The formation of pods is triggered by the pollination of the flowers, in turn occasioned by the visits of bees. So why did bees visit the lower-down flowers but not the high-up ones? Are they afraid of heights, I ask?! My theory is this: the lower-down flowers appeared first, when the weather was reasonably good, and they opened out fully allowing easy access to the bees. However, at the time when the upper flowers were out the weather was appalling and the heavy rain reduced many of them to a soggy mess, and the bees just couldn't get in to pollinate them.
I think my theory is confirmed by the fact that the very much shorter "Robin Hood" beans have experienced the same effect - lots of pods down near the ground; few pods up above. It's a timing issue, not one of height.
The good news though is that the later-developing "Masterpiece Longpod" plants seem to be forming a lot more pods higher up.
They may have got in just in time, because the absolutely torrential rain we had on many occasions last week would have destroyed any flowers that were open at the time.
I have also come to the conclusion that it would be better to space my Broad Bean plants a bit further apart. This year I have two rows, each of 12 plants, in a bed which is 2.4 metres long - so in other words the plants are roughly 20cm apart. This means that their leaves are inter-twined, which may make it harder for bees to navigate between them, or even to detect the flowers in the first place. Next year I might try growing fewer plants at a wider spacing.
I'm going to continue to grow them tied to individual canes though. I think this is a good method to use if you are only growing a few beans, because it keeps them upright and stops them becoming a tangled mess that bees can't get into. Obviously if you had loads of bean plants this method might be too laborious.
Well, the quantity of beans this year may be somewhat disappointing, but there's no faulting the quality. This is a batch we ate a few days ago - BBs mixed with peas. The beans here are "Robin Hood" ones, which are small, but very tender and a glorious bright green colour.