Ambient Brightness

or A Good Explanation for Everything

The nursery worker looked at me strangely.  "Where did you move here from?"  "Vermont," I answered.  "Ah," she said wisely, as if that explained everything.  "Forget the plant tags.  You can grow a lot of things here in less sun than they say, because of the ambient brightness."

"Ah, yes," I thought wisely, "the ambient brightness."  As if that explained everything.

I had just moved to New Mexico, to an apartment with a north-facing balcony, and was looking for suitable plants at a local nursery.  The first person I chanced on there was the kind of worker you trust automatically.  She may not have had a floppy hat—the gold standard for knowledgeable plants-people in these parts—but she did have the right sort of outdoorsy, weathered smile-lines around the eyes.  She suggested 'Winter Gem' boxwood (Buxus microphylla japonica), even though the plant tag recommended full to part sun and the balcony only knew sunlight by hearsay.  And she was right:  the boxwood bushes were perfectly happy and didn't resent the shade at all.  Not that they resent much of anything, but they seemed actively pleased with all that ambient brightness.

Over the next few months I paid close attention to the plants I saw growing and blooming smack up against the shady north sides of houses:  lilacs, roses, Russian sage.  The ex-Vermonter in me was astonished and took the lesson to heart.  When I moved to my townhome a year later and toddled off to a nursery specializing in plants of the southwest (Plants of the Southwest, it's called) looking for natives for my part-sun garden, a worker in a floppy hat stopped me from making a purchase:  "That one needs full sun."

"Oh," I said brightly, "Isn't that just what the tag says?  It ought to do OK in less sun, because of the ambient brightness."  He looked at me strangely.  "Where did you move here from?"  "Vermont," I answered, dimming a trifle.  "Ah," he said.  Because that explained everything.

Since then I've learned a thing or two about provenance.  "Full sun" means something different to a desert plant than it does to one from a milder climate, where skies can be cloudy all day (or even longer!) and sometimes ambient brightness is the most a light-hungry little photosynthesizer can hope for.

My appreciation for these nuances of meaning ("sun":  it's complicated) got bumped up another notch when I planted an ivy this past spring.  Ivy is well-behaved here, not invasive, and useful in full-shade areas where you have a wall to cover.  I'd been growing this one as a houseplant in the sunny upstairs bathroom for several years—the only place humid enough to keep spider mites at bay—enjoying its bright green leaves, but finally decided it would be happier in the Sanctuary for Shadows along the garden's north-facing wall.  It's settling in well, but it isn't bright green any more:

It's variegated.  I'd forgotten that.  For several years the ivy had received a couple of hours of direct sun a day, with bright indoor light the rest of the time, and even with all of that the variegation had long since disappeared.  Now, outdoors, in December, on the north side of a wall in full shade, it's well-enough lit that those beautiful highlights have come out again.

Sometimes you need a tangible object lesson to understand apparently simple things like "light."  On the scale of brightness, I would have put a half-morning's direct indoor sun about on a par with full, outdoor shade—maybe even a little ahead.  In terms of my own appreciation of light, I'd vote for a sunny east window over a shady northern exposure any day of the week.  Apparently, though, human perception doesn't have much to do with botanical reality.  Maybe the difference lies in the gap between luminous flux—the amount of light visible to the human eye—and radiant flux—the total power of light across its spectrum (if I'm using those terms correctly); maybe it's just a charming quirk of chlorophyll. 

But it's probably because of the ambient brightness.