Resembling, characteristic of, or suitable for a garden; having garden-like qualities.
From 'garden' plus the adjectival suffix '-y' (from Old English). The '-y' suffix is highly productive in English, turning nouns into descriptive adjectives easily.
English loves making informal adjectives with '-y' (like 'cloudy,' 'muddy,' 'cheesy'), so 'gardeny' makes perfect sense grammatically even though it sounds casual and poetic rather than technical.
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