Aussie pygmyweed in Southern California

You call that a pygmyweed? This is a pygmyweed...

What's this all about, mate?

In May of 2002, the first known collection was made of Crassula colligata ssp. lamprosperma in California. This small, annual Crassula species is native to southern Australia, occurring in several states there, with a range along the Great Australian Bight. The first collections of the species were all made in and around the area of the Santa Fe Dam Recreation Area, at the mouth of Azusa Canyon.

In the two decades since it was first discovered in California, it has been found over a larger, although still fairly restricted range. It appears to be most firmly established and most abundant in the San Gabriel Valley, where it was presumably first introduced. Large populations occur in the northern Pomona Valley, as well, and it has been documented around Hansen Dam in the San Fernando Valley, though it is unclear how widespread or numerous it is in the San Fernando Valley. The species has also been documented around the Santa Ana River in the Yorba Linda area, near Whittier Narrows, and in coastal Orange County, in Costa Mesa.

The seeds of Aussie pygmyweed are incredibly small, and it would be exceptionally easy for someone walking through a population of mature plants to accidentally transport them to suitable habitat far off from where they picked them up. The species is probably a lot more widespread in SoCal than is currently documented, and appears likely to become very widespread and common across most of SoCal in a few decades time.

Habitat preference for Aussie pygmyweed is very similar to that of the native Crassula connata, but it does occur with decent frequency on fairly high-organic soil. I have found it growing as a weed in typical suburban lawns, so it does not appear so confined to sandy, low-organic soils.

I have also found small and isolated populations of Aussie pygmyweed along fire roads and trails in the front range of the San Gabriels, and one locality has so far been found much higher and deeper in the San Gabes (see observation here: https://www.inaturalist.org/observations/160491608). Given the isolation of these populations, I suspect (on gut intuition alone) that these may be the result of shoe-dumps by hikers shaking pebbles from their shoes, and unwittingly dumping seeds that had fallen into their shoes while walking through patches of Aussie pygmyweed in the valleys below. There probably are populations of Aussie pygmyweed in much more remote parts of the San Gabes yet to be discovered, and likely some along the hiking routes of the other SoCal ranges.

How the heck did it get here, mate?

We don't know, and will probably never know, but I do have some ideas.

1. A somewhat likely explanation for how Aussie pygmyweed came to SoCal is that seed of the plant was somehow introduced in association with the quarrying operations near Santa Fe Dam (Vulcan mining operation). At least in the eastern San Gabriel Valley, where I have personally studied and searched out the species, it very often occurs on relatively recently laid beds of low-organic, xeriscape-type soils, like decomposed granite, to an extent that it makes me wonder if seeds of the species are being transported around the San Gabriel Valley in commercial supplies of such soil. It is not at all clear whether this is the case, and it may simply be that the species happens to thrive and establish well on these types of soils. Likewise, it is unclear whether the quarrying operations near the Santa Fe Dam deal with these kinds of soils, or sell them locally, but I think it is a hypothesis worth keeping on the table for the moment. I would love to go poking around the quarries and see if I can find the species on piles of stored materials, but I suspect the quarries may not appreciate the idea... What seems like a good idea to me now is to keep our eyes on any newly laid beds of decomposed granite and similar xeriscaping soils, and see whether new stands of Aussie pygmyweed emerge in the rainy seasons thereafter to an extent that seems implausible under natural subsequent dispersal.
2. An Aussie tourist, or SoCal hiker returning from an outback adventure may be the origin for this species in SoCal. Given the tiny size of the seeds, it is not hard to imagine someone hiking through a stand of these plants in Australia, not cleaning their boots well, and then hiking in the Santa Fe Dam area.
3. The introduction of this species to SoCal may be the result of a strategic surreptitious plot to exact revenge on Californians for introducing invasive species like California poppy and California Quail to their strange island nation. If this is the case, the threat presented by such a shadowy group of Aussie extremists is frightening, and I quiver to think of a future SoCal overrun by introduced koalas and kangaroos (sarcasm).

A dense stand of Aussie pygmyweed on a decomposed granite type soil in San Dimas, CA. The species is very common on areas that have been "paved" with this kind of material.

How do you distinguish them from the other wee, annual Crassula species, mate?

The best way to tell it apart from the other wee, annual Crassula species in SoCal is to count the number of petals/sepals. Aussie pygmyweed has five petals and sepals, while Crassula connata has fout, and (the also introduced) Crassula tillaea has three petals and sepals. For more ID tips see another of my journal posts here (https://www.inaturalist.org/journal/grmorrison/76252-id-tips-for-wee-annual-crassula-species-of-western-north-america).

The plants start off green, with little other coloring, and become more red-speckled and pink overall over time. "Pink" is the better way to describe the typical mature color, in my opinion, rather than "red", while the red color of mature specimens of Crassula connata and Crassula tillaea tend to be much more intense. The coloration can thus be helpful for ID. I have, however, discerned an apparent correlation between sun exposure and the pink/red coloration at maturity. I hypothesize that the non-green pigmentation is a stress response to something like solar radiation, heat, or soil dryness, and should not be seen as a necessary diagnostic character for identification.

The photo below shows a particularly large individual I found, growing under the shade of a tree, which lacked the pinky-red color entirely.

Crikey, mate. What'll we do about this introduced species?

The cat's probably out of the bag with this one. Given its small size it isn't likely to cause any ecological disasters, but it will probably present some negative impacts on C. connata, as it has such similar habitat preference, and is larger and can probably outcompete C. connata; hopefully it won't be too bad an impact. We should try to get a better handle on the distribution of this species, and I am making my own efforts at this since I first encountered the species. Please contact or tag/at me if you find it in areas that extend the range in SoCal, or elsewhere.

Note on the common name, mate.
I cannot find a documented common name for this particular subspecies or the species as a whole, so I am just calling it "Aussie pygmyweed" here. I would love for someone in the know to unearth an indigenous Australian name, though there are probably numerous indigenous names available, given the linguistic diversity of Australia and the size of its native range. I also like "giant pygmyweed", for another English language option.

Glen R. Morrison
25 Jan 2024

Posted on 26 January, 2024 00:37 by grmorrison