This is a stranger you smell first, long before any sighting. I was walking with my dogs on the mountain behind the tiny town of Smithfield this morning when the scent reached me: delicious, sweet and spicy. The strong wind blew the smell in all directions but I knew what I would find when we reached the top of a hill near the summit of the mountain:

Wattle on the Smithfield mountain – serious alien invader

With its bright yellow flowers the tree stands out strongly against the indigenous bush, a shock in the harmoniously coloured late winter veld.

We are lucky in Smithfield because the wattle problem here is not nearly as severe as, for example, in the Langkloof, where infestations are so dense and widely distributed that the water supply to Port Elizabeth is threatened.

In fact, in his book, Problem Plants of South Africa, Clive Bromilow includes a map that shows the Smithfield area as free of the major wattle threats: Acacia dealbata (Silver wattle), Acacia decurrens (Green wattle) and Acacia mearnsii (Black wattle). The black wattle, best known of the group, was introduced into what is now KwaZulu-Natal by immigrant John Vanderplank, who planted seeds from Tasmania on his farm at Camperdown. A. mearnsii was then cultivated for its tannic acid, used in the leather industry, while the timber is still used for pulp, firewood and the mining industry.

The wattles, with their legendary thirst, are now regarded as serious invaders of veld and indigenous bush as well as – crucially – of watercourses. They all produce huge numbers of seeds that can stay dormant for more than 50 years.

This morning’s sighting is a warning signal: in such a dry environment we can’t afford to lose a single drop of water to this unwelcome stranger and it’s time to get a hacking party ready for action.

The wattle’s bright yellow balls