On a cold day in January my thoughts turn to summer flowers. One of my favourites is the Dwarf Thistle, Cirsium acaule.

Dwarf Thistle, Cirsium acaule

Cirsium acaule is a member of the Daisy family, Asteraceae (formerly known as the Compositae). It is one of nine species of Cirsium growing in the wild in the British Isles (note 1).

Our other species of Cirsium have an upright growth habit. Marsh Thistle (Cirsium palustre) can reach two metres in height, Spear Thistle (Cirsium vulgare) 1.5 metres and Creeping Thistle (Cirsium arvense) 1.2 metres. But the Dwarf Thistle usually just reaches 10 centimetres tall. Because of its short growth form, Cirsium acaule is sometimes known as the Stemless Thistle. (There also is a long-stemmed form, Cirsium acaule var. caulescens, that reaches up to 30 centimetres tall when growing in longer grass.)

A slightly taller specimen of Dwarf Thistle, Cirsium acaule.

Like its taller relatives, Dwarf Thistle has purple composite flowers. These some 20 to 40 mm across and are produced from June to September. The flowers are followed by masses of fluffy, wind-borne seeds in late summer. Cirsium acaule is a perennial plant and its basal rosette of leaves persists throughout the year, blending in with the surrounding grass. The leaves are deeply lobed and spiny (Stace describes them as “strongly spiny”).

It is easy to sit on a Dwarf Thistle by accident, especially when it isn’t in flower, leading to a painful encounter. The thistles “look just like a nice flat patch of grass to sit on whilst you eat your butties. But automatic ejection is painful and abrupt!” In “Flora Britannica” Richard Mabey writes that Cirsium acaule is “widely known as ‘picnic thistle’ because of its fondness for favourite beauty spots on calcareous grassland and for giving no warning of its lurking spininess even when the flowers are out (note 2).”

Do look carefully at the ground (or take a foam mat to sit on) when picnicking or botanising, just in case.

Dwarf Thistle, Cirsium acaule

I see Dwarf Thistle most years but I have to travel west because it’s a plant of short chalk grassland and that is scarce in my part of Norfolk (note 3).

The photographs on this blog post were all taken at Warham Camp, an Iron Age hillfort not far from Wells-next-the-Sea in North Norfolk. Here, chalk is at the surface and it’s a good place to see chalk-loving flowers and their associated insects, such as an introduced colony of Chalkhill Blue butterflies (Polyommatus coridon) whose caterpillars feed on Horseshoe Vetch (Hippocrepis comosa). The site is grazed by sheep, which keeps the grass short and rich in flowers, including Dwarf Thistle.

At Warham Camp, the Dwarf Thistles are usually growing amongst grass but this one is on nearly bare ground. You can clearly see the white of the hillfort’s chalk ramparts.

Dwarf Thistle, Cirsium acaule

Thistle flowers are a rich source of food for insects and I’ve photographed several visitors, such as this Red-tailed Bumblebee (Bombus lapidarius).

Dwarf Thistle and Red-tailed Bumblebee (Bombus lapidarius).

Butterflies and moths appreciate the flowers too.

Dwarf Thistle and Essex Skipper butterfly

Dwarf Thistle and Six-spot Burnet moth

Cirsium acaule is a British native. It grows as far north as Yorkshire and Derbyshire, with outlying colonies in the Arnside and Silverdale area of north-west England. On the northern edges of its range it is almost wholly confined to south-west-facing slopes. Dwarf Thistle benefits from grazing reducing sward heights to less than 10 –15 cm, or frequent mowing.

Dwarf Thistle’s distribution in the British Isles is determined by the occurrence of short, heavily grazed chalk or limestone grassland, though the plants can occasionally be found on mesotrophic grasslands on deeper soils as well (note 3). Its habitat requirements mean it is absent Scotland, Ireland, most of south-west England and much of Wales (note 4).

Distribution map of Cirsium acaule from BSBI Online Plant Atlas 2020.

Outside the British Isles, Cirsium acaule is native in much of Europe, from Spain, Italy and the north-west Balkans in the south, to Norway and Sweden in the north and Romania and  the Baltic States in the east.

In Britain, Dwarf Thistle has declined since the 1960s, in spite of some new discoveries in Carmarthenshire and Glamorganshire. This is partly because of agricultural “improvement” involving the ploughing-up of grassland or its degradation through the addition of artificial fertilisers. The relaxation of grazing can also harm Cirsium acaule and other plants of short grassland, if tall grasses and scrub take over.

Cirsium acaule is also destroyed by heavy trampling. And, as Eeyore wisely observes: “It don’t do them any Good, you know, sitting on them… Takes all the Life out of them. Remember that another time, all of you. A little Consideration, a little Thought for Others, makes all the difference.” (note 5).

Wise words indeed. The world would be a better place with a little Consideration, a little Thought for Others.

Notes

Note 1 – Clive Stace (2019). “New Flora of the British Isles“.  Fourth Edition.

Stace lists two hybrids between Cirsium acaule and other thistles:

C. x kirschlegeri (C. acaule x C. palustre). This hybrid with Marsh Thistle has shortly spiny-winged stems to 40cm and intermediate leaves and capitula. Found rarely with the parents in southern England but not since 1951.

C. x boulayi (C. acaule x C. arvense). This scarce hybrid with Creeping Thistle has branched stems up to 60cm tall and leaves like C. arvense but with intermediate leaves and capitula.

Note 2 – Richard Mabey (1996). “Flora Britannica”. Sinclair-Stevenson, 1996.

Note 3 – Mesotrophic soils are soils with moderate fertility.

Note 4 – The author of the Wild Flower Finder website, Roger Darlington, wrote that he “has never knowingly seen it in flower, although he may have attempted to sit on a basal rosette of it in his youth whilst walking the hills”. Peter Llewellyn wrote on his UK Wildflowers website: “This is not an uncommon thistle but it prefers basic soils and so until this occasion [5th September 2010] I had never seen it in flower.”

Note 5 – A.A. Milne (1926) “Winnie-the-Pooh“. (From chapter VIII: “In Which Christopher Robin Leads an Expotition to the North Pole”.)