Wattles (Acacia)

Nothing announces the ranges’ cool season quite like a wattle in full flower — a shrub or small tree suddenly amok with fist-sized, egg-yolk puffballs of bloom, so bright it looks less like ordinary plant business than a decision someone made. Dozens of wattle species are threaded along the gulf-to-range gradient, from wiry range-front shrubs to taller gully trees, and South Australia has long claimed one of them, golden wattle, as its own emblem. Look past the flower, and past the flat, blade-like “leaf” — on a mature wattle usually not a true leaf at all but a phyllode, a flattened leaf-stalk doing a leaf’s job at a fraction of the water cost, the same sclerophyll accounting that runs through so much of this bush (Keeley et al. 2012) — and the real business of a wattle is going on in the seed.

Each wattle seed is small, dark and hard-coated, and glued to one end is a small parcel of fat and protein a botanist calls an elaiosome: cheap for the plant to build, irresistible to an ant. An ant finds the fallen seed, hauls it into the nest, strips off and eats the parcel, and abandons the seed itself — now cleaned, buried and planted in exactly the rich, undisturbed soil a seedling wants. Wattles, in the pea family Fabaceae, are among the anchor genera of this trade: something like 1,500 Australian plant species have independently evolved the same fatty bribe, the richest ant-dispersed flora on the planet, and a scrubby hillside of wattle is one of its most ordinary working examples (Berg 1975).

Being carried underground by an ant matters twice over, because the same hard coat that survives an ant’s mandibles also survives fire — and fire, not the parent plant’s calendar, usually decides when a wattle seed germinates. Buried and undamaged, a seed can sit dormant for decades, until the heat of a blaze cracks its coat or smoke through the ash tells it the coast is clear, at which point seed nobody knew was there germinates across the burnt ground within weeks (Auld 2020). Wattles themselves tend to be quick, short-lived, disturbance-loving shrubs, which is why the seed bank underground matters more than any one shrub above it: a slope cleared, grazed hard or burnt through can go from bare ground to a thicket of gold-flowering wattle within a couple of seasons, running on seed that had been waiting since long before the disturbance arrived.

Read it that way, and a “weedy” flush of wattle after roadworks or a fire stops looking like an accident of good rain — an old bargain finally called in, an ant that buried a seed and a fire that, sooner or later, woke it.