It is generally assumed that all cacti are succulents. That is also true, but the inverse conclusion that all succulents are cacti is not true. Let’s address the differences between cactus and succulent in today’s post. I will also show some examples of succulents that look like cactus.
What’s the Difference Between Cactus and Succulent?
The difference, which is decisive, is determined by the aureols. These are pillow-like and reserved for cacti. The spines grow from the aureols. Succulents, on the other hand, do not have these aureols. Moreover, succulents are not native to America like cacti, but originate from the arid areas of Africa.
Below are some examples of succulents that look like cactus.
“But these are two beautiful cacti”…
… would surely proclaim many at the sight of the two pictures – and would be wrong with that. We do not see two cacti here, but on the left a spurge plant (Euphorbiaceae) and only on the right a cactus plant (Cactaceae). And where is the difference?
Although the two plants seem to look so similar, they are not more closely related to each other. Strictly speaking, they are even far from being called “nearly related” by a biologist. In the following I will try to illustrate the differences between the two families Cactaceae and Euphorbiaceae.
The first fundamental difference between the two families lies in their place of origin. In present times, both plant families are no longer only found in their place of origin but are widely planted (for example in the Mediterranean), mainly due to human intervention.
Cactaceae originally came from the American continent, while Euphorbiaceae was originally found only on the African continent. Since the tectonic plate movement that caused the original continent of Pangea to break apart is already a long way back and evolution has progressed at full speed since then, an origin of two different continents is a clear indication that the two plant families cannot be more closely related to each other.
Flowering is the ultimate criterion when it comes to the systematic classification of plants. Let’s take a look at the flowers – to the left of a spurge plant and to the right of a cactus:
First of all, the “flower” that you see on the left is not a flower, but an inflorescence. The spurge family all have more or less similar-looking inflorescences. The two large “petals” are none at all, but either transformed bracts or leaf-like appendages of the nectar glands. The flowers themselves are greatly reduced and always uni-sexual. The female flower mostly consists of the naked ovary, the male one of a single stamen.
The flower on the right, the flower of the cactus family is a “real” flower. The visible petals are just as real petals as the stamens and the (mostly) inferior ovaries. The flowers are hermaphrodite and often radial symmetrical. There are also funnel-shaped and zygomorphic flowers, as we will see when the species are introduced.
But how do we differentiate the two families when they are not blooming?
Other Distinguishing Factors
The spurge plants usually carry a maximum of 2 thorns (left photo), which often protrude from a “shield”, the thorns are mostly converted stipules. Cactus plants, on the other hand, almost always carry 3 or more thorns (right photo) that come from an “areole”, a reduced short shoot:
Furthermore, the Euphorbiaceae have their name for a reason, because when you cut open a spurge family, a white, milk-like substance comes out of the inside of the plant. On the other hand, if you cut open a cactus, it contains a clear, watery liquid. There is only one exception to cacti, that is the Mammilaria genus, which also contains milk sap.
Therefore, one way to make the right assignment is to poke the plant a bit in one place. If whitish juice squirts out, it is a Euphorbia (succulent). However, be careful, as the juice is toxic and can lead to inflammation or poisoning if touched or swallowed.
More photos of Euphorbia that are often mistaken for a cactus:
Pachypodium is a genus of succulent spine-bearing trees and shrubs, native to Madagascar and Africa. It belongs to the family Apocynaceae.
Pachypodiums are tall and spiny like some cacti; especially when leafless. The spines of Pachypodiums rarely arise from a bump, and most smoothly taper from the surface of the plant without interruption as though they were mere pointed extensions of the plant’s surface.
When it comes to flowers, Pachypodium can have large, colorful flowers, but usually these plants also have large, non-succulent leaves (something cacti almost never have).
Agaves are regularly referred to as cacti by some plant lovers. This classification may well have its reasons, but strictly speaking it is not really correct.
The botanical classification of agaves and cacti
Agaves are not cacti, but they share various characteristics and location needs with so many cactus species. This is probably not least due to the fact that both the different agave species as well as the cacti belong to the so-called succulents. This generic term generally subsumes plants that, due to the drought prevailing in their natural locations, store a lot of moisture in their parts of plants and can thus survive even long dry periods without special care. But there are also other peculiarities that the agaves share with many cactus species.
The rarity of flowering in agaves
Many cacti are known to flower very rarely. The same is true of the agaves: While some agave species used as a houseplant can bloom after only a few years, for other agave species it sometimes takes decades to first and sometimes only flowering. It may even be that a magnificent agave with a life age of several decades after flowering almost inevitably dies. This rarity in the flowering of agaves makes this plant genus all the more interesting and challenging for some gardeners.
Similar climatic needs: Agaves thrive in the cactus house
For private agave lovers, older specimens are often brought from the summer location to the winter quarters and back under great effort. Since some agave species are very hardy, these (similar to cacti) are often cultivated in correspondingly large planters. In botanical gardens, this effort is usually spared: Agaves are grown in the cactus house under year-round mild climate conditions. In contrast to a greenhouse with tropical conditions, there is generally a much lower humidity here, which is very conducive to the plant health of the agaves.
As a substrate, agaves need cactus soil or similar
Agaves are not only grown under similar climatic conditions as cacti, they often lead their owners to the same shelf as cactus growers in the garden centre. If you choose not to mix a suitable agave soil together from various porous and coarse-grained materials, you can also simply use commercially available soil for cacti. This is characterized by:
- porous materials for adequate root ventilation
- a non-perfect storage capacity for moisture
Although aloes and cacti are not botanically related, they have many similarities. Both are succulents, carry thorns and are able to survive without water for a long time due to their pronounced water storage organs.
The Aloes are a genus of their own family in the family Asphodelaceae. The cacti plants form a separate family with over 100 genera and between 1500 and 1800 species. The cacti belong to the so-called stem succulents, i.e. they store water in their sprout. The aloes, on the other hand, use their leaves as water storage organs, they are leaf succulents.
While the wild aloe species grow in the deserts and rocky regions of Africa and on the offshore islands, the cultivated areas for the extraction of the gel from Aloe vera can be found all over the world. The wild cacti occur in nature only on the American continent.
There is a great similarity in appearance between Aloe and Agave. However, aloe plants do not die after flowering, as is the case with agaves.
The genera of plants within the tribe Stapelieae are all to varying degrees stem succulents. The spines on the plants make them look a lot like cacti, though are not closely related, as an example of convergent evolution.
Stapeliads are most abundant in warm, dry climates. Many of the flowers also bear some physical resemblance to rotting animal carcasses, leading to their popular name of Carrion Flowers. However, not all stapeliads smell bad, or attract flies.
Conclusion: Succulent vs Cactus
It is important to know exactly whether the plant purchased is a cactus or a succulent, because the care requirements of both are different, for example, they need different amounts of fertilizers. A test is therefore unavoidable if you cannot get precise information from the seller.