Riddell's ragwort

Riddell's ragwort (Senecio riddellii) is a perennial herbaceous plant belonging to the Asteraceae family. In agricultural settings, it is primarily recognized as a harmful weed of rangelands and pastures. Its significance lies in its toxic properties, specifically the presence of pyrrolizidine alkaloids that cause chronic liver damage in livestock such as horses and cattle.

The plant originates from the western and central regions of North America. It is natively distributed across dry, sandy plains and foothills, showing a remarkable resilience to aridity. In agricultural landscapes, it tends to colonize areas where soil has been disturbed or where the native grass cover is sparse, making it a persistent concern for land managers.

Botanically, the species is defined by its erect, branched stems that grow between 30 and 80 cm in height. Its leaves are thread-like or very narrow, serving as an evolutionary adaptation to arid environments. The plant produces abundant yellow flower heads arranged in flat-topped corymbs, which bloom from mid to late summer, contributing to significant seed dispersal.

Management of Senecio riddellii requires integrated control strategies, as the plant is difficult to eradicate once established. Agricultural practices focus on preventing seed production and reducing soil disturbances. Key management approaches include:

• Mechanical removal or mowing before the flowering stage;
• Application of selective systemic herbicides in spring;
• Improving grazing management to prevent overgrazing of desirable forage;
• Reseeding with competitive, perennial native grasses.

While the plant has virtually no direct economic use, it plays a role in local ecosystems as a late-season pollen source for various wild pollinators. However, its economic negative impact remains high due to the necessity of livestock exclusion from infested pastures. Common pests include specialized insects that feed on ragwort, while diseases such as rust fungi may affect foliage, though these rarely control the population sufficiently for agricultural needs.