Pest of the Month: Fireweed

A primary concern brought about by this weed is its ability to rapidly spread and its danger to livestock, if ingested.

By Damon Adamson.

Fire weed, yellow flowers

One of more than 1200 species of Senecio globally, Senecio madagascariensis, fireweed, or Madagascar ragwort is of specific concern to Hawai‘i. Having been established in Hawai‘i in the early 1980s, it has continued to climb the chain as an invasive, dangerous, and noxious weed.

S. madagascariensis, an upright, branched mostly annual herb, growing from seed, that generally grows to heights between four and 24 inches tall. Its narrow, bright-green, alternating leaves can reach two to five inches in length and one-quarter inch across, often lobed and can demonstrate a smooth or serrated edge. The inflorescence is terminal and comprised of both disc and ray florets. Dull to bright yellow in appearance, similar to a common daisy, mostly displaying a 13-patterned ray floret count. Additionally, each type of floret, disc and ray, produce seed.

As stated previously, fireweed has annual tendencies, however it has been identified as reproducing vegetatively—this is illustrated when the stem is trodden or trampled close to moist soil, resulting in new root and shoot growth at stem nodes, as a perennial often demonstrates. Producing a shallow, yet highly fibrous tap root adds to the weeds efficacy in various substrate types. Each flower has the potential of producing 150 seeds, yielding individual plant production of approximately 30,000 seeds. Seeds or achene, are slender, cylindrical, and about one-tenth inch long. Tufted with white fine hairs, the seeds are easily dispersed by air and waterborne methods.

A primary concern brought about by this weed is its ability to rapidly spread and its danger to livestock, if ingested. Along with its ability to produce large amounts of seed per plant, its seeds are viable for several years and often grows prolific in pasturelands, roadsides, and disturbed areas. Twenty-five out of 1200 separate species of Senecio have been determined to be toxic to animals. It is strongly believed that S. madagascariensis could be number 26 on the toxic list.

The principal toxins in these species are compounds known as pyrrolizidine alkaloids (PA). PA concentrations vary with the species and their growth stages. Generally, young plants are more toxic than older plants. Agriculturally important animals in Hawai‘i that have susceptibility to PA are pigs, poultry, cattle, horses, goats, and sheep. Studies have been conducted with tansy ragwort (S. jacobaea), threadleaf groundsel (S. douglasii), and Ridell’s ragwort (S. riddellii), among others, for effects of latent PA levels in livestock from ingestion.

Pyrrolizidine alkaloids are secreted in the milk of cows and goats and in low quantities can cause mild liver changes in calves and kids consuming the milk. There is no evidence that humans are adversely affected by consuming milk that contains PA. Pyrrolizidine alkaloids are readily absorbed from the digestive tract of the animal and transported to the liver. Enzymes in the liver convert PA to toxic pyrroles, which are the principal toxins causing liver cell damage. The pyrroles biodegrade following reaction in the liver and thus do not accumulate in the animal. The greater the exposure of the liver to pyrroles, the more liver damage occurs.


UHM CTAHR: Fireweed Control