Sequential flowering of sympatric plant species is a commonly observed phenomenon and is thought to contribute towards maintaining the pollinator community. Sequential flowering is also interpreted as a potential way to reduce competition for pollinators and to reduce interspecific pollen transport.

Within-season sequential flowering is well-known for many species assemblages and usually shows a staggering over weeks and months. Here, we report on the flowering behaviour of two unrelated sympatric cacti (both Cactaceae — Cactoideae) from N Argentina, Acanthocalycium thionanthum (Cereeae — Trichocereinae) and Parodia microsperma (Notocacteae). At the start of the flowering season, the two taxa exhibit a temporally stable sequential mass-flowering that is triggered by the first rainfall event after the dry season: On the 6th or 7th day after the rainfall, Parodia microsperma starts a 3-day “big-bang” flowering period, and on the 8th or 9th day after the rainfall, Acanthocalycium thionanthum also starts a 3-day “big-bang” flowering period, with 1 day of overlap with that of Parodia. The sequence and duration, including the 1-day overlap, has been found to be temporally stable in all study years, with c. 90% of all flowers buds of a population opening synchronously in years with average spring rainfall. Later in the season, after the fruits derived from these initial flowerings have ripened, opportunistic flowering (including sporadic local small-scale mass flowerings) is exhibited by both species with no apparent synchronization or sequence. Only once was a second “big bang” flowering observed, likely caused by above-average dry conditions. The species pair is notable for the massive “big-bang” flowering triggered by rainfall with a tightly staggered synchronization, as well as for the change in flowering strategy later in the season.