RAGWORT - Senecio jacobaea

 

An article in the Chorley Guardian for 14^th August, 2002, headed 'Poison weed patrols to combat roadside killer' brought back memories of nearly fifty years ago when I wrote a thesis on 'The ecology and control of Senecio jacobaea (Ragwort) in south-west Wales'.  The article went on to say that 'a deadly weed which can kill grazing animals is being rooted out throughout Lancashire by Highway Agency patrols.  Ragwort….grows on the verges of the area's roads and is one of the most frequent causes of livestock poisoning in Britain….'

 

The article betrays a lack of understanding of the ecology of the plant.  The genus Senecio is a large one with over 2,000 species spread over much of the world.  Ragwort itself is native to all of Europe and a wide sweep of western Asia but is also an introduced weed in the Americas, Southern Africa and Australia and New Zealand.  Its natural habitat is wherever it does not face much competition - sand-dunes, waste land and overgrazed pastures where the sward is exceedingly short.  Ecologically, it plays its part in the complex interrelationships of plants and animals.  It provides food for a wide range of bees and flies, which visit the flowers, and the plant is the main food source of the caterpillars of the cinnabar moth, which also absorbs the toxic alkaloids and uses them in its own defence, so that predatory birds are repelled by the bitter flavours imparted.

 

It is poisonous to cattle, sheep and horses, when eaten in quantity, but significantly rabbits appear to be unaffected.  Fortunately, Ragwort is very unpalatable and domestic livestock will not eat it unless they are desperate with hunger.  It contains alkaloids which cause a form of cirrhosis of the liver called Pictou Disease from the Island of Pictou in the mouth of the St. Lawrence River in Canada where sheep were first diagnosed as suffering from the ill effects many years ago.  The island had been heavily overstocked with sheep which, having exhausted the grass cover, turned in their hunger to the Ragwort which also grew there.

 

Ragwort is usually a biennial, germinating and establishing a rosette of leaves in its first year and sending up the familiar flowering stem to a height of as much as four feet in its second year.  Mowing, once the rosettes are established, is of little value as a control measure as the rosettes are so close to the ground that mower blades pass harmlessly above them.  Nor are hormonal weed killers certain to kill the plant as the rosette leaves are difficult to wet thoroughly.  In the second year the flower stalk shoots up and the rosette quickly dies, its work completed.  However, if the flower stalk is cut down at an early stage, then the rosette persists and the plant is well able to become a perennial and flower in a following year.  Moreover, as I discovered by laboriously germinating seeds from many flower heads, even if sprayed by hormonal weed killers, which will kill the plant, a proportion of the flower heads produce viable seeds, which can perpetuate the species.

 

I was researching during the first great outbreak of myxamotosis in rabbits and quickly discovered that, with the death of practically all rabbits, grazing pressure reduced dramatically, the grass grew longer and shaded out seedling Ragwort, preventing it from establishing.

 

The moral is that pastures should not be overgrazed and the Ragwort will not establish in quantity.  There will be grazing for domestic livestock and they will not, in their hunger, turn to Ragwort.  Follow this rule and cinnabar moths can graze happily!

 

Robert Yates