Everyone argues about grow-light spectrum. But the setting that usually decides whether a plant thrives or fries on your shelf is intensity — so I tested it properly, from Echeveria to Caladium to a carnivorous butterwort.
Walk into any houseplant forum and you'll find people debating light color — "full spectrum," "red and blue," "warm vs cool." Spectrum matters. But I've watched far more plants suffer from getting too much or too little light than from the wrong color. Intensity is the dial most of us set by guesswork.
So I ran a controlled test. One light. One spectrum. The only thing I changed was how much light reached the plants — four levels of it — and then I watched 12 very different species respond over about three months.
The plants
I deliberately picked species that "want" different things — sun-loving succulents next to shade-dwelling jungle aroids, plus a fern and a carnivore to stress-test the extremes:
| Plant | Type | Reputation |
|---|---|---|
| Echeveria 'Chihuahua' & 'Amber' | Succulents | Sun lovers |
| Caladium 'Hilo Beauty', 'Scarlet Lady', green form | Fancy-leaf caladiums | Shade / understory |
| Alocasia 'Green Velvet' (+ variegated) | Jewel alocasias | Bright shade |
| Anthurium — two hybrid seedlings | Aroid | Adaptable |
| Curly staghorn fern (Platycerium) | Epiphytic fern | Bright, indirect |
| Butterwort 'Tysje' (Pinguicula) | Carnivore | Bright but sensitive |
| Dwarf red tomato | Edible (spectrum side-trial) | Full sun |
The actual test plants, from sun-loving succulents to deep-shade jungle foliage.
The light
Every plant in the intensity trial grew under the same fixture: a commercial full-spectrum LED with a red-to-blue ratio of about 3.2:1, hung 90 cm above every group. Its spectrum stayed constant across the whole experiment, so the only thing that differed between groups was how much of that light reached the leaves. (The dwarf tomato ran in a separate side-trial under five different fixtures — that one tested color, not intensity.)
The variable: intensity, measured in PPFD
Here's the one thing worth learning if you take away nothing else. Your phone's light-meter app reads lux — brightness as the human eye perceives it. Plants don't photosynthesize with your eyes. What they care about is PPFD: the number of usable photons landing on a leaf each second (µmol/m²/s). Two lights can show the same lux yet deliver wildly different PPFD, which is exactly why lux-based advice leads people astray. So every number in this experiment is PPFD.
I set four intensity levels, metered in PPFD at the center of each group — from a dim windowsill to a bright greenhouse:
The four PPFD levels used throughout. Same light, same color — only the amount changes.
How I ran it
Most species got three plants per level (so I could average out the odd runt or overachiever); the 'Scarlet Lady' caladium and the variegated alocasia got two, and the staghorn fern just one, so read those results as weaker signals. Everything shared the same fixture, the same 90 cm hanging height, and the same room. Every one to two weeks from late April to mid-July, I measured plant spread, height, leaf count, and leaf size.
Two notes on timing: the Echeverias joined in mid-May, and the caladium and alocasia charts stop in late May to early June. After that, those plants declined at every light level, so the later readings no longer separate the levels cleanly.
Here's what one species looked like at the end — four groups lined up from lowest to highest PPFD. Even in a single frame the effect of intensity is obvious:
Echeveria 'Chihuahua' on 17 July, about nine weeks in. Left (82 PPFD) stays blue-green; toward the right (267–533 PPFD) the rosettes flush pink — the stress coloration succulent growers actually chase.
How to read the charts
Every result uses the same chart style: four lines, one per PPFD level, tracked over time. A line climbing = the plant is growing; a line sagging = it's shrinking or stressed. Here's a real one — the butterwort's rosette width — so you know what you're looking at:
Rosette width of Pinguicula 'Tysje' at four PPFD levels. Notice the 133 PPFD line pull away from the pack — that's its sweet spot.
Reading tip: don't just look at who's tallest at the end — watch the direction. The highest light (533) often starts fine, then flattens and dips. The lowest (82) never gets going. The winner is usually somewhere in the middle, and it's different for every plant.
More from this experiment
- Part 2 — The brightness map: which plant goes where
- Part 3 — Echeveria: compact rosettes and pink color
- Part 4 — Caladiums and alocasias: the shade-lovers
- Part 5 — Butterwort and staghorn fern
- Part 6 — Five light colors on one dwarf tomato
Lighting we'd recommend
- Barrina T5 grow lights — slim, linkable full-spectrum tubes for a shelf of mixed houseplants; raise or lower them to move each plant into its PPFD band.
- Barrina Orbgrow 9W puck with timer and stepless dimming — the easiest way to put this experiment into practice: dial the intensity up for a succulent or down for a caladium, one plant at a time.
- Barrina plant stands with grow lights — shelving with lights built in, so sun-lovers and shade plants can each have their own tier.
