The poem opens with trees "coming into leaf," which means that it's spring and that everything is beginning to blossom. The speaker describes this in line 3, saying that the "recent buds relax and spread." The word "relax" hints at a sense of relief, as if the buds have been tensely waiting all winter to finally open up and "spread" themselves into lush green leaves.
Because poems about spring are, very often, celebratory or happy in nature, many readers will probably assume going into "The Trees" that it will be a good-natured poem about the beauty of budding trees and the excitement of spring. However, things take an unexpected turn at the end of this opening quatrain ("The trees are [...] of grief"), when the speaker metaphorically suggests that the "greenness" of the new leaves is "a kind of grief." At this moment, the speaker projects feelings of discontent onto the otherwise happy, care-free sight of budding trees.
This actually makes sense alongside the simile the speaker uses in the poem's first two lines:
The trees are coming into leaf
Like something almost being said;
By comparing the budding trees to "something almost being said," the speaker subtly implies that there's something lurking behind their beauty. In the same way that it's possible to sense when people are just barely holding themselves back from saying something meaningful, the speaker feels as if the buds are hiding something.
Behind the buds' surface-level beauty, the poem intimates, lies "grief" and sadness. It's not yet clear why, exactly, the speaker feels this way. At this point, then, the only thing that's clear is that "The Trees" will take an unconventional, nuanced look at the sense of renewal associated with spring.
These opening lines also establish the poem's use of iambic tetrameter, a meter in which each line contains four iambs: feet consisting of an unstressed syllable followed by a stressed syllable (da-DUM). Look, for example, at the first two lines:
The trees | are com- | ing in- | to leaf
Like some- | thing al- | most be- | ing said;
This gives the language a consistent, plodding rhythm, perhaps reflecting the speaker's lack of enthusiasm about the changing seasons. Whereas free verse might allow the language here to feel exuberant and frolicking, the poem's use of iambic tetrameter gently calms it down, giving the speaker's tone a somewhat subdued, structured quality that matches the hesitance to view spring as a time of joy and excitement. The ABBA rhyme scheme at play in this stanza also contributes to this musical yet methodical feel.