Oh, dude, nature words that start with "U"? Like, seriously? Okay, let me think... Oh, got one! "Umber" is a color, right? And, uh, "underbrush" is a thing in the woods. So, there you go, two whole nature words that start with "U." You're welcome.
Unemployment and urbanization are social study words. They begin with the letter u.
Ultimatum, uncle, undertaker and unicorn begin with the letter u. Union, utopia, usual and usage begin with the letter u.
unique
no!
up, us
Unscrew and undertow begin with the letter u. They end with letter w.
Uncertainty, undertow and ulcer are scary. They begin with the letter u.
Ugly
uranus
Some Christmas words that begin with "u" are unwrap and under (as in under the tree). The words yule and yuletide have a "u" sound, but they actually begin with "y."
Urginea is a flower. It begins with the letter u.
nilgau, nilghau, nylghau