It’s the season for searching for the first signs of spring: early snowdrops, the first hazel catkins, precocious primroses. But this scruffy cluster of groundsel plants, colonising bare earth by a roadside, has been flowering all through winter. A wild flower for all seasons, blooming in every month. In January it was frozen in ice, flattened by snow, sprayed with a mist of road de-icing salt by passing vehicles. Still it rises, releasing plumed seeds wafted away in the traffic slipstream. To the gardener, groundsel (Senecio vulgaris) is a persistent weed. To a botanist, it’s a ruderal – a ubiquitous, pioneering coloniser of soil disturbed by human activity, adapted to a life of vagrancy. Its inconspicuous yellow flowers need no pollinator, its seeds are carried everywhere by wind. Stunted, droughted and nutrient-starved, in a crevice in a pavement, it might produce one flower; lush specimens in a farmyard produce hundreds, releasing thousands of seeds. Within three fast-growing generations in a single summer, one typical plant could easily give rise to a million descendants. Wherever Europeans travelled, groundsel followed. Early settlers accidentally took it to North America and the antipodes, then brought it back again from Australia with an interesting addition. The leaves of these roadside verge plants are speckled with yellow spots, each smaller than a pinhead – the fruiting bodies of an Australian rust fungus called Puccinia lagenophorae. First noticed in Britain in 1961, it spread rapidly. Like any efficient parasite, the fungus weakens the plant without killing it, ensuring a future supply of hosts. I took some home for a closer look. Magnified 50 times under a microscope, those lesions were revealed as structures of exquisite beauty, tiny cups erupting through the leaf surface like miniature suns, with sunbeams formed by chains of countless golden spores. No wonder it is so contagious among dense groundsel colonies with no social distancing. Senecio vulgaris is a traveller, part of an increasingly globalised weed flora, now with its own pandemic in tow. In 2000, the rust caught up with it in the US. Groundsel lacks the botanical celebrity charisma of, say, an orchid or carnivorous Venus flytrap, but this weed’s story is part of our story too.