Hi Luke. By coincidence I just posted yesterday about my tall sunflowers. These self-seed every year, and last year they were extraordinary. The tallest was 14 feet, which I was able to measure when it toppled over from being top-heavy in a rain. I don't know exactly what they are, but they are well adapted to this high arid area.
This was the 14-footer:
They come up each spring and I pull out most of them and leave others where I want them that year...a movable feast! Last year they were so thick in the side path I trimmed the lower leaves and made a tunnel to the patio.
What else in in your hugelkultur bed? Other than the massive sunflower, are you happy with the result?