One bonus experiment changed the light's color instead of its intensity. Red-rich light grew the tallest plants; a hemp full-spectrum grew the most compact.
The experiment: 12 species · 4 intensity levels · ~3 monthsUnits: PPFD (µmol/m²/s)
Everything so far changed how much light the plants got. This side-trial did the opposite: it fixed the plant — a dwarf red tomato — and changed the spectrum, running five different grow-light "colors" at broadly similar intensity. If you've ever wondered whether the color of your grow light actually matters, this is the cleanest look I have.
The five spectra, from a hemp/full-spectrum white to a heavily red-shifted recipe:
The five light spectra tested on tomato, shown as representative power-distribution curves. Like a fingerprint, each mixes blue, green and red differently — note how the red-rich recipe stacks energy at the far right.
Spread barely cared about spectrum
Plant width came out remarkably similar across all five spectra — roughly 27–30 cm of cumulative gain, whichever color the plants grew under. If your goal is a bushy plant, spectrum was almost a non-factor here.
Cumulative spread by spectrum — the five lines cluster tightly. Color didn't move width much.
Height is where the spectrum showed up
Height was a different matter. The red-rich recipe grew the tallest plants by a wide margin (about +23 cm), while the hemp full-spectrum kept them the most compact (about +15 cm), with the warm-white and pinkish recipes in between.
Cumulative height by spectrum. Red-rich (purple) stretches tallest; hemp full-spectrum (green) stays shortest.
That fits the horticulture: red-heavy light pushes stem elongation and flowering. Great if you want to drive fruit set — but for a dwarf tomato that's supposed to stay compact on a shelf, it's a mixed blessing. Here's what the five canopies looked like, each under its own light color:
The five tomato canopies, each photographed under its own spectrum — from warm hemp/white light to deep red-rich.
About the mid-summer dip: in the height and width charts every line turns down after early June. That's harvesting and pruning, not a lighting effect — don't read it as the plants failing.
What to actually do
Tomato spectrum, practical settings
- Want a compact fruiting dwarf? A balanced hemp/full-spectrum white keeps height in check
- Want to push flowering and fruit set and don't mind taller plants? A red-rich spectrum does that.
- For plain leafy bulk, spectrum barely mattered — intensity is the bigger lever.
And that's the thread running through this whole set of experiments: get the intensity right first — matched to the plant in PPFD — then fine-tune with spectrum. Not the other way around.
