Literally: one – tree – one – grass
Alternately: Everything in the area; every last tree and blade of grass.
In contrast, this phrase can also mean something of which there’s only a very small amount – a mere single tree or single blade of grass instead of a whole forest or prairie.
Notes: The order of the elements may be reversed, although 一草一木 seems to be the less common usage. However, reading 一木 as ichimoku or ikki is an error.