The plants of the genus Senecio often produce attractive flowers and foliage, and their appealing characteristics lead gardeners to bring the plants into their gardens. While some Senecio species are well-behaved annuals, others are pernicious invaders with sinister hidden qualities that make them a potential threat to the animals and people that encounter them.
The genus Senecio is one of the largest of all flowering plant genera, containing well over 1,000 species. The species are very diverse, ranging from herbaceous annuals, to shrubs and succulents. Many Senecio species are problematic; they can be invasive and difficult to control outside of their native habitats, and many species are toxic to animals and humans. Some Senecio species, such as dusty miller (Senecio cineraria), are popular and harmless garden annuals, however.
Ragworts, Groundsels and Succulents
Some of the most common Senecio species are the annual ragworts and groundsels. Common ragwort species include tansy ragwort (Senecio jacobaea), oxford ragwort (Senecio rupestris) and Barkley's ragwort (Senecio salignus). Common groundsel (Senecio vulgaris) is also widespread. Several succulent species of Senecio have recently been moved to their own genus, Kleinia, but some, such as succulent bush senecio (Senecio barbertonicus) are still considered by some botanists to belong to Senecio; this species is perennial in U.S. Department of Agriculture hardiness zones 5 and above.
Some Senecio species, including tansy ragwort and common groundsel, contain pyrrolizidine alkaloids, naturally occurring substances that are potentially deadly to livestock and toxic to humans. These alkaloids accumulate in the livers of animals that eat the plants, eventually causing scarring and, if enough of the plant is consumed over time, potentially fatal complications. The plants are especially dangerous because they grow well in pastures and fields where cows and horses typically graze. Fortunately, livestock don't find the plants particularly tasty and will likely avoid them unless no other palatable plants are available.
Because of their toxicity, many Senecio species are unwelcome in areas where they are introduced, and their prolific reproductive strategies make them difficult to control. Common groundsel, for example, may produce a million seeds per plant in a season, and the seeds are easily dispersed by the wind, allowing the plant to spread widely across fields and along roadsides. The cinnabar moth is sometimes used as a biological control since its caterpillars like to eat both tansy ragwort and common groundsel.