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The highly invasive Fallopia complex ( Fallopia japonica s.l., Fallopia japonica var. japonica, Fallopia sachalinensis and the hybrid Fallopia × bohemica) is known to spread primarily vegetatively by rhizomes and stem fragments in its adventive range. Sexual reproduction has rarely been shown in Europe, despite a high production of germinable seeds. To elucidate this contradiction the aim of the present study was to determine which environmental conditions prevent seed germination or seedling establishment. Several laboratory experiments were conducted focusing on water and light supply as well as the influence of frost. According to morphological and cytological analyses of the parental plants paired with morphological analyses of the seedlings, the seeds used for these experiments were assumed to belong predominantly to the hybrid Fallopia × bohemica. Laboratory experiments revealed that seedling establishment is hampered by severe drought or strong late frost, which are, however, rare events in the study region and could not be crucial in explaining the rarity of seedlings in the field. We discuss the possible role of weather conditions that might lead to a loss of seeds or seedlings under field conditions and include a comparison of several other cited studies carried out in the adventive range of Fallopia japonica s.l. that observed either the presence or the absence of Fallopia seedlings in the field. [ABSTRACT FROM AUTHOR]