What's the plural form of underbrush? Here's the word you're looking for.
Answer
The noun underbrush can be countable or uncountable.
In more general, commonly used, contexts, the plural form will also be underbrush.
However, in more specific contexts, the plural form can also be underbrushes e.g. in reference to various types of underbrushes
or a collection of underbrushes.
