> I've seen plenty of knotweed. I've never seen seed heads on knotweed.

There are vast stands of knotweed in my township, have been for many years. It occurred to me that I never saw seeds, didn't even know what they looked like. I found pictures of the seeds and fixed them in my mind, but several more years went by until I actually saw plants with seeds, somewhere in Pennsylvania, in and around a creek bed. Since then, I have seen plants with seeds on two occasions in my county, small patches of them. 

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> More than 100 years ago, Japanese knotweed was introduced to North America. Given its vigorous rhizome system and capability to grow from rhizome and stem fragments, it persists and spreads locally, forming monotypic stands. The Japanese knotweed clone originally introduced was a male sterile female clone; thus, early in the invasion, reproduction from seed was not an issue. 

> However, recently, widespread hybridization between Japanese knotweed and Sakhalin (giant) knotweed has been reported, with the hybrid species, Bohemian knotweed, forming the majority of knotweed plants in many areas and possessing higher variability than the parent species. 

> The hybrids produce large numbers of wind-dispersed viable seeds that germinate at rates approaching 100% in some populations. As temperatures increase, knotweed is predicted to expand its range farther north and to higher elevations. With the ability to regenerate from vegetative fragments and disperse via seeds, invasive knotweed species are on the move.

* the earliest recorded presence of the plant in North America was as a cultivated specimen in Yorkville, NY, in 1873

* it has gone from being a prize-winning horticultural species to one of the world’s top 100 most invasive species

* Originally from Japan, Korea, China, and Taiwan, Japanese knotweed is found on volcanic slopes as an early seral species suitable for colonizing disturbed areas

* Flooding events are important in the spread of knotweed. Colleran and Goodall (2014) found that 70% of new plants. originated from rhizome fragments and 30% from stems carried by floodwaters.

* Seed production and germination of knotweed is now commonly observed in North America.

* The male-sterile Japanese knotweed is now no longer limited to pollen from the less numerous giant knotweed because introgression has produced a diverse hybrid swarm of invasive cytotypes of F. x bohemica, most of which can produce viable pollen.

Gillies et al.: Knotweed (Fallopia spp.) invasion of North America Utilizes Hybridization, Epigenetics, Seed Dispersal (Unexpectedly), and an Arsenal of Physiological Tactics. Invasive Plant Science and Management 2016 9:71–80

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