The species is dioecious and over much of the extant British range there is a considerable disparity between the numbers of male (frequent) and female (rare) trees, a consequence of the deliberate selection of male clones (they do not cause problems through the production of debris associated with flowering/fruiting); in some parts of lowland England (eg. Sussex) ratios approaching 1:1 are still found but tree numbers are low and the sexes are not intimately associated. Natural sexual regeneration is thus extremely uncommon, not least because the conditions necessary for germination and early establishment are rarely present. The seeds are short-lived and must fall on bare, moist competition-free surfaces that are neither dry, or become flooded. Where conditions do allow germination, seedlings are likely to be hybrids as pollen contamination from the more abundant cultivated varieties is probable. The estimated radius for pollen travel is about 16km (Jones, 2004).
Although the tree doesn’t sucker readily, natural vegetative propagation from layering of fallen individuals, debris washed down rivers, etc. occurs, but is less likely to happen now that watercourses are so managed and river and floodplain systems have become less dynamic. Damage to root systems through agricultural and other activities may encourage suckering.
Water Poplar can be propagated from hardwood or softwood cuttings, the latter more easily. Early advice suggested using lengths of 15-20cm, from new growth made in the proceeding summer, taken while the tree is dormant; vigorous epicormic shoots are particularly suited to this purpose. These cuttings were then planted such that at least half their length was underground. Recently the use of longer truncheons, 1-2m lengths of vigorous vertical terminal shoot growth, trimmed at the base and rapidly planted to at least a depth of 30cm is recommended. Past bunch planting of cut shoots has given rise to dense clonal stands in some localities.