"绿草如茵"或"绿树如茵"是成语，是说草或树长得绿油油的，非常茂盛，像铺上了绿色的被褥。茵，means "blanket", or something like that. "如"是"好像"。"如茵"，就是"好像绿色的毯子一样"。

如 means "as" and 菌 is something green, it just means that the garden is very green.

形容树木很多，非常旺盛

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