Spooner's to stay and work in garden extensively (4,3,3)

I believe the answer is:

high and low

'extensively' is the definition.

I cannot understand how the remainder of the clue works.

(Other definitions for high and low that I've seen before include "all over the place" , "everywhere" , "Where to search" , "Suffering from mood swings" , "Like varieties of churchmanship" .)

I've seen this clue in The Guardian.

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