Well, no one's tried to answer this, so let me see what I can do.

There are many plants known -- formally or informally -- as "Knotweed." Most (maybe all?) are in the Polygonaceae family.

You're probably familiar with Common knotweed, Polygonum arenastrum [don't know if that Latin name is up-to-date: many plants formerly in the Polygonum genus are now classed as Persicarias], which can be flat as a pancake and is commonly found in sidewalk cracks and what are called "waste places." It's one tough plant, a real survivor, and was common in the Bay Area when I was growing up. (We have that or something similar here; I think I mostly see it on the side of the road near my mailbox.)
http://www.ipm.ucdavis.edu/PMG/WEEDS/common_knotweed.html

And you may know Japanese knotweed, Fallopia japonica, a shrub which is trying to take over most parts of the country, including here.
http://nyis.info/index.php?action=invasive_detail&id=43

Many of the Persicarias may be referred to as "knotweed," just as they're often referred to generally as "smartweeds" or "lady's thumbs."

There are also garden cultivars, such as the older Kiss-me-over-the-garden-gate and all sorts of modern varieties, which I gather are more common in Europe than in the U.S. Here are some examples:
http://www.finegardening.com/bistort-persicaria-amplexicaulis

https://www.rhs.org.uk/Plants/67494/Persicaria-affinis-Superba/Details

http://www.paghat.com/persicaria.html

http://www.bethchatto.co.uk/o-r/persicaria-polygonum/

So what is the flower in your photo? To me, it looks like it might be something classified as Persicaria, or some relative of the Persicarias -- but I can't say what it is.

Could it be Plectritis macrocera?
Doesn't look like Polygonaceae.

Lysimachia clethroides also looks "similar" and is fairly aggressive. But not sure if it is in California.

You might send a message to the California Native Plant Society through their website and ask if they know. Unfortunately native plant folks have to learn about invasive plants too.

Looks like a Valerian (Valeriana species) to me, check that out if it turns out the Plectritis doesn't seem to fit.

Thank you for all the posts. My best bet is Plectritis macrocera, as suggested by nonmember.