Lines 20-24 Summary

Get out the microscope, because we’re going through this poem line-by-line.

Today my son told me

Big Moment Alert! Our speaker's son told her something (which we'll get to in a moment), but what's really pivotal here is the entrance of the son in the first place.
The appearance of the son here is a big shift. We've been so focused on our lonely speaker, and the loss of her husband, that it's almost shocking to learn she has a son around to talk to.
We don't know whether they talked on the phone, or are actually in the same place, but still that contact should be a reason for joy, right?
And yet, the tone stays pretty level. The entrance of the son doesn't make any noticeable impact on the slow, sad pace of the language. It feels like just another fact thrown into the (flower) pot.

that in the meadows,
at the edge of the heavy woods

Her son is describing the location of something in the meadows at the edge of the woods (we haven't gotten to what yet).
And what about those woods? Well, for one thing, they're "heavy." That heaviness is everywhere in this poem.
Perhaps the woods are dense? Or maybe she's just projecting again. The woods are heavy because she's feeling the weight of her grief.
Our speaker is being a bit coy, though. She just won't tell us exactly what she means. Just like earlier in the poem, our speaker is keeping the focus on the natural world. She mentions her son (as, earlier, she mentioned her husband), but immediately zooms right in on an image of the natural world.
This means that when we read the poem, we have to decode her descriptions of the world around her, to see what they tell us about her inner world. And wouldn't you know it? That's what we've been doing all along.

in the distance, he saw
trees of white flowers.

Near those heavy woods (which we now learn are far off in the distance), the speaker's son saw trees with white flowers, growing in the meadow.
Hmm. That sounds familiar, doesn't it? Could these be more plum trees, growing by the woods?