Garden flower that’s female outside, male inside (7)

I believe the answer is:

verbena

'garden flower' is the definition.
(flower that might be grown in a garden)

I don't understand how the remainder of the clue works.

(Other definitions for verbena that I've seen before include "Herbaceous plant" , "Garden plant" , "Ornamental plant with showy flowers" , "Vervain (plant)" , "Ornamental pant" .)

I've seen this clue in The Times.

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