The development of hormonal contraceptives stands out as a key contribution of biochemistry to the 20th century, part of the wider ‘sexual revolution’ that dramatically changed society in many Western countries. But unbeknown to them, the pioneers of the contraceptive pill had been beaten to the idea by a few hundred million years, by a rather unlikely group of organisms that have been using hormones as contraceptives since their own sexual revolutions back in the swinging Palaeozoic. Since their successful conquest of land in the Ordovician, land plants had been restricted in the genetic mixing and expansion of populations by their relative immobility. A series of key innovations in the seed plant group, and in particular in flowering plants, enabled plants to mate and to disperse their offspring over much longer distances, by harnessing the wind or animals to provide mobility. However, all this ‘outsourcing’ created new challenges; coordinating and optimizing reproductive effort is not straightforward when it depends on a third party. Here, I discuss some of the key signalling molecules – sex hormones, as it were – that plants use to plan their families and manage their fertility, and why this matters to us, now more than ever.