Actually, the title ("Bearded Oaks") has more going on than you might think, just like Spanish moss has much more going on in it than you probably want to think about. (Chiggers make their nest in there, so look out. They'll eat your skin and leave nasty, itchy bites—not good.)
The title immediately roots the poem in a specific region. We know we're in the south, that we're going to be confronted with nature (nice try with wearing a beard, nature, but that disguise fools no one). The oaks are both literal and, as we are to discover, metaphorical, and their beards hold much of their meaning (along with, you know, those chiggers).
Spanish moss does in fact look like a beard. Neither Spanish, nor moss, this plant is actually related to the pineapple (no need to thank us for the trivia, gang). While it hangs on the tree, it's not a parasite at all. It's an epiphyte. That means, though it attaches itself to the tree, it doesn't draw its nutrients from the tree. It absorbs water and food from the air.
So, what does that have to do with this poem? Well, you might say that this moss—the thing that gives those oaks their "bearded" appearance—is in tune to its environment. It's connected—to the trees, to the air—in a way that the speaker seems to be into. Keep in mind, though, that our title is "Bearded Oaks," not "Spanish Moss" or even "Stuff that Grows on Trees." The title, then, right away identifies two things that are connected (moss, tree), much like those lovers are in the poem).
We mentioned that these trees situate the poem in the south (check out "Setting" for more on that), but they also, more generally, put us in nature. Remember that this is a poem that is thinking Big Picture, the biggest possible picture, in fact (the eternal universe, for starters). Nature, of course, is intimately tied up with that sense of existence, and these trees put us as readers in that mindset, just as it puts the lovers into a similar reflective state.
Finally, oak trees with beards tend to be old (hence, you know, the beards). These aren't puny saplings that haven't even shaved yet. Nope, these are old, established trees that give off a sense of time's passage. And, in a poem that is encouraging us to ponder Time as a concept—even if we just do it for an hour—these venerable old oaks are reminders of Eternity.
The title, then, captures the Big Ideas of the poem for its readers. Wowzers, that Warren is one smart fella—no moss on him (sorry).