Variation in annual flowering effort is described for 16 long datasets from 11 species of Chionochloa (Poaceae) in New Zealand. All populations exhibited extreme mast seeding. The most variable species was C. crassiuscula (coefficient of variation, CV=3.02) over 26 years at Takahe Valley, Fiordland, which is the highest published CV we know of worldwide. The other populations also had high CVs (lowest CV=1.42, mean CV=1.84) which were higher than for other well-studied genera such as Picea, Pinus and Quercus. There were also frequent years of zero flowering (mean across all populations was 37.2% zero years; maximum 53% for C. rubra and C. crassiuscula over 19 years) whereas zero years are rare in other published masting datasets.Flowering was highly synchronous among species within a site (mean r=0.886), and also (though significantly less so) among sites. Among sites, synchrony was not significantly higher within-species (mean r=0.711) than between-species (r=0.690). Warm summer temperatures led to heavy flowering the following summer. Flowering synchrony increased with increasing synchrony in local deseasonalised summer temperatures, and decreased with increasing distance between sites.Mast seeding has been shown in Chionochloa to reduce losses to specialist flower or seed predators. Among-species synchrony may be adaptive if species share a common seed predator. Developing seeds of at least 10 Chionochloa species are attacked by larvae of an undescribed cecidomyiid. In Takahe Valley, where masting is most pronounced, cecidomyiids attacked all six Chionochloa species in all four years studied. Mean annual losses were almost constant (10.0 to 13.4%) while flowering effort varied 100-fold. The invariant losses are consistent with other evidence that the cecidomyiid may have extended diapause, which would make it harder to satiate by mast seeding. We hypothesise that one possible factor favouring such extremely high levels of mast seeding in Chionochloa is that its seed predator is very hard to satiate.