When you think of hardy, long-lasting plants, you probably think of trees with expansive root systems. In comparison, many cacti species can live for more than 100 years. But their roots are much more shallow than you think—why though? Let’s see!
Cactus roots don’t grow deep, rarely growing more than 5–10 inches below the soil. Their roots stay close to the soil surface to better absorb rainfall, gather more nutrients, and get established faster. Shallow roots enable cacti to survive drought and nutrient-poor climates for several years.
Although deserts are typically barren and unforgiving, cacti have evolved so much that these lands have become their realm. While it can be difficult to study roots in-depth, here are a few reasons why scientists believe cacti have such shallow roots!
Cactus roots do not grow very deep. The average depth that cacti roots reach is usually no more than 10 inches below the soil surface. Columnar cacti are the only exception as they grow the roots approximately 30 feet deep in the soil.
Cacti are fascinating plants that quickly evolved to survive droughts and harsh desert conditions that many other plants cannot tolerate. Much of this is thanks to their roots.
While Shepherds trees in the Kalahari Desert grow roots more than 200 feet deep, most cacti roots do not extend deeper than 5–10 inches below the soil’s surface.
However, some cacti roots can extend more than 30 feet away from the plant. Columnar cacti are a good example of this and have some of the deepest roots. These deep taproots help anchor the tall cacti and prevent them from falling over.
This is the deepest, and widest, that cactus roots have been known to spread. If you enjoy growing columnar cacti outdoors for their impressive height, it’s probably safe to say its taproot is growing far deeper than other cacti. Keep reading to learn how to select the best pot for this type of cactus!
But otherwise, most cacti rarely grow more than a few inches underneath the soil. Here are the average root sizes for a couple of popular cacti you might be familiar with!
Opuntia cacti grow taproots and lateral roots between 5–10 inches deep in the soil. Because of this, Opuntia cacti do not require much space to grow and have become invasive in Australia.
The bunny ear cactus comes from the Opuntia family and is probably one of the most iconic cacti. The very same cactus family produces a majority of the world’s prickly pear too.
To meet the demand for exotic cactus fruit, thousands of prickly pear cacti must be grown commercially. Its shallow root systems help make cultivation easier.
Established opuntia cacti will grow taproots that are an average of 5 inches (12.7 cm) long.
The taproot is considered the primary root and does not grow very deep. Lateral roots, by contrast, tend to extend further out but opuntia roots seem to rarely extend deeper than 10 inches.
Surface-growing roots allow cacti to stay drought-tolerant! However, the downside to this is that Opuntia spreads much faster. It is now even considered a weed in Australia.
The Rebutia cactus family often consists of small, globular cacti that do not have root systems deeper than 4 inches.
One of the reasons behind this is that this cactus species is quite small compared to other cacti. For example, the Rebutia minuscula cacti are only 4-6 cm wide!
Due to its size, it does not require extensive root systems. Its shallow root systems tend to grow slowly. Many Rebutia taproots only go about 4 inches or 10 cm deep into the soil.
Cacti grow shallow root systems for 3 main reasons: to absorb more rainwater near the soil surface, to gather more nutrients, and to establish faster than competing plants.
In dry soils, it’s common to find plants growing deep root systems in search of minerals and water. So why do most cactus roots only grow a few inches below the ground?
It might be confusing, but it’s for the same reasons!
While other plants utilize deeper root systems, the cactus has evolved to use more shallow roots to gather the moisture and nutrients it needs. Keep on reading to learn how they do it.
To gather more rainwater, cacti grow roots 5 inches below the soil’s surface. Short and shallow roots have a higher chance of absorbing rain, allowing cacti to survive droughts and irregular precipitation.
For the cactus to survive, it’s crucial to have a root system that can tolerate and quickly adapt to desert climates. Scientists have discovered that established cacti have two different root systems: the primary roots and the horizontal roots.
The primary roots are one of the first roots to form. They typically grow downwards to help anchor the cactus in the soil. Often, they are called the taproot. Even the smallest cacti pups can be found growing their primary roots!
Learn more about this in our article on How Do Cacti Roots Look?
Once the plant and the primary root increase in size, the cactus will begin to develop horizontal roots.
Rainfall is usually the primary source of water for cacti. It’s also quite rare in the desert. More often than not, rain water will only penetrate the first 4–5 inches of the soil.
So how does the cactus adapt to this?
To gather more water, horizontal cacti roots will grow up to about 4–5 inches into the soil to absorb as much rainfall as possible. This is why cacti have such shallow root systems!
Because of this system, there’s no reason for the roots to descend further.
Shallow cactus roots allow the roots to access more nutrients and moisture under desert boulders. Decaying plant debris and animal feces within the upper part of the soil can also provide the cactus with more nutrients.
