Garlands collectively; a collection or arrangement of garlands.
From garland plus -age (suffix forming nouns meaning a collection or system of things), similar to how 'baggage' means a collection of bags.
The -age suffix is a sneaky way English creates abstract nouns—'garlandage' works the same way as 'baggage,' 'luggage,' and 'tonnage,' grouping individual things into one concept.
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