Plant that's more costly than the silver birch? (9)

I believe the answer is:

goldenrod

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'plant' is the definition.
(goldenrod is a kind of plant)

I cannot understand how the rest of the clue works.

(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" .)

I've seen this clue in the Evening Standard.

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