From a Light Reading to a Plant List: Using Your Measured Light Level

Taking a light reading is only half the job. The other half is turning that reading into an actual, filtered list of plants that will survive in your specific bed — and that means bridging two tools that don't quite speak the same vocabulary. Here's the exact pipeline, worked through with real numbers.

Two calculators, two slightly different vocabularies

Our light level calculator classifies a reading into one of five categories, based on Daily Light Integral: full sun, light shade (partial sun), partial shade, full shade, and deep shade. Our shade plant selector, by contrast, only accepts three light values — partial, full, or deep — because it's built specifically around genuinely shaded, shade-garden-appropriate conditions. The three narrower categories map directly onto three of the five DLI bands by name: a "Partial shade" reading feeds the selector's "partial" option, a "Full shade" reading feeds "full," and a "Deep shade" reading feeds "deep." If your reading instead lands in "full sun" or "light shade (partial sun)" territory, that's a useful result in its own right — it means the spot is genuinely too bright for most of the shade perennials this site covers, and you're better served by sun-perennial resources than by forcing a shade plant into a spot it was never meant for.

Worked example: a full-shade reading

Take a reading of 4,200 lux sustained for 6 hours — a fairly ordinary reading for a bed with bright ambient light but no real direct sun. Running that through the light level calculator: PPFD of 77.7 µmol/m²/s, DLI of 1.68 mol/m²/day, foot-candles of about 390.2, and a category of "Full shade." That DLI sits just above the 1.5 threshold that separates full shade from deep shade — close enough to the boundary that it's worth a second reading on a different day before treating it as settled.

Feed "full" into the shade plant selector at, say, zone 6 and medium soil moisture, and the result is a long list: hosta, Christmas fern, astilbe, Lenten rose, Japanese painted fern, coral bells, lungwort, Solomon's seal, foamflower, bigroot geranium, Siberian bugloss, lily-of-the-valley, sweet woodruff, wild ginger, and barrenwort — essentially the entire curated roster, since full shade at medium moisture in a mid-range zone is one of the most forgiving combinations the selector covers. Narrow the same query to dry moisture instead, and the list collapses to just two: bigroot geranium and barrenwort. Same light reading, same zone — a completely different practical answer once the real soil condition is factored in, which is exactly why light alone is never the whole story.

Worked example: a deep-shade reading

Now take a dimmer reading: 1,200 lux over 5 hours, giving a PPFD of 22.2, a DLI of just 0.4 mol/m²/day, foot-candles around 111.5, and a category of "Deep shade" — comfortably under the 1.5 threshold this time, not a borderline case. Feed "deep" into the selector at the same zone 6 and medium moisture, and the roster shrinks from fifteen plants to ten: hosta, Christmas fern, Japanese painted fern, lungwort, Solomon's seal, foamflower, lily-of-the-valley, sweet woodruff, wild ginger, and barrenwort. Five plants drop out entirely at this darker tier — astilbe, Lenten rose, coral bells, bigroot geranium, and Siberian bugloss — because their published light tolerance doesn't extend into genuinely deep shade, even though all five were perfectly happy at the brighter full-shade reading above.

What the drop-out list tells you

It's worth actually looking at which plants fall out between one light tier and the next, not just at the plants that remain. Astilbe and coral bells dropping out at deep shade lines up with what both are typically grown for — a distinct summer bloom that needs more light than deep shade provides to perform well, even if the plant itself might technically survive in dimmer conditions. Bigroot geranium's drop-out is a reminder that drought tolerance and light tolerance are separate axes — a plant can be excellent in dry soil and still have real limits on how little light it will take. None of this is visible from a single "shade-tolerant" label on a plant tag; it only shows up once you filter by an actual measured light tier and watch the list change shape. A generic "shade plant" list, sold as one undifferentiated group at a nursery, is really several different light tolerances bundled under one convenient marketing word — the filtering step is what un-bundles them back into an honest answer for your specific bed.

When the shortlist gets very short — or empty

Combine a demanding light tier with a demanding moisture requirement and an edge-case zone, and the shortlist can shrink to one or two plants, or occasionally none at all — deep shade, dry soil, and zone 3 is a real combination the selector returns nothing for, since the two genuinely dry-tolerant plants in this curated set both have a published zone minimum of 4. An empty result isn't a bug in the tool; it's an honest signal that the specific combination of conditions you've measured is genuinely difficult, and it's worth double-checking each input rather than assuming a plant must exist somewhere. Re-verify the light reading on a second day, confirm the soil moisture assessment by hand rather than by eye, and consider whether the "dry" classification is really accurate or whether some supplemental watering during establishment could shift a plant from "dry" into "medium" territory long enough to get it settled in.

Why the boundary readings deserve extra caution

The 4,200-lux example above landed a DLI of 1.68 — close enough to the 1.5 full-shade/deep-shade boundary that a slightly different day, a slightly different hour count, or even a different phone app could plausibly tip it into the other category. Boundary readings like this are exactly where it's worth taking the extra step of a second or third reading on a different day before committing a plant list to the result, since the two adjacent categories can produce meaningfully different shortlists — the difference between fifteen candidate plants and ten, in the worked examples above. A reading that lands comfortably in the middle of a band, the way the 1,200-lux deep-shade example did, is more trustworthy on a single measurement, simply because it would take a much larger swing in conditions to actually cross into a different category.

Don't forget the other two inputs

It's easy to get focused on the light reading, since it's the input that requires actual measurement rather than a quick lookup, but the selector's other two filters — hardiness zone and soil moisture — matter just as much to the final result and are worth getting right rather than guessing. Your USDA hardiness zone is a fixed fact about your location, easy to look up once and reuse for every planting decision going forward; get it from a current USDA zone map rather than an old memory of what zone a region "used to be," since published zone boundaries have shifted in some regions as climate data updates. Soil moisture is the input most worth double-checking by hand rather than by eye, using the same finger test described elsewhere on this site: push a finger a couple of inches into the soil a day or two after the last rain or watering, and judge dry, medium, or wet from what you actually feel rather than what the bed looks like from a distance.

Re-checking as the seasons and the bed change

A pipeline built on a single measurement is only as good as that measurement staying accurate over time, and a bed's light level is not fixed. A young tree's canopy thickens over the years, a neighboring structure gets built or torn down, an old tree comes down in a storm — any of these can shift a bed from one light category into another, sometimes dramatically. It's worth re-running this whole pipeline every few years, or any time you notice existing plants in the bed starting to stretch, bloom poorly, or scorch in a way that suggests the light they were planted for no longer matches the light they're actually getting. Treat the pipeline as a repeatable check rather than a one-time exercise, and it will keep earning its keep well past the first planting.

Putting the pipeline together

The full sequence, start to finish: take a lux reading and an hour count at the bed in question, ideally more than once and at the time of year that matters most; run it through the light level calculator to get a DLI and a category; translate a "Partial," "Full," or "Deep" shade category directly into the matching selector input; check your soil by hand to classify it as dry, medium, or wet rather than guessing; and run the shade plant selector with your zone, translated light tier, and moisture to get a real shortlist. Our shade level reference lays out this same pipeline as a single reference table, with spacing and watering guidance layered on for each plant the selector returns.