Driving through the suburbs and along the highways of Durban over the last few weeks, I found myself constantly greeted by bursts of brilliant star-yellow blooms hanging over fences, verges, and medians throughout the city.
What a wonderful winter welcome, but what species of flower are these? Turns out these gorgeous banks awash with yellow are not beautified by a native species; these are Mexican sunflowers or Tithonia diversifolia for the fancy people.
Invasive Species South Africa describes the Mexican sunflower as an annual or perennial shrub which has a woody, tree-like appearance at the base. The bush can reach heights of up to 1.5 metres to 3.5 metres. The shrubs' pale and velvety leaves are deeply lobed and five-fingered and grow up to 150mm long and 120mm wide.
The flowers, which are large and strikingly yellow, bloom up to 100mm across, growing solitary on long stalks like a traditional sunflower or together in branched clusters, and appear in autumn from April to June.
The Invasive Species Compendium explains that the Mexican sunflower is a herbaceous flowering plant which has been widely introduced as an ornamental flower but has escaped from cultivation to become invasive, mostly in disturbed sites, along roadsides, and in rural areas near cultivation.
When I say “escaped”, I do not mean that they uprooted themselves and hopped the fence, although that would be pretty amazing. Due to their brightly coloured flowers, which are easily pollinated, seeds are plentiful and just need a little more than a strong breeze to spread to areas where they establish a kind of a colony of sorts.
The resourceful shrub is a successful invader of new habitats through its tolerance to heat and drought, its rapid growth rates, and its large production of lightweight seeds, which are easily dispersed by wind, water, and animals.
Dormant seeds also remain viable in the soil for up to four months. Once established, it quickly forms dense stands, which quickly out-compete native vegetation and thus prevent the recruitment and growth of native plant species. Allelopathic activity has also been reported for this species.
Allelopathy is simply a natural process of inhibition or stimulation of plants by the action of allelochemicals that are produced and released into the environment by different plants.
In this case, the Mexican sunflower has been known to release allelochemicals to inhibit the growth of surrounding vegetation to allow itself the opportunity to grow.