Plant that's more costly than the silver birch? (9)
I believe the answer is:
goldenrod
'plant' is the definition.
(goldenrod is a kind of plant)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for goldenrod that I've seen before include "bloomer" , "Plant with tall spikes of yellow flowers" , "Autumnal flower" , "Yellow-headed garden plant" , "Plant with spikes of tiny golden flowers" .)