Dioscorea - Cinnamon Yam - not flowering?

Inspired by the book Paradise Lot, in spring 2015, I planted three Cinnamon Yam (labeled Dioscorea polystachya) in a large west-facing planter box. They grew well, made underground tubers, survived the winter in place, and grew again this year, getting about 9 feet tall. However, they never flowered last year or this year, and I'm wondering - do they take several years to get established before flowering? Do they need more summer heat than I have? (My garden is fairly cool for zone 7/. Maybe this is the wrong species for the aerial bulbils?

Any ideas/experience gratefully received

Kevin
Powell River, BC, Canada

I've grown Dioscorea for many years, but I've only had it produce aerial tubers a couple of times in more than a decade. My Dioscorea is in a warm very dry spot on the south side inside the drip line of a large fir tree next to a big fig tree. It flowers more often then it develops aerial tubers, but the vine doesn't even come up every year. This year I didn't see any growth on it, but this summer was rather cool. It often comes up big on years after it was a no show, and it seems to do better in years with warmer springs. The spot it's in is really dry most of the year, so I know I'm not getting optimal growth out of it. I plan to move some of it, but I haven't decided on a new location. I sort of planted it as an optimistic curiosity shortly after moving in, then mostly left it to it's own devices.

I haven't spent a lot of effort with the plant over the years, but a couple of times I've gotten huge numbers of aerial tubers. Some folks miss the blooms because they are so tiny. The flowers are tiny pale balls about the size of a seed bead, but they smell amazing. The aerial tubers look like itty bitty potatoes, ranging in size from a dried pea to a large hazelnut.

A former neighbor of mine did a lot of work on perennial vegetables when he lived in my neighborhood, and he ate the ariel tubers, leaving the big tubers in the ground. He has a blog from his days in the PNW and he mentions Cinnamon Vine, but doesn't go into great detail about it. If you search his blog you'll find a bit more info.   http://farmerscrub.blogspot.in

My Dioscorea is planted in the ground, so I don't know if being in a planter helps or hinders the vine. My vine, on good years, will turn into a massive sprawling vine that tries to take over the fig tree, so they can get quite big in optimal conditions.

I'm in Zone 8, but my yard is a bit of a cold patch on a north facing slope above the Columbia River, with gorge exposure.

Thanks Moyra! Your info on the flower size made me go out and examine my vines more closely, and now I see that in a very few places there are actually tiny bulbils in the leaf axils - as you say of the flowers, about the size of a seed bead. So they are producing, but in a very limited way.