Dwarf Thistle, Cirsium acaule

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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

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.)

Dwarf Thistle, Cirsium acaule

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

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

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 Bombus lapidarius

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

Butterflies and moths appreciate the flowers too.

Dwarf Thistle and Essex Skipper

Dwarf Thistle and Essex Skipper butterfly

Dwarf Thistle and Six-spot Burnet

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

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”.)

Zythia resinae (aka Sarea resinae)

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It’s always worth looking at tree stumps, as they can host a range of fungi.

This coniferous stump was in a Norfolk wood and is probably the remains of a Douglas Fir, Pseudotsuga menziesii, for they were planted in the wood as a timber crop, amongst the native deciduous trees. The bottom of one side of the stump was covered in a dark patch of resin.

A tall stump of a conifer in a Norfolk wood (May 2025).

The resin was covered in lots of pinky-orange, stalkless cup fungi, each one about a millimetre in diameter.

Conifer resin covered in light orange cup fungi.

I took some photographs and used a knife to remove a small piece of resin with the fungi attached.

Stalkless cups of Zythia resinae on conifer resin.

Back home, I had a look at the cups under the microscope.

The fungi were clearly cup-shaped ascomycetes (sometimes known as discomycetes), with asci (chambers containing spores) and paraphyses (sterile hairlike filaments).

Paraphyses (sterile filamentous hyphal end cells) of Zythia resinae. x1000, stained with Congo red.

I’m used to asci that contain a small number of moderately sized spores (often eight) but these contained a hundred or more tiny spores, each one roughly two micrometres in diameter.

Two asci of Zythia resinae, containing hundreds of tiny spores, each about 2 micrometres in diameter. x1000, stained with Congo red.

I posted my photos on the “Ascomycetes of the World” Facebook page and the genus Zythia/Sarea was suggested. This led me to identify the fungi as Zythia resinae (also known as Sarea resinae).

My find was a “first for Norfolk”, so I dried and froze a sample of fruitbodies and passed them to the Norfolk Fungus Study Group’s DNA team for DNA extraction and sequencing. The results came back in November 2025, confirming my provisional identification. The fungus will be added to the Fungal Records Database of Britain and Ireland under its current name of Zythia resinae but the DNA sequence will be submitted to Genbank as the older Sarea resinae.

I found the fungi on the stump in mid April and revisited at the end of May, when there were many more fruitbodies, in spite of the dry spring.

Zythia resinae

The genus Sarea was thought to contain two species complexes: the black Sarea difformis and the orange Sarea resinae (both sensu lato). But in 2021 Mitchell et. al. examined the morphology and ribosomal DNA (rDNA) of 70 specimens of Sarea and their conclusions supported the split of Sarea difformis s.lat. and Sarea resinae s.lat. into two distinct genera, with Sarea resinae becoming Zythia resinae (note 1).

Zythia resinae is illustrated in Volume 1 of ‘Fungi of Switzerland’ by its older name of Biatorella resinae (on page 240). It features as Sarea resinae in Volume 2 of Laessoe and Petersen’s two volume “Fungi of Temperate Europe” (on page 1461). Other synonyms are Pycnidiella resinae, Cytispora resinae, Cytospora resinae, Sphaeria resinae and Pycnidiella resinae (note 2).

Zythia resinae is known as Sarea resinae on several websites. Jan Thornhill, who is based in central Ontario in Canada, describes her encounter with the fungus on her enjoyable “Weird and Wonderful Wild Mushrooms” blog (‘”Another Fluke: Sarea resinae”, 23rd March 2014). Nearer to home, Chris and Christine Johnson have some excellent photographs on their “Fungi Outer Hebrides” website. There are some good photographs on the AscoFrance Forum and on the Fungi of Great Britain and Ireland website, as well as in the paper “Sareomycetes: more diverse than meets the eye.” (note 1).

Distribution

There are scattered records for Zythia resinae throughout much of the British Isles.

Zythia resinae can also be found in many European countries and also in North America. “Fungi of Temperate Europe” describes it as being widespread and rather common.

This is the current distribution in the British Isles:

Distribution map for Zythia resinae from The British Mycological Society Fungal Records Database of Britain and Ireland. [Accessed on 28th December 2025].

(My record will be submitted to the FRDBI database in 2026, so does not appear on the map above.)

Zythia resinae is small and easily overlooked but lack of a suitable habitat (conifers with patches of resin) probably means it is genuinely scarce in parts of the country with mostly deciduous trees, such as East Anglia. Many woods (especially in Norfolk) are privately owned and lack public access, and this must surely be a factor as well.

