Research shows that grass pollen is by far the most common cause of hay fever in Victoria.

Plants release their pollen when they are flowering, and different types of plants flower at different times of year. Many deciduous trees such as birch, plane and elm flower in late winter and early spring, for example, whereas many grasses flower in the period from October to December.

Grass pollen data collected in Melbourne for more than 20 years shows that the highest levels start occurring, on average, in mid-October and fluctuate daily until the end of December. By the end of November, grasses start dying off and the levels of grass pollen in the air begin to reduce. The exact start and finish of grass pollen season varies slightly from season to season, but is typically from 1 October to the end of December.