Our days put on such reticence These accents seem their own defense.
That line break between 18 and 19, separated by a comma, indicates a change in focus but since there's a comma there and not a period, we can assume that both lines relate to one another.
And the best connection we can find is one that links those puzzling experiences with the speaker's own "days." So again, it looks like Ashbery is moving from the more general experience of many to a more specific experience the speaker seems to be having. We get more bridging of the individual and the larger world.
But what's the deal with line 19? First up, it's another fancy word: "reticence," meaning a kind of shyness when speaking, as if a person can't really say what he or she feels. This reminds of the idea of speech from way back in line 2. The trees put the speaker in mind of the idea that speaking is a kind of performance. It comes with difficulty, which explains the "reticence" here.
So what do we make of all this shyness and reservation and that final idea about "defense"? And what the heck are "these accents"?
Again, accents remind us of speaking. Yet it's important to remember that the days are the things putting on this hesitant, reserved speech. Another way to express your life is as your "days" (as in "I haven't see a cat do a backflip like that in all my days"). So, one way to read this is that our lives are filled with speaking, with accents, with performances. All of that, however, seems pretty far off from the reality of our life, as if talking about life was a kind of defense—but against what?
Well, consider that "puzzling light"—consider the fact that the more we try to "explain" things, the more puzzling those things become. But when we allow things (and ourselves) to simply exist and not douse them in meaning or artificiality (speaking and accents), the experience is far better. So don't try to jam all of life into a noisy box of rational speech-making. Just let it be, like the trees do. Words to live by, gang (ironically enough).
More irony comes with this last rhymedcouplet, which really brings this final stanza together and maintains the orderly form that Ashbery uses in his poem. In the face of puzzling reality, the poem itself puts forward a brave face in terms of its ordered form. Check out "Form and Meter" for more on that.