Found this one in London Farm, Richmond. Roughly 10 feet tall. Not certain if it's Giant or Japanese.
I believe your photos show Reynoutria japonica (Fallopia japonica or Polygonum cuspidatum), commonly known as Asian Knotweed or Japanese Knotweed. Polygonum sachalinense (Fallopia sachalinensis), also known as Giant Knotweed, is the largest of 3 closely related knotweeds in the area. A third knotweed, Polygonum x bohemicum, is a hybrid of the first two.
The leaf is closer to Reynoutria sachalinensis (R. japonica has shorter, broader leaves, with less prominent veins). I'll not rule out the hybrid as I'm not familiar with it.
Scratching my head ! I have taken this one at Garden City Park, Richmond, august 2018. What is this ? Giant, Japanese or Hybrid !!!
Will the real Knotweed please stand up! Because the plant in David Tang's photos seems to have features of both Reynoutria japonica and Polygonum sachalinense, it could very likely be the hybrid, Polygonum x bohemicum. I have no idea how you can say for sure except by a process of elimination.
I can say for sure because the hybrid is definitely what the plants asked about look like. With sachalinensis producing quite apparently jumbo leaves, x bohemica a percentage of leaves more than 20 cm long and japonica no leaves more than 18 cm long. And with those of x bohemica being visibly intermediate in shape - closer to ovate. Otherwise the 3 differ in details of the leaf undersurfaces, when viewed using magnification.
Just edited my own post to update the names; both Giant and Japanese Knotweeds are now classified in Reynoutria, following the split of Fallopia into 3 genera. Neither (nor their hybrid) is in Polygonum.
Here is a document with the Fallopia names (though updated in June 2019) but it does have a comparison of the three, along with Himalayan knotweed. Identification and Management of Invasive Knotweeds — TechLine Invasive Plant News It has a chart which mentions an erect flower arrangement only under Bohemian knotweed. This one has a lot more detail, still with the Fallopia names. https://www.extension.uidaho.edu/publishing/pdf/pnw/pnw0610.pdf
The 3 species brought up on this thread are treated within Fallopia by Peter Zika and/or David Giblin in the Second (2018) Edition of Flora of the Pacific Northwest (University of Washington Press, Seattle).
The problem is that inclusion of Reynoutria within Fallopia leaves Fallopia s.l. paraphyletic with respect to Muehlenbeckia. The option to have a Fallopia "sensu latissimo" also including Muehlenbeckia restores monophyly, but has the problem that a number of new combinations of Muehlenbeckia species in Fallopia would need to be made; no-one has done them (yet). Ref: Schuster, T.M., Wilson, K.L. & Kron, K.A. 2011. Phylogenetic relationships of Muehlenbeckia, Fallopia, and Reynoutria (Polygonaceae) investigated with chloroplast and nuclear sequence data. International Journal of Plant Sciences 172 (8): 1053-1066 [pdf file].