The Dark Side

Zythia resinae’s relative, Sarea difformis, also grows on conifer resin. It consists of black discs and was once thought to be a lichen. A black fungus growing on black conifer resin is difficult to see, so it is even easier to overlook than Zythia resinae.

Sarea difformis is illustrated on page 1445 of Volume 2 of Laessoe and Petersen’s two volume “Fungi of Temperate Europe”, where it is described as “widespread, probably common but easily overlooked”. As with Zythia resinae, there are scattered records throughout much of the British Isles, but not from East Anglia.

Its worldwide distribution is similar to that of Zythia resinae, with the addition of a handful of records from Asia.

Gary Walton, who lives in Minnesota in the northern United States, writes about it on his October 2025 blog post “Sarea difformis: A fungus that lives on conifer resin“.

I will keep a lookout for Sarea difformis in Norfolk – now I know that it exists.

Resin eaters?

Zythia resinae and Sarea difformis are known as resinicolous, as they grow on resin from pines and other conifers. They have ancestors that date back to the late Jurassic or Cretaceous periods (about 140 million years ago), around the time that pine trees (genus Pinus) were diversifying (note 1).

Conifers (particularly those in the families Araucariaceae, Pinaceae and Cupressaceae) produce resin in response to damage. This seals wounds, protects against herbivores and inhibits the growth of microbes. Resin is runny when first produced (about the same viscosity as honey) but it soon solidifies. Resin forms a physical barrier to penetration by fungal hyphae, especially when hardened but when it still liquid it can flow around fungal hyphae and spores, trapping them. It also protects chemically, as it contains terpenoids with anti-fungal and anti-bacterial properties. It is a pretty specialised habitat for anything to grow on (note 3).

Zythia resinae‘s specific name resinae comes from the Latin word resina which means “resin” and genus name Zythea is derived from the Greek zythos meaning beer or ale. On the Mushroom Monday substack, Aubrey Carter wonders whether this refers to the idea that the fungus is fermenting some sugar in the resin.

There is still a lot to learn about the physiology and ecology of Zythia resinae and Sarea difformis.

For Zythia resinae, Aubrey Carter says “little is known about how the fungus lives. So little that we’re uncertain where the fungus obtains nutrients — perhaps the mycelium goes inside the tree, perhaps they derive nutrition from the resin itself, or perhaps it’s a different mechanism entirely.”

And for Sarea difformis, Gary Walton says “I have looked for research papers on how Sarea can live on conifer resin, but the results have not yielded much information. Does it actually break down the resin components into simple carbohydrates? If so, then does it do this on its own, or is there a symbiotic relationship with bacteria or another fungus? Are the fungal hyphae growing in the resin or merely on top? How does it disperse? A lot of questions, and I’m sure there are answers somewhere.“

In his 1941 paper “Biatorella Resinae: The Perfect Stage of Zythia Resinae” Theodore Ayers describes how inoculation tests with artificial cultures of Zythia resinae on Pinus strobus failed to show that it was parasitic on the conifer. Since then, many researchers have tried to determine whether whether Sarea and Zythia are parasites, but the conclusions have been contradictory. Mitchell et. al. sum it up: “The varying results and generality of these tests leave unresolved the question of pathogenicity of species in Sareomycetes; some authors assume pathogenicity and others accept a saprobic lifestyle” (note 1).

The conifer stump where I found Zythia resinae was most definitely dead, so presumably the fungi were living saprobically. But the tree must have produced its resin when it was alive and presumably very poorly. When did Zythia resinae colonise the resin? I will never know.

Notes

Note 1 – Mitchell JK, Garrido-Benavent I, Quijada L, Pfister DH (2021). “Sareomycetes: more diverse than meets the eye.” IMA Fungus Vol 12, article 6. The paper is freely available to download at https://link.springer.com/article/10.1186/s43008-021-00056-0. Sarea difformis s.lat. was split into Sarea difformis s.str. and Sarea coeloplata. A new genus was also formed, Atrozythia. Most importantly for us, Sarea resinae s.lat. became Zythia resinae, a single though genetically and morphologically variable species.

Note 2 – These synonyms are listed in the Species Details for Zythia resinae on the British Mycological Society’s Fungal Records Database of Britain and Ireland website. A login is required to access the information, which includes the distribution map shown above, as well as a facility for exploring individual records. New users can register to use the website free at https://www.frdbi.org.uk/user/register.

