A carnivore with one clear optimum — and a fern whose "best" result comes with an asterisk.
The experiment: 12 species · 4 intensity levels · ~3 monthsUnits: PPFD (µmol/m²/s)
Two of the trickier plants in the trial gave the two most instructive results: one textbook-clean, one that makes you ask what "doing well" even means.
Butterwort: a clean single peak
The carnivorous butterwort (Pinguicula 'Tysje') produced the tidiest response of the whole experiment. At 133 PPFD its rosette expanded dramatically — over 4 cm wider. Below that (82) it barely moved; above it (267) it grew more slowly, and at 533 it stalled and showed stress. One clear winner, dim on both sides — exactly the single-peak curve textbooks promise and plants rarely deliver.
Butterwort rosette width. The 133 PPFD line separates from the pack and keeps climbing.
The 133 PPFD rosette (left) is broad and open; the 533 PPFD plant (right) stayed small and tight-stressed.
Staghorn fern: it depends what you want
The staghorn fern refused to give a simple answer. Measured by frond length, its best result was at the highest light, 533 PPFD — the fronds elongated most there. But long fronds aren't automatically healthy fronds: strong elongation can be reaching, the same stretch response we saw etiolate the succulents.
Frond length climbs most under the strongest light — but read that as "stretch," not automatically "thriving."
Flip to a different metric and the winner flips too. At the lowest light (82 PPFD) the fern produced the most spore fronds — the fertile fronds a mature, settled plant makes. So "best" depends on your goal: raw length at high light, or fertility and form at low light.
Spore-frond count peaks at the lowest light — a different definition of a happy fern.
The fussy ones, practical settings
- Butterwort: aim for 133 PPFD — a genuine sweet spot, not too dim, not too strong.
- Staghorn fern: 133–267 PPFD is a sensible middle; go brighter only if you want length and can watch for over-stretch.
This is the useful lesson hiding in the fern: "best light" is only meaningful once you've decided which outcome you're optimizing for — size, form, color, or fertility.
