We analyzed the effects of temperature, precipitation and the age of the mother plants on the change of an important seed quality parameter, the thousand-grain weight (TGW) in the case of three Hungarian cultivars of different grass species (’Tomaj’ red fescue, ’K-50’ tall fescue and ’K-51’ smooth brome). Examining the seed samples of six years from the stocks maintained at the MATE Georgikon Campus (Keszthely), we found that the age of the stocks did not affect the value of TGW. The age of the stocks did not affect the TGW. Only the heat sum lower than average showed negative effect to the TGW in the linear model applied the heat sum and total precipitation calculated between 1st of May to the harvest date, and the age of the stocks as independent factors. Precipitation had a positive but weak effect, the amount of precipitation calculated from 1st of November of the previous year to harvest correlated better with TGW. We also found that proper purification and the TGW-data correction for the same moisture content are necessary in such a comparative study of TGW values of grass seeds.