactus.
This included cacti like Stenocereus Thurberi and Mammillaria Fraileana, which also are pipe-like cacti, just like the Cardon.
The cactus even propagates this bacteria into its offspring to ensure that they can also survive on the basis of this symbiotic relationship. They transfer the bacteria through their seeds to continue the symbiosis.
Can You Grow Cactus on Rocks At Home?
In the wilderness, the sturdy cacti can make do with the littlest of resources.
But in the home environment, it is in almost all cases impossible to grow cacti solely on rocks for various reasons.
First of all, typical desert cacti like the ones mentioned above, are hardy enough to survive on whatever little amount of nourishment they can procure from the weathered rock.
Even with the bacteria dissolving the rocks, the nutrients made available for the cactus are not sufficient.
These desert cacti also love being under the scorching sun for extended periods. They also face the brunt of abrupt temperature drops.
Home Cacti Need More Attention
However, most home cacti need relatively more care and nourishment to be at their best. At home, you will need to ensure that you are using the right soil, with a good combination of organic and inorganic components to give your cactus a healthy home.
The care your cactus requires also depends upon the climatic conditions in your region. In the summer days, you will have to be vigilant with how much sun your cactus gets.
During the cold waves of winter, you might have to shift it indoors to prevent it from freezing.
Can You Plant Them In Gravel?
If a cactus can grow on rocks given certain conditions are met, one might wonder if you can plant it in gravel.
We always talk about how cacti need excellent drainage. Well, gravel provides excellent drainage. So, you can indeed grow a cactus in gravel.
However, your cactus and its green heydays will be short-lived, since its nutritional needs won’t be fulfilled.
Gravel has the advantage of being an excellent drainer of water, but like rocks, faces the same issue with the nutrients it holds. These nutrients are tightly bound and cannot be utilized by the plant.
The plant will be able to make do in the initial stages – thanks to its resilience, but will soon face acute scarcity of nutrients. This eventually leads to weakness, discoloration, diseases, and even death of the spiky succulent.
Rocks vs Soil – The Best For Your Cactus?
This question might pop into your mind while thinking over cacti’s viability growing on rocks.
The answer is pretty straightforward: soil is the better medium to grow cactus on.
First of all, soil provides better nourishment than rock. It can contain trace elements like nitrogen, potassium, and phosphorus that are beneficial to the cactus. These elements are also readily accessible to a cactus plant in soil, unlike the case with rocks.
The rock may have the said nutrients, but they are inaccessible to the cactus. Unless there’s a third-party present, the cactus cannot grow, as is the case with the Cardon cactus.
Although, some cacti may be able to make do with a little amount of soil on gravel. Even then, they would require a lot of personal care and attention to grow. You might also have to supply them with the necessary nutrients periodically in the form of fertilizers.
Can You Use Gravel In The Potting Soil?
Growing your cactus in just gravel may not be the best idea, but you can use its fast-draining abilities to your advantage. You can use gravel as an inorganic layer in the soil mix you are planting the cactus in.
Cacti appreciate soils that dry within a day or two. With gravel, you can allow a better passage of water while ensuring that the cactus gets enough moisture for retention.
You can either use the gravel as a bottom layer, which will allow the water to pass before it damages the roots. Or, it can also be used as a top layer to ensure that the water stays in contact with the roots only.
In addition to the good drainage, gravel also gives the plant a rustic look which can be a definite plus for your indoor decor.
In A Nutshell
Cacti are very sturdy, self-dependent plants for the most part. However, their hardiness does not take them as far as growing on bare rock.
Although some species of cacti are able to survive in the desert with this substrate, trying to simulate the same in one’s home can be tricky and might end up harming the plant.
While rocks are pretty much out of the question, gravel tends to be a relatively more acceptable medium for cactus to grow in. But alone, they fail to provide adequate nutrition to the plant, without which the plant will perish.
Gravel can be used in layers as a fantastic inorganic medium to make your soil drain faster. You can also improve the aesthetics of the spiky plant with it.
Home cacti may be tough, but do require an efficient and periodic care routine to thrive. Trying to grow them in an ineffective substrate is a direct precursor to looming diseases for them.
For more guidance on planting cacti and taking appropriate care of them, check out our recent blogs!