The noun feathery false lily of the valley can be countable or uncountable.

In more general, commonly used, contexts, the plural form will also be feathery false lily of the valley.

However, in more specific contexts, the plural form can also be feathery false lilies of the valley e.g. in reference to various types of feathery false lilies of the valley or a collection of feathery false lilies of the valley.