A delight Comes sudden on my heart, and I am glad As I myself were there! Nor in this bower, This little lime-tree bower, have I not mark'd Much that has sooth'd me.
The speaker has done such a good job in wishing for beautiful sights for Charles that he imagines the scene as if he were there. He has recreated it in his mind.
This process is not gradual; it happens all of a sudden, like an epiphany. Maybe the "veil" from line 43 has been lifted and he can see the true spirit of God.
Really, he feels like he is in two places at once, because he also notices that the garden contains its own wonders. He notices or "marks" them as he takes a closer look around.
These wonders "soothe" his frustration at being left behind.
Pale beneath the blaze Hung the transparent foliage; and I watch'd Some broad and sunny leaf, and lov'd to see The shadow of the leaf and stem above Dappling its sunshine! And that walnut-tree Was richly ting'd, and a deep radiance lay Full on the ancient ivy, which usurps Those fronting elms, and now, with blackest mass Makes their dark branches gleam a lighter hue Through the late twilight; and though now the bat Wheels silent by, and not a swallow twitters,
The speaker switches into the past tense to describe some of the things he remembers having seen over the last couple of hours, and then he goes back to the present to describe things he sees now.
Among the things he has noticed in the past: "foliage" (leaves) that looked transparent when lit up by the sun's "blaze" (try holding a piece of white paper up to a light bulb and you'll notice the same phenomenon); the shifting of shadows and sun caused by the movement of leaves and stems (similar to the "speckled" appearance of light in line 11); the sunlight that illuminated a walnut tree and "ting'd" it with a glow; and finally the "radiance" of light on ivy plants that grow next to elm trees.
Now he notices that the ivy has grown dark and looks like a black "mass" of leaves compared to the taller elm trees, which still receive a little light from the "late twilight."
Phew, that was a lot of imagery related to light and leaves. Let's move on, shall we?
He also notices bats circling around in the approaching darkness. The bats have replaced the "swallows," which have stopped twittering now that the sun has gone. The bees have mostly gone, too, but – look! – there's one left buzzing around ("singing") in the flower of a bean plant. That bee is a true survivor.
Yet still the solitary humble-bee Sings in the bean-flower! Henceforth I shall know That Nature ne'er deserts the wise and pure; No plot so narrow, be but Nature there, No waste so vacant, but may well employ Each faculty of sense, and keep the heart
Ah, so the speaker has had an epiphany. He has learned something that he will never forget from this moment on, or "henceforth."
The thing that the speaker has learned is that Mother Nature never leaves people who are "wise and "pure" enough to appreciate her. This is a fancy way of saying that people who know how to look for natural beauty will always be able to find it, wherever they are.
Even if a space ("plot") looks small, or a barren spot ("waste") seems to have no living things, you can still use your senses to discover sights, sounds, and smells that will open your heart to the emotions of "Love" and "Beauty."
If Coleridge had to work in a modern-day office building, he would probably go around burying his head in people's houseplants and admiring the clouds through the window. Still, we bet he wouldn't choose to work in an office…