Carduus tenuiflorus – November 4th 2024 – Slender Thistle

Thistles have always had ‘bad press’, because as weeds they infest gardens and farms, and can be hard to control. The poet Ted Hughes presents them as defiant and resilient, symbols of conflict and endurance:

In Scotland, of course, we rather enjoy our thistles and they have been adopted as a national emblem since first shown on the coinage of King James III in 1470.  Maria Chamberlain tells the story of the Scottish thistles elsewhere. Their prickles saved Scotland from invasion by sea-faring Vikings in the eleventh or twelfth century (hence the mention of ‘decayed Viking’ in the poem). Evidently, we were fortunate that thistles were so common in our land at the time of the invasion.

Carduus tenuiflorus, showing the sharp spines. Two of the achenes are detached; note their pappus of hairs with simple filaments. Image: Chris Jeffree.

This week’s plant is not common, and can easily be overlooked or mistaken for the common Creeping Thistle, Cirsium arvense. However, it is not even in the same genus. Cirsium and Carduus can be distinguished by their pappus-hairs i.e. the tuft of hairs forming the tiny parachute of each seed (forming the ‘thistle-down’). In Cirsium the hairs are feathery, in Carduus they are simple filaments. Both belong to the Daisy-family, the Asteraceae: as in all daisies the ‘flowers’ are made up of many tiny flowers.

Growing against a wall near the sea, one of its preferred habitats. Note the spiny wings from base of the stem to the flower-head. Image: Chris Jeffree.

Carduus tenuiflorus is called the Slender Thistle. I prefer its alternative name, the Sea-side Thistle. Yes, it grows by the sea-side and in recent years it has extended its range inland, mostly along roads that have been salted. It is usually an annual plant but like many annuals it can take two years to complete its life cycle. It is native throughout much of Western Europe and parts of North Africa. Outside its native range, especially Australia, New Zealand and parts of the USA it has become a troublesome weed.

Detail of flower-head, showing the extremely spiny bracts (‘phyllaries’). Image: Chris Jeffree.

Its features are clear in Chris Jeffree’s photographs. The stems are erect, up to 80 cm tall; the flowerheads (‘capitula’) are more slender (almost cylindrical) than for most thistles and with very short stalks, and they are in bunches of 3-10; the leaves are greyish to white-cottony on the lower side. The stems have spiny ‘wings’, affording extra defence against grazing or hand-pulling, and increasing the green photosynthetic area. Further spines can be seen on the bracts that surround each flower-head.

The BSBI Atlas 2020 describes the habitat as dry, coastal grasslands, seabird colonies, sea-walls, the upper edges of beaches, sandy waste ground and roadsides. Brian Ballinger’s checklist of the Scottish Urban Flora says “rare in towns and mainly coastal”.

Carduus tenuiflorus, distribution in Britain and Ireland, from BSBI/Maps

The global distribution pattern shows how remarkably well it has done outside its native European range. Most of the science papers I found are about how to control it. In some parts of the USA this plant is considered a ‘noxious weed’. Easy-to-read accounts of the biology are written for farmers by weed specialists from the USA and Australia (Roche and Burrill, 1992, Weedsaustralia, 2019). I remain intrigued that so many species we have featured in Plant of the Week perform better (or worse, depending on your point of view!) when they are growing outside their native range. The usual explanation is that when the plant is an alien, it is no longer held in check by its natural enemies.

Carduus tenuiflorus, global distribution, from GBIF.

There is a closely related species C. pycnocephalum known as the Plymouth Thistle. It is an alien, first recorded on limestone cliffs at Plymouth in 1868 and still there. It came from Southern Europe perhaps in wool or birdseed. It hasn’t done well in Britain, and there is only one record for Scotland. However, like C. tenuiflorus, it is a troublesome weed in USA and Australia. Its stem and leaves have more of the cottony coating than C. tenuiflorus, and it has fewer flower-heads per cluster.

References

Stroh PA et al. (2020) Carduus tenuiflorus Curtis in BSBI Online Plant Atlas 2020. https://plantatlas2020.org/atlas/2cd4p9h.n7 [Accessed 30/10/2024]

Roché C and Burrill LC (1992) Slenderflower Thistle (Carduus tenuiflorus Curt.), Italian Thistle (Carduus pycnocephalus L.), Plumeless Thistle (Carduus acanthoides L.). A Pacific Northwest Extension Publication • Washington • Idaho • Oregon.

Weedsaustria (2019) Slender Thistle, Winged Slender-thistle, Slender Winged Thistle, Shore Thistle, Winged Thistle, Italian Thistle, Rabbit Thistle, Sheep Thistle Carduus tenuiflorus Curtis.

©John Grace