Thursday, December 8, 2011

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.


  1. P is always going on about ambient heat and radient heat and the difference. So I guess ambient and radiant is the same theory applied to light... but I would rather have a bit of sun coming in the window than reflected from anywhere.
    Stacy, do you know a lovely garden in Vermont called North Hill. The guys that own it are called Joe Eck and Wayne Winterrowd. I first saw it in a magazine and fell in love with it. I have the book...

  2. Well, I never heard the term ambient lighting when referring to plants, but I'm going to use it from now on! There are a lot of plants in Texas that are full shade here, and full sun in the north. And a lot more "full sun" plants that need afternoon shade. Each area, like each garden, has its own quirks and idiosyncrasies. Just another fun challenge for the gardener!

  3. Stacy...Ive been noticing these mottled, bleached out spots on my forearms since moving activities mostly inside in the past month. You dont suppose...?

    Have to say I find the light that emanates from your writing invariably ambient/radiant


  4. This just proves what I've said all alone - throw away the tags and ask someone who's grown it! Experience tops text.

  5. That explains why on the verandah the light is too bright for the laptop. It is shaded with a solid roof, and house on two sides. My Streptocarpus is happy, but I cannot read the screen against the sun out there in the open garden.

    I saw neighbours growing roses against the south facing shady house walls, and went ahead with planting in the walled garden. Slightly shaded? Yes, thank you!

  6. Stacy - Your comment about ivy not being invasive in NM, put a picture of rampant cacti in my head. I really need to get a floppy hat.

  7. Your post explains everything :D

  8. The plant tag text is too generalized and even microclimates only meters apart affect growing potential. I rarely follow the tag and the way the weather has become, many plants do well in locations they otherwise would not. Coincidentally, two days ago, I saw the brightest sun I have ever seen and thought to myself this is a weird occurrence. It shone bright, stark white with the rays radiating further than I have ever seen before. It really seems as if changes are afoot.

  9. just never know...I have the floppy hat and I still don't just makes me look like I do...sunlight is a funny thing...plats that seed themselves in shade but are full sun plants seem to do fine and I still don't get it...

  10. Janet, I never came across North Hill while I was there, alas. I looked up their website just now—what a beautiful place it must be. A New York Times article quoted Joe as saying that he wrote the drafts of their books and Wayne “put the mice in.”

    Holley, I suspect that some little town in central Ohio has been chosen by secret committee to be the “plant tag standard,” and the rest of us are trying to figure out how to translate central Ohio into our own climates... (Not to go all conspiracy theory or anything.) I'm not surprised things need a LOT less sun in Texas, with that combination of light and heat!

    Oh, my, yes, Steve, that does sound like the natural variegation coming out... And what a lovely thing to say—thank you, and welcome!

    Karen, the folks at local nurseries are better than plant tags hands down! We're “lucky” in a way to have such a difficult climate—the specialty nurseries with all their knowledgeable staff really seem to thrive here.

    Yep—ambient brightness is definitely the culprit, Diana. That's the most frustrating thing about laptops—otherwise they're just tailor-made for working outside! Piggy-backing onto Karen's comment, sometimes What Grows at the Neighbor's is the best garden guide of all.

    b-a-g, I'm glad there are still a few hours before bedtime, or I'd end up dreaming about cacti creeping up the walls and over the windowsills, thank you very much... Floppy hats are the Be All and End All. (I need to get one, too.)

    klbrowser, see how handy it is?? :D I only wish I'd known about ambient brightness when I was still teaching, but it wasn't a phrase often used in Vermont.

    Donna (GWGT), I've begun to understand why people who aren't really gardeners but just want a few nice plants get discouraged—the tags give so little to go by. There are a lot of signs around that things are changing, though few as striking as the sun you saw. I've been trying to keep track of records we've broken this year—heat, cold, drought, most rain in one day, wind, etc. I wonder what next year will bring...

    Donna (GEV), well, my Mom occasionally suggests the “Act as though...” philosophy: “act as though” you enjoy an irritating chore, and eventually you might come to enjoy it; “act as though” you care about something, and maybe you will. So maybe if you “act as though” your floppy hat means you know everything, you will...?

  11. I have a floppy hat to wear when it's raining. It's called driza-bone – which is misleading as it doesn't keep me dry as a bone at all. (It does work better though in ambient rain). I like the idea that people might think I'm a knowledgeable plants-person because of it. And I also like the standard SW response to "Vermont." Made me smile - as you so often do.

  12. It might take more than a hat to keep someone dry, really, even a hat with an optimistic name. Ambient rain--ha! I love it--well, I love the idea more than the reality. Obviously. I'm glad to know the post made you smile, especially since you so often do the same for me.