The stem leaves are irregular and ragged edged from which comes the name, ragwort, and they are dark green in colour. The flowers are densely packed together into large flat-topped heads and are bright yellow. The cluster of daisy-like heads are soon replaced by large numbers of white downy seeds, which are carried away by the wind, rapidly multiplying the problem. Seeds may lie dormant for several years before germinating. Close grazing and low fertility of land, both of which are common where horses are intensively grazed, appear to encourage the growth of ragwort.
Sheep are also susceptible, but they are perhaps more resilient.
As death in sheep from ragwort may be delayed for several months the cause may be overlooked. The practice of grazing sheep on rosette stage ragwort to keep clear pastures is not to be recommended. The toxic effects are mainly in the liver and digestive tract of the recipient. Symptoms of poisoning can appear quickly or be delayed for weeks or months. Similarly, death can be within days or with a delay of several weeks following declining stages of health.