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The development of summer-dormancy under natural conditions was studied in Poa BulbosaL., transferring plants from a sandstone hill in the central coastal plain of Israel to controlled photo-thermal conditions. Plant transfers were done several times in 1981–2 and 1987–8 during autumn and winter, the active growing season. The proportion of plants exhibiting dormancy (presence of bulbs, leaf senescence, and cessation of leaf appearance) increased the later the transfer to controlled conditions. Under controlled conditions of 16-h photoperiods (long days, LD), this increase started by the end of autumn, while under controlled conditions of 8-h photoperiods (short days, SD), dormancy was observed only in plants transferred after mid-winter. Plants in the field showed external signs of dormancy (leaf senescence) by the end of winter. At higher temperature regimes plants became dormant faster than at lower ones. Dormancy-enhancing effect by high temperature was found in LD for all transfers, and at SD only after the plants in the field had attained such levels of induction, by mid-winter, that dormancy development continued even under this non-inductive photoperiod.The results indicate that induction of summer-dormancy in Poa bulbosaunder natural field conditions starts quite early, probably during early winter, since an increasing proportion of the plants becomes dormant even under SD at later dates of transfer. Plants transferred from the field by late autumn, when days are shortest and temperatures are decreasing, show an increased responsiveness to induction of dormancy by LD, compared to plants transferred earlier. Hence, in P. bulbosathe induction of summer-dormancy by LD might be enhanced by a pre-exposure to SD alone or in combination with low temperatures.