A carnivore with one clear optimum — and a fern whose "best" result comes with an asterisk.
The experiment: 12 species · 4 intensity levels · ~3 months · Units: 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) the group ended where it started — two of its three plants shrank badly in June; 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.
Before and after: the butterwort at 133 PPFD (top) more than doubled its rosette area; the staghorn fern's fronds lengthened by up to 10 cm (bottom).
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. Spread points the same way as the spores: by mid-July the fern at 82 PPFD had kept its width, while the plants at 267 and 533 had lost a third or more. (One caveat: we had only one fern per level, so treat this as a strong hint rather than proof.)
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: 82–133 PPFD kept it fullest and most fertile; go brighter only if you want longer fronds 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.
More from this experiment
- Part 1 — The setup: 12 houseplants, 4 light levels
- 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 6 — Five light colors on one dwarf tomato
Lighting we'd recommend
- Barrina T5 10W magnetic grow lights — even, moderate light for a tray of butterworts at around 133 PPFD.
- Barrina Orbgrow 9W dimmable puck — magnetic and easy to aim, so it suits a staghorn mounted on a board or wall; keep it at a gentle setting.