Note 3 – Resins are solid or highly viscous liquids that can be converted into a polymer. Amber is fossilised tree resin. It sometimes contains insects that were trapped in liquid amber and preserved as it solidified.

Golden Conecap, Conocybe aurea

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This August I found some rather striking small, bright yellow-orange fungi in pots of compost. They were Golden Conecaps, Conocybe aurea.

Golden Conecap, Conocybe aurea

Golden Conecap, Conocybe aurea

Golden Conecap on the Allotment

My allotment neighbours Karin and Simon grow some fantastic fruit and vegetables and in summer their greenhouses are full of healthy and productive tomatoes, cucumbers and peppers. I sometimes water their plants when they’re away, and they do the same for me.

On 6th August 2025 I was watering some of their pots when I noticed some rather pretty fungi growing in the compost. I didn’t have my camera with me, so I picked a couple of them and brought them home for a closer look.

I was reasonably sure they were a species of Conocybe (conecaps), a genus of saprotrophic fungi that grow on soil, dung, compost heaps, herbaceous litter or woody debris. Funga Nordica has a key to 62 species found in Northern Europe and Geoffrey Kibby illustrates 30 of them (note 1).

Species of Conocybe are not very easy to identify but I was pleased to see an illustration of a brightly coloured species that might be a match for my find, Conocybe aurea, in Volume 4 of Geoffrey Kibby’s “Mushrooms and Toadstools of Britain and Europe”.

Golden Conecap, Conocybe aurea

Golden Conecap, Conocybe aurea

I did some microscopy and checked the spores and cheilocystidia (cells on the gill edges) and caulocystidia (cells on the stem of the fungus). Everything fitted Conocybe aurea.

Spores of Golden Conecap, Conocybe aurea.

Spores of Golden Conecap, Conocybe aurea. (In water, x1000.)

Golden Conecap, Conocybe aurea

Cheilocystidia of Golden Conecap, Conocybe aurea (stained in Congo red, x1000). The caulocystidia (cells on the stem of the fungus) are the same shape.

I wanted photographs of the fungi in situ but I was out the next day and when I visited the allotment on 8th August, armed with a camera, there was no sign of any.

Luckily the original fruitbodies kept well in the fridge and I took them along to the Norfolk Fungus Study Group AGM the following day and showed them to Tony Leech, the Norfolk County Recorder for fungi. He had seen the species before, in September 2023 when his friend Donna found them growing in compost around a newly planted grape vine in her garden at Melton Constable. That was the first record of Conocybe aurea in Norfolk. He agreed with my identification (note 2).

More fruitbodies appeared during August and the first half of September and I managed to take pictures of Golden Conecaps at various stages of growth in several different pots, growing with cucumber, mint and basil plants in a commercial peat free compost.

Golden Conecap, Conocybe aurea

Golden Conecap, Conocybe aurea, mid August 2025.

Golden Conecap in the British Isles

Conocybe aurea is a rather scarce fungus in the British Isles and just ten records are listed on FRDBI (the Fungal Records Database of the British Isles) at the time of writing. (My record should reach the database next year.)

Not everyone records fungi, even when they’ve identified them, and although Conocybe aurea is very pretty I suspect its identification is often dismissed as being too difficult. (And it doesn’t feature in many illustrated guides to fungi.)

Here is the distribution map for Conocybe aurea in the British Isles:

Distribution map for Conocybe aurea from The British Mycological Society Fungal Records Database of Britain and Ireland. [Accessed on 15th November 2025]

Distribution map for Conocybe aurea from The British Mycological Society Fungal Records Database of Britain and Ireland. [Accessed on 15th November 2025]

The earliest records on FRDBI are from September 2010, when Andy Overall found Conocybe aurea on Hampstead Heath in London.

In 2014, he co-wrote an article with Vivien Hodge in Field Mycology entitled “Conocybe aurea, a rare British native or another coloniser?” (note 3) and described how he “spotted some bright yellow fungi fruiting inside a large clay pot, which was being used for growing herbs in a rich compost”.

The article concludes that Conocybe aurea “is recorded from soils rich in nitrate, fertilised meadows, old compost, bark mulch and in greenhouses, as well as freshly sown lawns. Whether this is a genuinely rare British species or an occasional coloniser from elsewhere is not known at present. Any further information that clarifies the origin and status of the species would be of great interest to the authors.”

