Ragwort is an aggressive colonizer of any bare soil on which seeds may alight. It is most commonly found on set-aside agricultural land, where there is no established sward following cultivation or on pasture land especially if over grazed, poached by the feet of livestock, or otherwise poorly maintained.
Neglected and under grazed pasture land conversely loses sward density and admits ragwort. It grows widely on wasteland, and has spread along the motorway network where embankment plantings and establishing swards give it opportunity.
Ragwort has also gathered a large community of insect species; which can be divided into those 30 which wholly depend on it, those 52 which use it as a substantial part of their food, and those 117 for whom it is a major nectar source. Exotic invasive species, however attractive, should be eradicated. Ragwort, which is native and an important part of our fragile tapestry of flora and fauna, should be controlled wherever it poses a threat to livestock or where it appears to be becoming dominant, but eradication is not a desirable option.
Seeds germinate mainly in the autumn when they are shed, and the plant grows as a rosette of leaves from an eventually spreading rootstock, which may develop adventitious offshoots around the parent plant. The rosette builds sufficient size and energy to produce the flowering stalk in its second growing season or sometimes later. Winter chill is required to initiate the development of the flowering stage. Damage, such as pulling that does not remove the whole rootstock, encourages the plant to produce adventitious shoots and to perennat, eventually producing multiple flowering stalks. Seeds can then germinate in the space left by the pulled main crown.
The flowering and seed- producing season can be extremely prolonged, from mid June until November. A large plant can produce over 2000 flowerheads in a lifetime, each producing 70 or more seeds. It is recorded that some plants produce as much as 200,000 seeds. Seed produced is of two types. The central seeds with their parachute of pappus can be distributed by wind up to 70 metres or so from the parent plant. The heavier seeds from the edge of the disc are designed to remain in situ until shaken free, and can often germinate in the gap in the sward produced by the death of the parent plant. Seeds that do not germinate in the autumn of their shedding can persist in the top 4cm of soil for 4-6years, but seeds buried to a greater depth than this, can survive for at least 16 years.
Ragwort is undoubtedly toxic to horses and cattle, especially so to horses with the alkaloids causing damage to liver DNA, leading to death of cells. It is the damage which is cumulative, not that the alkaloids themselves accumulate in the liver.
Repeated ingestion causes repeated asymptomatic damage until a total threshold is passed and symptoms show. At this stage, it is usually too late. The lethal dose may be ingested all at once, over a few months, or over 10 years! It is this irreversible cumulative effect that makes Ragwort so dangerous. The lethal dose for horses is stated to be between 3-7% of body weight, though some researchers claim 20%. Such discrepancies are probably because of variations in gut flora, which can help break down the toxins, from case to case, the amount of water in the ragwort, or its stage of growth. It is not difficult to see how a 500kg horse could easily ingest the equivalent of 15 kg of weed over a season or two, especially when dried and palatable in hay. Arguments over how toxic ragwort is to horses, and the number killed each year in the UK, also seem to me to express a polarization of opinion, rather than interest in scientific truth or simple common sense.
Annual deaths are reported from 10 to 6500 (!) depending on which evidence is employed. Death from liver destruction is an appalling end for any animal and correct identification, management and eradication of Ragwort must be practised, by horse keepers and stockmen, to avoid it. Proper paddock management is also important, as overgrazing and lack of good pasture care gives rise to the thin swards that the weed can easily colonize.