Despite how hardy most members of the Cactaceae family are, they are still plants that need nutrients to live long and prosper.
As you can imagine, nutrients can be scarce in the desert. Instead of burrowing in the soil, however, it is possible that cactus roots stay near the soil surface because they are more likely to find nutrients there.
Rocks and boulders are common in desert environments. While you might think most cactus would do their best to avoid these, these rocks can provide support for cactus roots!
This is because, after a downpour, moisture is more likely to remain under rocks. Additionally, there may be more nutrients and minerals available in these rocky areas.
Desert animals, like reptiles, will also find rocks attractive to hide under and leave nutrient-rich feces. All these things together reward the shallow-growing roots of cacti and help them thrive.
Cacti will grow shallow roots to enable them to harvest more water and grow faster than other competing plants. These shallow roots aid the plant in establishing itself sooner and prevent the cactus from dying.
Not only do wild cacti have to battle with their own harsh terrains, but they must also compete for resources with other plants!
The slow growth rates of cacti can make their survival a challenge. However, their shallow roots enable them to capture more water and grow faster than other plants.
Additionally, cacti can grow new root systems over and over again as needed. This helps give them an edge when competing with other desert plants that do not replace their roots as readily as cacti.
It can be puzzling at first glance, but there is a method to this madness.
Shallow roots have the same goal and function as other deeper plant roots—to ensure the cactus absorbs adequate moisture and nutrients. But by growing near the surface, they simply grow differently than other plant roots!
Because cacti have wide and shallow roots, it is best to plant them in 4-inch pots that are wider than they are deep. Conversely, columnar cacti like the Saguaro have deeper root systems and require 5-gallon or 11-inch containers.
When it comes to selecting pots for cacti, it might be confusing. Is it better to get large deep pots or small shallow ones?
As mentioned previously, cacti grow shallow roots that are typically wider than they are deep. Because of this, it’s better to grow cacti in wider pots than in deeper ones.
But the depth and size of the pot should always depend on the individual cactus species. Smaller cacti like the moon cactus or Rebutia cactus can be kept in 4-inch pots. The Saguaro cactus, however, will require deeper planters.
Like other columnar cacti, the Saguaro is known to grow some of the deepest cactus roots. A young, 3-foot Saguaro can form 11-inch taproots and grow roots more than 5 feet long.
So if you’re considering owning a Saguaro, keep it in a pot that is at least 5 gallons or 11 inches deep. Be sure the pot is heavy– you don’t want the cactus to fall over and hurt someone!
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Some cacti like the Opuntia species and the Christmas cactus can be rooted in water. However, the most common ways to propagate cactus is via stem cuttings or replanting of offsets.
It might sound surprising but cacti can develop roots in water, although some root more readily than others.
This is one of the best ways of rooting Christmas cactus cuttings. Opuntia leaf pads can also be placed in water to grow roots after several weeks.
Aside from rooting in water, cacti can even form new plants after having their stems cut in half! While this would kill most other plants, this is commonly known as beheading, a popular method of propagating cacti.
As you can see, the cactus is a resourceful plant that can survive in a wide range of situations. Studying cactus roots—and plant roots, in general—can be difficult. But luckily, many scientists are already studying them for everyone else’s benefit!
From learning how to harvest dew from spines to developing highly absorbent material inspired by cactus roots, there really is no shortage of amazing discoveries when it comes to cacti.
Do cacti have aerial roots?
Aerial roots can develop on cacti but are generally uncommon. These roots are usually only found on epiphytic species like the fishbone cactus and the mistletoe cactus. Epiphytic cacti are known to grow on other plants and can use their aerial roots to cling to other surfaces.
Are cactus roots invasive?
Cactus roots are typically shallow and slow-growing and are typically not a threat to the environment. The Opuntia cactus, however, is known to be invasive in Western Australia—overtaking native plants and disturbing established local ecosystems.
Many cacti grow horizontal root systems that extend only 5-10 inches into the soil. But because these roots are shallow, the plant can harvest more nutrients and rainwater collecting near the surface of the soil, helping them grow faster and outlive competing plants.
Due to the horizontal growth of cactus roots, many cacti species will benefit from being planted in pots that are only 5 inches deep. Columnar cacti, however, require pots that are 11 inches deep to contain their extensive root systems.
- “Cacti: Biology and Uses” by Park S. Nobel in University of California
- “Biology of Cacti” by n/a in Dalhousie University
- “Cacti and succulents” by Deborah L. Brown in University of Minnesota
- “Root Succulence in Cacti” by J. D. Mauseth in University of Texas
- “Care of Non-Hardy Cacti & Succulents” by n/a in Cornell University
- “Prickly Pear Cactus Production” by n/a in University of California