Giant hogweed (Heracleum mantegazzianum), Sosnowsky's hogweed (Heracleum sosnowskyi) or possible hybrides of Heracleum spp. are highly invasive plants. Where I’ve met them they grew mainly at stream banks. It means that their seeds may be carry by waterways and not disperse only gravitationally and by the wind, as they usually do, as we can observe fresh formations of rosettes at the rims of flowering ones.
However, they very soon occur also hundreds meters away, even at the tops of nearby hills in isolated stands. What mechanisms may be responsible in yours opinions for their dispersion there?
Hogweed seeds usually fall within 4m of plants, but they can inadvertently be transported by cars, people, and animals. However, I think it is by wind. When the seeds dry out, they are extremely light and easily blown long distances :-)
"When the seeds dry out, they are extremely light and easily blown long distances."
After due consideration I have to agree. There are some violent weather events capable to move trees, not talking about seeds.
In USSR, H. sosnowskii has been cultivated as a feed crop. Of course, that was a precondition to its subsequent unaided self-dispersion. Now I find somewhere it in St. Petersburg city between houses. A possible way may be a unpremeditated introduction of seeds with garden soil used in the planting of greenery.
Seeds of hogweeds are easily dispersed by wind. Along roads fruits of hogweeds are easily dispersed by air movement created by vehicles.
Definitively wind is one of the main dispersal factors - but as mentioned above also zoochory may play an important role (also, if not mainly: humans and their machines, cars).
Indeed Heracleum mericarp is flat, light, almost winged, anemochory probably is part of its dispersion syndrom.
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