In pot and field experiments, the emergence and bud appearance of Fat hen (Chenopodium album L.) were observed. The quantitative relationships between the day length and temperature, and the duration of the period between emerence and bud appearance were determined. Fat hen accelerates budding with shorter days. Under similar thermal and photoperiodic conditions buds appear earlier in autumn than in spring; no reason has been found for this regularity. Multiple regression equations describing the relationships studied are presented.