In a manner relating to or resembling ivy plants; in a way characteristic of hederaceous plants.
From 'hederaceous' plus the adverbial suffix '-ly.' This creates an adverb from a botanical adjective, following standard English grammar patterns.
Hederaceously might be the most specialized adverb in English—it's so rare that it probably appears in fewer than a dozen published texts. It's the kind of word that exists in the language's back pocket, just in case a botanist needs maximum precision.
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