If we want to dismiss something as irrelevant, we say it’s “as boring as watching the grass grow.” Yet grasses — including corn, wheat, and rice — supply the majority of the plant-based calories humans eat, along with most of the calories fed to livestock. New research led by biologists at the University of Massachusetts Amherst, published in Current Biology, suggests we should have been paying closer attention: grasses grow according to temperature, not light, unlike most other plants.

Scientists have long known that most plants respond to light cues that trigger their growth cycles, growing fastest at particular times of day and particular times of year in response to seasonal and daily light changes. Grasses, it turns out, don’t respond to light at all — only to temperature.

Samuel Hazen, who previously led a team showing that temperature alone governs how grass leaves grow, said the pattern is easy enough to see: each leaf’s cells divide and elongate when the temperature is right, making the leaf longer.

Watching a Gene Glow Inside a Living Stem

The harder question was what happens inside grass stems — the woody structures that let wheat, corn, and rice grow tall enough to be harvested. Stem rigidity comes from a thickening of secondary cell walls, a process that unfolds on a microscopic scale that researchers previously had no way to observe in real time.

Graduate student Greg Gregory, the study’s first author, and co-author Dave Follette, director of advanced digital design and fabrication and device characterization at UMass Amherst’s Institute for Applied Life Sciences, designed a custom real-time bioluminescence imaging system that could take dozens of time-lapse images of a model grass called purple false brome as it grew under varying conditions. The team engineered the plants with a genetic reporter linking CESA8, a key gene involved in building rigid secondary cell walls, to firefly luciferase — the same enzyme that makes fireflies glow. Whenever CESA8 switched on, the cells emitted a faint glow the imaging system could detect.

“For the first time, we are now able to watch a gene responsible for building the plant’s structural support turn on and literally glow inside a living stem. By making this process visible, we can now finally uncover how the grass responds to the environment in regard to building secondary cell walls.”

Samuel Hazen, University of Massachusetts Amherst

An Unexpected Rhythm

Testing a range of light and temperature conditions, the team found that temperature alone drives the growth rhythm of grass stems — but not in the way they expected. Grass grew fastest during warm pulses within cool nights, and slowest during cool pulses within warm days. Cold temperatures initially suppressed cell-wall thickening but promoted it if prolonged; the inverse was true for warmth.

Mathematician co-author Didier Gonze, of Belgium’s Université Libre de Bruxelles, modeled the pattern as an “incoherent feed-forward loop” — a system in which temperature both activates grass growth and sets up a delayed response that eventually turns growth off.

The findings raise an open question the study doesn’t yet answer: what happens to this delicate temperature-driven rhythm in a world where nighttime temperatures are rising due to climate change? Any future work addressing that question, the researchers say, will build on this foundational look at how grasses actually grow.

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The study was published in Current Biology (DOI: 10.1016/j.cub.2026.06.034).