Your "blades of grass" argument is totally a non-sequitur in the face of Hume's quote. Hume would  argue against this blades of grass argument because nobody would pursue an arbitrary goal that doesn't satisfy their passions. Indeed, Hume's point is exactly the opposite of what you've proposed! It is something more like "we only have reason to do that which we are passionate about"! Thus, the completely reasonable statement "Reason is a slave to passion". Even the drive to understand the physical world is a passionate act, because it enables us to enact our desires and drives in a more competent way. That is to say, the more we understand, the more smoothly we can physicalize or actualize the content of our imaginations.