The story starts earlier than 2010, as Overall & Hodge note that Conocybe aurea was recorded at South Queensferry in Scotland in September 1969 and this led to the species being listed as British (note 4).

Funga Nordica describes the habitat of Conocybe aurea as “on nitrogen rich soil, dung, sawdust, bark litter and fertilized meadows”.

The ten records on FRDBI list the substrate as “soil” (seven records) and “compost” (three records) and the associations are given as “grass” (three records), “Tulip (Tulipa)” and “Spruce (Picea)” (one record each).

Andy Overall included Conocybe aurea in his book “Fungi: Mushrooms & Toadstools of parks, gardens, heaths and woodlands” (2017), which illustrates over 750 species of fungi with beautiful photographs. He describes the habitat as “On nitrogenous soil but also amongst woodchip mulch, in gardens, parkland, cemeteries or open woodland”. He notes that Conocybe aurea isn’t edible (note 6).

Further Afield

Records of Conocybe aurea on the Global Biodiversity Information Facility (GBIF) mainly come from Europe and North America, but the fungus has been found in South America, Australia and some parts of Asia.

At the time of writing, the United States has most records (163), followed by the Netherlands (85 records) and Switzerland (51). Germany and Denmark are next, followed by the United Kingdom. September and October are the most likely months to find Conocybe aurea.

Golden Conecap, Conocybe aurea

Golden Conecap, Conocybe aurea. The same specimens as in the last photograph, but a couple of days older.

I do hope I find Conocybe aurea again. But even if I don’t, I feel lucky to have found such these beautiful fungi so close to home.

It was more than enough reward for watering some pots.

Notes

Note 1 – “Funga Nordica: Agaricoid, Boletoid, Clavarioid, Cyphelloid and Gastroid Genera” (2008), edited by Henning Knudsen and Jan Vesterholt (Nordsvamp, Copenhagen). A fantastically detailed book with keys and line drawings of spores, cystidia etc. Sadly out of print but available to download as a PDF.

“Mushrooms and Toadstools of Britain & Europe – Volume 4” (2023), by Geoffrey Kibby. Part of a four volume set of books, beautifully illustrated by the author. Still in print.

Note 2 – Tony Leech wrote about this first for Norfolk in “Wildlife Report 2023: Fungi” in Transactions of the Norfolk & Norwich Naturalists’ Society Volume 57, Part 1 (2024).

In October 2023 Geoffrey Kibby posted a lovely picture of Conocybe aurea growing in a post of Cosmos on the British Mycological Society (BMS) Facebook page. I think this is the second record for Norfolk, which would make mine the third.

Note 3 – Field Mycology. Volume 16, Issue 1, January 2015, Pages 14-15.

Note 4 – Watling, R. (1982). British Fungus Flora Vol. 3: Bolbitiaceae. Edinburgh, HMSO. The fungus was growing on a track in woodland by the estuary of the River Forth. The fungus was “distinctive in its beautiful colours… saffron to apricot”. The species was first described by the mycologist Julius Schäffer in 1930 as Galera aurea and renamed Conocybe aurea in 1962 (note 5).

Note 5 – Julius Schäffer (3rd June 1882 – 21st October 1944) was an eminent German mycologist.

Schäffer enjoyed eating Brown Rollrim fungi (Paxillus involutus) at a time when they were considered to be a good edible mushroom. However, in early October 1944 he and his wife ate a meal containing Brown Rollrims and after about an hour he developed vomiting, diarrhoea, and fever. He was admitted to hospital the following day and died of renal failure just over a fortnight later.

It is now known that Paxillus involutus contains an antigen that can eventually (often after consuming the fungus many times over many years) trigger an autoimmune reaction causing the body’s immune cells to attack its own red blood cells.

Modern books on fungi describe Brown Rollrims as “not edible – toxic” but the real danger of eating them has only been understood for the last forty years. Earlier field guides “Collins Guide to Mushrooms & Toadstools” by such as Lange & Hora (1963, but reprinted into the 1980s) say “harmless if cooked, of little value; slightly poisonous to some when raw”.

Wikipedia cheerfully notes: “Despite the poisonings, Paxillus involutus is still consumed in parts of Poland, Russia, and Ukraine, where people die from it every year.”

Note 6 – Sadly, Andy Overall’s book is now out of print but I managed to find a secondhand copy earlier this year. It is too heavy to take out into the field but it has some of the best photographs I’ve seen of a range of fungi found in the London area (but also further afield), accompanied by some very useful text.