Solidago (9)
I believe the answer is:
goldenrod
'solidago' is the definition.
(I know that solidago can be written as goldenrod)
This is the entire clue.
(Other definitions for goldenrod that I've seen before include "Plant with tall spikes of yellow flowers" , "Yellow-headed garden plant" , "Autumnal flower" , "Yellow-spiked garden flower" , "Tall-spiked plant" .)