In the winter period with prevailing low light intensities and short days, the forcing of Iris “Wedgwood” can easily result in flower bud abortion (bud blast). Consequently a high proportion of plants fail to produce flowers. To elucidate the cause of bud blast, plants were subjected to various treatments —12C02, 14C02, growth substances — in the period of rapid stem elongation about two weeks before flowering. Plants grown under weak illumination and normal C02 concentration (leading to 57% flowering) were compared with plants grown under strong illumination at low C02 concentration (leading to 100% flowering). As the dry weight of whole plants of both these groups was almost the same at flowering, bud blast under low illumination cannot simply be attributed to a lack of recent photosynthetic products. The distribution pattern of photosynthates appears to be changed. It is suggested that a hormone imbalance plays a role in the distribution pattern and consequently in bud blast, since injections of cytokinins, such as Ar6-benzylaminopurine, zeatin, and kinetin, during dark treatment can increase the flowering proportion.