Haru is a pivot-word here -- "bend/pull" for the bow (which is the only reason the bow is here at all -- it's a jo to set up the pivot) and "spring" attributively for the foothills/mountain region. As such, the bow is an unmarked direct object and yama-be a marked one, what he crossing over.

My current draft translation is:

When I cross over
(with my catalpa bow drawn)
the mountains of spring,
I cannot keep to the road --
the flowers are scattering.

A bit of gallantry to the ladies-as-flowers, spiced with the wit of a pivot-word.

---L.