This study produced maps of temporal and spatial variability of the photothermal quotient in the state of Rio Grande do Sul (RS), Brazil. The possible relations between wheat grain yield potential and climatic risk zoning for this state are discussed. The results show that there is temporal and spatial variability of the photo-thermal quotient in RS and also that it affects the expression of wheat grain yield potential. Such circumstances indicates the need to find a technology especific for each environment, in order to optimize the wheat grain yield potential for each county in the state. However, one can not forget that even the best sowing date recommendation for each county carries a certain risk of frost at anthesis.