How Can You Tell a Lysiloma from an Acacia?

Is this a plant in my yard a Lysiloma or an Acacia? That was the question from a Substack newsletter reader of mine. Complete with two pictures of the plant in their front yard. Feathery leaves, creamy pale puffball flowers. It could be a Lysiloma or is it an Acacia?  Acacia and Lysiloma are both common landscape plants in desert areas, but they have very different care needs.  So how can you tell them apart?

Why it Matters

Why would it matter? Lysiloma can be trimmed into a nice screening hedge. Lysiloma also tends to need more water in summer, and it is less cold tolerant. So let’s get the correct name on the plant so it can get the correct care and grow to the best version of itself that it can be.

Which Is It?  It’s Complicated

Bear with me here as we start with some history and botanical exploration.  If you get bored you can skip to the final paragraph. But then you would miss reading about botanical battles and infighting!

Acacia are a group of trees in the Legume or Pea Family.    Their pufball flowers put them in the Mimosa subfamily.    The genus was first described by the Swedish botanist Carl Linnaeus in 1773. He based the name on the African species he called Acacia nilotica.    The name “Acacia” derives from the Greek word for thorn = “akis.”    The species name “nilotica” was from this thorny tree’s best-known range – along the Nile river.  More about Linnaeus and the language of naming plants.

After 1733, as botanical explorers wandered the globe, they found many plants with highly similar flowers, fruit, and thorns.    These similar plants were found in similar habitats and so were tucked in with the thorny Nile loving Acacia.    Then the botanists got to Australia (remember Botany Bay?).    Anyway, there were a number of similar plants with puffball flowers, only they lacked thorns.    Still, they seemed similar enough so they got placed in the Acacia genus as well.

One of These is Not Like The Other One

Meanwhile, here in the Americas, botanical explorers found this new plant (now Lysiloma).    By the way, botanical explorers are not generally the same ones as the botanists naming plants.    Explorers collect specimens, dry them, and ship them back to botanists who sit around in herbariums naming plants (or herbaria if you are a purist).

On a side note, botanical explorer Sara Lemmon collected many plants and drew and painted lovely images of them as well. Asa Gray sitting in the herbarium (now names after him) named a number of plants “lemmonii” in her honor. Tucson has a mountain named in her honor as well, Mount Lemmon. Botanical explorers love wandering around in the wilderness. They are generally very different people from the ones sitting indoors staring through microscopes.  

Bentham the Botanist Loved Precision

Back to the maybe Lysiloma that is growing in that yard. The botanist Bentham was not happy about thorny plants and non-thorny plants getting stuck together in the same genus – even if they all had creamy puffball flowers. Bentham wanted his world to have better order. He came up with a new genus with a new name that fit this pretty plant with delicate feathery foliage.    He came up with a name to celebrate the lacy foliage of the plants. Lysiloma.    Lysiloma comes from the Greek “lysis” for loosening and “loma” for edge or fringe.  I think he nailed it because the plant does indeed have a lacy fringy aspect.

Bentham only gave the name Lysiloma to a small group of feathery trees and shrubs from the New World, mostly Mexico, Central America, and the Caribbean – not those Australian ones. He left them in Acacia.

So the Answer Is?

Is that it?  Is the answer that Acacia have thorns and Lysiloma don’t?  Sorry. Not any more.

Those Pesky Australian Acacias

Here is where it gets convoluted.  As of 2005, most of the world’s acacias aren’t acacias any more.    Scientists in laboratories staring at DNA sequences decided that – even though acacia flowers all look the same, and the plants all grow alike, and even and have similar mycorhizal relationships, and you can’t see any differences with your naked eye or even a hand lens – they are not in the same genus.

In 2005, the Acacia genus was divided into five separate genera by the International Congress of Botanical Nomenclature. Now we have Acacia, Vachellia, Senegalia, Acaciella and Mariosousa.    But don’t change your plant labels yet, or even bother to learn the new names, debate rages on and may take a while to settle.    Even botanical politics can get quite heated.  The International Botanical Congress meets every so often, and since that meeting was held in Australia, they were able to pack the vote and all the Australian Acacia stayed Acacia and all the rest got changed.  

The argument against this is that the thorny plants from Africa had the name first, so they have the rights to it. Meanwhile while the Aussie plants don’t have thorns and are newcomers to the nomenclature game, and so should get a new name all their own. It’s been 20 years and the debate can still get more than a little heated.

So the Real Question

The real question people have to ask is not if it is Lysiloma or Acacia – but rather is it Lysiloma, or Acacia, or Vachellia, or Senegalia, or Acaciella, or Mariosousa?

Vachellia, Senegalia, Acaciella, and Mariosousa all have thorns.    Currently, Acacia in the strict sense come from Australia and they don’t have thorns.

So how can you tell?    Australian Acacia have a chemical complex that all other “acacia” do not.    This gives them a blueish green cast that you will not see in those others.    You will see that color in Senna and Cassia which tells us that the Australian plants have the common ancestor of Cassia – but that’s a whole can of worms we are not going to open today.    Along with that bluish cast the Australian Acacia have leaves that are generally thicker and more leathery than the plants that used to be Acacia.

Final Answer – The Million Dollar Answer!

* Feathery green leaves, soft delicate leaves, puffball flowers, no thorns = Lysiloma
* Feathery green leaves, soft delicate leaves, puffball flowers, thorns = Vachellia, Senegalia, Acaciella, or Mariosousa
* Blue-green leaves, tough leathery leaves, puffball flowers, no thorns = Acacia

Another post will have to cover Vachellia, Senegalia, Acaciella, and Mariosousa.

A Final Note

The name Acacia is applied to certain plants, but you can still call them all acacia.  The difference is if they are designated a scientific name with the use of italics (or underline) plus a capital letter, or if they are a common name, with no capital letter.  This happens with many other scientific names that have come into our languages, like Iris and iris. O en Espanol, Rosa y rosa. Of course, when they are name of ladies, they would be capitalized but with no italics or underline, Iris and Rosa.

Cover image: Lysiloma microphyllum variety thornberi in cultivation, courtesy J. Pawek

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