In a manner relating to gardens; concerning gardens or gardening.
From 'garden' plus the directional/manner suffix '-wise' (from Old English 'wīse,' meaning 'way' or 'manner'). The '-wise' suffix is productive in English, creating adverbs like 'otherwise,' 'likewise,' 'clockwise.'
The suffix '-wise' had a massive revival in corporate jargon (businesswise, timewise, weatherwise) in the 1990s, to the point that English teachers started forbidding it—but 'gardenwise' is actually the kind of straightforward usage the suffix was originally meant for.
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