Their greenness is a kind of grief.
The rhythm in this poem impressed me, for some reason. It's all iambic tetrameter, with only a few substitutions, but there's a more subtle rhythm going on. Maybe it's with the longs and shorts of the syllables, or something. I find it most strongly in the last two lines of each of the first two stanzas.
The Trees, by Philip Larkin
The trees are coming into leaf
Like something almost being said;
The recent buds relax and spread,
Their greenness is a kind of grief.
Is it that they are born again
And we grow old? No, they die too.
Their yearly trick of looking new
Is written down in rings of grain.
Yet still the unresting castles thresh
In fullgrown thickness every May.
Last year is dead, they seem to say,
Begin afresh, afresh, afresh.