The document states that the seeds of Ambrosia spp. can spread naturally by water (including heavy rains, floods, rivers, canals), wind and animals (Opinion of the Scientific Panel on Plant Health on the pest risk assessment made by Poland on Ambrosia spp.). Most seeds fall near the parent plant, but some can disperse long distances with water, animals, and human activities. Seeds are rarely consumed by animals. Birds and other animals do not commonly transport seeds of A. trifida, apparently because they are not an attractive food source, partially because of the large size of the seeds. One of the primary mechanisms of spread in crop areas is by cultivation and harvesting equipment which move from field to field and farm to farm. The most common means of introduction of A. trifida to countries or regions appears to be by seeds as a contaminant in other seeds. Once introduced to a region, this weed can flourish, especially in moist, heavy soils. Intentional introduction is considered highly unlikely (CABI). Seeds fall from maternal plants typically in autumn, but a portion remains on the maternal plant into winter. Incorporation of seeds into the soil occurs by rainfall, cryoturbation, and earthworm activity (Regnier et al. 2001) (Biology and ecology of Ambrosia trifida L. seedling emergence). No evidence was found to demonstrate that seeds are regularly dispersed long distance by animals.

For A. trifida the likelihood of seed dispersal is considered to be higher since the seeds float better than A. artemisiifolia. The document states that the seeds of Ambrosia spp. can spread naturally by water (including heavy rains, floods, rivers, canals), wind and animals (Opinion of the Scientific Panel on Plant Health on the pest risk assessment made by Poland on Ambrosia spp.). Most seeds fall near the parent plant, but some can disperse long distances with water, animals, and human activities. A. trifida produces seeds, and though the plants can be destroyed in early growth stages, the seeds retain their vitality and are scattered far and wide by flood waters and with blowing and drifting snow (CABI). A. trifida grows in its natural habitat in North America where it initially inhabited mainly river valleys, floodplains, drainage channels, moist disturbed open sites and others (AMBROSIA TRIFIDA (ASTERACEAE), A NEW NON-NATIVE SPECIES FOR THE BULGARIAN FLORA). Seeds fall from maternal plants typically in autumn, but a portion remains on the maternal plant into winter. Incorporation of seeds into the soil occurs by rainfall, cryoturbation, and earthworm activity (Regnier et al. 2001) (Biology and ecology of Ambrosia trifida L. seedling emergence). Large populations growing along waterways and evidence of water-aided dispersal and seeds that float mean this is a "yes".

Likely: North American publications report significant yield losses in maize and soya bean caused by A. artemisiifolia and A. trifida, with weed density and time of emergence being key factors. For example, according to OMAFRA10, in maize A. artemisiifolia in densities of 1 plant/m2 caused yield loss of 5 % while 25 plants /m2 decreased the yield by 45 %. The figures for A. trifida were 13 and 56 % respectively. The data were obtained in conditions of known weed populations, assuming that all weeds emerged with the crop. Harrison et al. (2001) reported on losses of maize yield between 13.6 to 90 %, for weed densities varying from 1.7 to 13.8 plants/m2, when the weeds emerged concurrently with the crop. In a study by Baysinger and Sims (1991), A. trifida was shown to reduce the yield of the soya seeds dramatically: a density of two A. trifida plants for every 9 metres of soya beans in each row caused a reduction of 46 % and 50 % in the two consecutive years 1988 and 1989. The document states that the seeds of Ambrosia spp. can spread naturally by water (including heavy rains, floods, rivers, canals), wind and animals. The import of plant products (seeds and grain) originating in areas where Ambrosia spp. occur is considered to be the major source of infestation and data are given on the amounts of such grain imported into Poland from other European countries, Canada and the USA in 1998. Examples of interceptions of A. artemisiifolia and A. trifida seeds are given from the Russian Plant Quarantine Inspection and Finnish Plant Quarantine Service suggesting that the main sources of the infestation are seeds and grain of wheat, maize, soya bean, grasses and rice and soya meal from different countries (North America, Argentina, western Europe, Japan). Interceptions of the weeds in Poland, in grain of maize and soya bean originating from the USA, grain of soya bean from Canada, cereals, soya bean and sunflower from Hungary and other European countries (Opinion of the Scientific Panel on Plant Health on the pest risk assessment made by Poland on Ambrosia spp.). Spread will be primarily as a seed contaminant. Further spread of A. trifida is highly likely, due to the risks of both accidental movement in agricultural equipment (locally) and as a contaminant of crop seeds. Humans move seed in or on cultivation and harvesting equipment (CABI). Seeds fall from maternal plants typically in autumn, but a portion remains on the maternal plant into winter. Incorporation of seeds into the soil occurs by rainfall, cryoturbation, and earthworm activity (Regnier et al. 2001) (Biology and ecology of Ambrosia trifida L. seedling emergence).