Planting ideas for narrow bed alongside path and under trees

[OrangeSunset]

Had such a lot of lovely ideas to my recent request for ideas of planting round a patio, that I've started another thread for the next area of our garden to be planted...

It's a long narrow bed, running alongside a path on one side and a lawn on the other. At one end there are 3 silver birch trees in a line.

The bed itself is only 650mm wide, about 11 meters long. I've planted some sanguisorba offininalis and astible, but not really happy with it. I think it's too sunny for the astilbe, so I'll move those elsewhere. Also, as they are both the same height, it looks too linear.

Any suggestions of that might go with the sanguisorba? Would ideally like something that is green for most of the year to provide ground cover. It's sunny from 10ish in summer.