Thursday, June 06, 2019

Smooth Sow-thistle and Prickly Sow-thistle

Prickly Sow-thistle (left) and Smooth Sow-thistle (right)




These two extremely unglamorous plants have been occupying my thoughts recently. They're very common, easy to tell apart, and superficially share the same growth-forms, habitat and lifestyle. And that raises a question in my mind. Why, in short, doesn't one of them drive out the other?

Superficially, they appear to occupy the same niche, and are often found growing within a few yards of each other, or even right next to each other. Nevertheless, my supposition is that there must be micro factors that favour Smooth Sow-thistle in some places and Prickly Sow-thistle in others. How else to account for our confidence that when we walk out for a five minute stroll we're certain to find both of them?

(They can hybridize, but the hybrid is extremely rare and extremely infertile.)

BSBI remarks that Prickly Sow-thistle (Sonchus asper) can tolerate wetter conditions than Smooth Sow-thistle (Sonchus oleraceus). And that accounts for the only significant difference in the UK distribution maps: Sonchus asper penetrates deeper into the Scottish Highlands. Elsewhere, both species are omnipresent, but BSBI notes that Sonchus oleraceus does better near to the coast. Presumably its glaucous leaves are more salt-resistant. Otherwise, both species seem to have the same tastes: aggressive colonizers of bare ground, fond of dryish sandy soil when they can get it, and unable to withstand grazing.



SMOOTH SOW-THISTLE (Sonchus oleraceus)





It's easy to see at a glance that this is Smooth Sow-thistle (Sonchus oleraceus) : but the ease arises from a combination of habitual features none of which is individually infallible: for instance, these plants' deeply-lobed glaucous leaves, flat (not crisped), with weak prickles but no spines.

But it's a matter of moral certainty, not scientific certainty. (The really diagnostic differences are two small details, neither of which we'd usually feel the slightest need to examine, as described later.)




Sonchus oleraceus usually has this unique flower colour, with pale outer ray florets giving a general impression of a pineapple colour.




But by no means always! This is Sonchus oleraceus too...


Here's another pointer to Sonchus oleraceus. The phyllaries (bracts that compose the green casing of the composite bud) usually -- though by no means always! -- have softish prickles, like those shown above. (Inevitably, there are sometimes pricklets on the phyllaries of S. asper, but not often.)




And yet another non-infallible pointer: glandular hairs, especially on the stem just below upper leaves and flowers. Only some of the individuals have this, but it's distinctive when it occurs.



Fruiting flower heads, a pure and attractive white, like the fur on a cat's belly.




Here's one of the diagnostic features: and it's a very tiny one indeed. The achenes, with around 12 ribs, are transversely rugose -- or wrinkly, if you prefer. (Ignore photobomb by a tiny spider!)





The other diagnostic difference is that Smooth Sow-thistle (Sonchus oleraceus) auricles (=leaf-bases) are pointed, whereas those of Prickly Sow-Thistle (Sonchus asper) are rounded. When you look at these leaves of Sonchus oleraceus from above, you might be inclined to call them rounded, but...





.... you need to flip them up and view them from the side / below. Then the pointed auricles become apparent. (The pointedness arises because the auricle is prolonged round to the far side of the stem.)

I should clarify that here we are only talking about the auricles of the flamboyantly clasping leaves on the upper stem. The lower leaves appear "stalked" and the auricles -- if that's what they should be named -- are fairly insignificant.




Note the normally bluish, matt surface of the leaves of Sonchus oleraceus. But inevitably, this isn't invariable. On many individuals the leaves are green and shiny. And occasionally S. asper is a bit glaucous...

These loopy markings, the work of some small creature whose name I don't know, are very distinctive.  They are fond of both S. oleraceus and S. asper.





Sonchus oleraceus on the beach at Bexhill-on-sea. Like the others I saw, it had very pale flowers and very slender leaves (apart from the auricles).


PRICKLY SOW-THISTLE (Sonchus asper)





A stand of Prickly Sow-thistle (Sonchus asper). Characteristic features at a distance include the crisped leaves, rather reminiscent of Creeping Thistle (though the spines are in fact rather feeble); also the relatively little-branched habit, and the apical crowns of spent flower-heads, mostly springing from a single point.




A closer view of one of those apical crowns...



Flowers, always deep yellow, never (?) pineapple-coloured. And usually there are no prickles on the phyllaries.




Pappus-head of Sonchus asper. For some reason they seem to me less noticeable than on S. oleraceus. My working hypothesis is that the seeds are apt to blow away before the phyllaries have fully reflexed. (In the photo above, you can see that the tips of the phyllaries are still extended outwards, not bent back on themselves.) That said, the phyllaries are fully reflexed in the photo below!





Later on, the dispersal makes a right old mess!



Here's that tiny diagnostic detail. The achenes of Sonchus asper are smooth (not wrinkly), with pronounced wings on the two edges and three shallow ribs on each side.




Leaves of Sonchus asper, typically crisped and spiny, like a thistle.




... But not always. Sometimes the leaves are flat.



Diagnostic detail no. 2. The auricles (leaf bases) are less pronounced in Sonchus asper. Their overall shape (ignore the spines), is rounded, not pointed.






A specimen of Sonchus asper that, for whatever reason, has declined to grow a stem.


Monster Sonchus asper, growing in a nutrient-rich stream.


Another mighty group of Prickly Sow-thistles, with normal-sized Smooth Sow-thistles on the right. Seems to me that Prickly Sow-thistle has the potential to grow taller than Smooth Sow-thistle, when conditions suit.

Sonchus asper. Swindon, 6 August 2019.

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