The weedy Fireweed (Senecio madagascariensis) is often confused with the native locally found Fireweed Groundsel (Senecio linearifolius). Hopefully, the following information will reduce this confusion.
Differences between native and introduced species
The native species is up to 900mm higher than the weed species. Also the petals of the native species are shorter and fewer (8 or fewer, compared to 13) with much larger leaves that are conspicuously veined on the upper surface and the margins are fine, regularly toothed and recurved. The differences in the leaves are apparent in seedlings as well as mature plants.
Introduced Fireweed (Senecio madagascariensis)
Fireweed is a highly invasive and opportunistic weed native to SE Africa.
Fireweed is able to grow on most soil types and in all aspects. It forms a persistent seed bank if not controlled before flowering and can rapidly take over cleared areas. One plant is capable of producing 5,000 to 30,000 seeds in one season depending on conditions.
Fireweed is a daisy-like plant that grows from 100 to 600 mm high. It is usually a low, heavily branched, annual or short-lived perennial plant.
Leaves: Generally bright green, fleshy and narrow, 20–70 mm long, alternately arranged on the stem, with serrated, entire or lobed margins. Broader leaves usually clasp around the stem.
Flowers: Small, yellow and daisy-like, flowers are 10–20 mm in diameter and arranged in clusters at the end of each branch. Each flower has 13 petals and 21 bracts forming the ‘cup’ under the flower.
Seeds and dispersal: Seeds are small (1–3 mm long), light and slender. Most seed will fall within five metres of the parent plant but some seed can be spread to greater distances in updrafts and whirlwinds.
Native Fireweed Groundsel (Senecio linearifolius)
Perennial herb or shrub 500–1500mm high, stems few-branched, glabrous to cobwebby.
Leaves: variable, either linear or elliptic. Mostly linear to lanceolate or narrow-elliptic (oval and flat, broadest at middle and tapered at ends). 60–150 mm long and 5–15 mm wide with margins ± recurved and entire to sparsely toothed, lower surface mostly cobwebby, base tapered and petiole-like.
Flowers: throughout the year. The structures supporting the flowers arise at different points on the stem but, the flowers are at the same level resulting in a flat top arrangement. There are numerous cylindrical, 2–3 mm diameter flower heads that are glabrous except at the apex The seed is compressed, 2–2.5 mm long, brown, glabrous with a 5–8 mm long pappus.
All photos are from Plantnet