Tree pollen season is already triggering hay fever in the national capital, and the start of grass pollen season could be just weeks away, according to experts.

Experts from the Canberra Pollen Monitoring Centre are warning we could be in for an above average season for grass pollen.

Pollen can increase symptoms of hayfever and asthma, and scientists say increased growth in vegetation could mean more will be in the air this Spring.

Grass pollen season usually starts at the end of September or early October and runs until late December.