Why thistles are a menace for livestock but are ecologically important - Roger Ratcliffe

Farmers mostly hate thistles, regarding them as one of the most pestilential weeds of all since they often grow vigorously in fields which are grazed by livestock or used for silage.

Picture by Bruce Rollinson.

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As a result there are fewer to be found in the countryside these days, and one footpath in Airedale where I have long had to negotiate jagged thistles with care has recently been purged of them.

The commonest variety is the creeping thistle – Cirsium arvense – a perennial which spreads its rootstock widely to create big colonies on grassland and pretty much anywhere else where the ground remains undisturbed.

Arguably the biggest nuisance to farmers, though, is the spear thistle (C. vulgare) – a biennial that establishes itself unnoticed in the first year and can then sprout an impressive spread of prickly fronds more than three ft wide and stand over six feet tall.

This is the species widely regarded as the “Scotch thistle”. The reason for its adoption as the national symbol north of the border, apparently, is that on the eve of the Battle of Largs in 1263 Viking invaders led by King Haakon Haakonsson of Norway crept up on the sleeping Scots army commanded by Alexander III but found themselves in a field of jagged thistles. Their cries awakened the Scots who, thus alerted, were able to defend themselves and send the Vikings packing.

Another species, the melancholy thistle (C. Heterophyllum) can be found even by roadsides at this time of year and I have seen its purple heads bristling along grass verges in Upper Wharfedale.

It has been shown that 10 per cent infestation on grassland causes 10 per cent loss in grass yield. Thistles are also held responsible for helping to spread between sheep the orf virus, also known as sore mouth. Farmers often deal with the problem by chopping thistles on the surface but find that they soon grow back again, sometimes more vigorously than before, so the latest line of attack is to spray them with a dedicated herbicide.

Personally, I am always saddened when I find thistles have disappeared from one of my favourite rural haunts because they perform a vital ecological role. For example, the spectacular six-spot burnet moth is often found on thistles as are several species of fritillary butterflies. As a birdwatcher, my biggest regret at their loss is that I am denied the sight and sound of goldfinches.

Goldfinches are as synonymous with thistles as blue tits and peanuts. In some places, in fact, the bird is known locally as the thistle finch and in Anglo-Saxon times it was called the thisteltuige or thistle-tweaker. Goldfinch beaks are longer and sharper than other finches in order to pull the seeds from thistle heads.

There are few more delightful experiences in nature than hearing goldfinches attacking thistles. As they eat they keep up a conversation with their lovely tinkling call, which is said to be as delicate as Chinese bells. No wonder then, that the collective name for one of our most colourful birds is a charm of goldfinches.