A new study led by UBC researchers suggests that the summer solstice—the longest day of the year—may serve as a key cue for plant growth and reproduction. But in a changing climate, relying on this fixed signal may no longer help plants to survive or thrive.

“Plants are known to use temperature to time things like leaf growth and flowering,” says lead author Dr. Victor Van der Meersch, a postdoctoral researcher at UBC’s faculty of forestry. “This study shows that the solstice is an optimal growth period and could also be an important trigger for reproduction.”

Scientists have long known that plants use daylength to time important seasonal events. But only recently have they begun to explore whether the solstice itself—typically around June 21 in the Northern Hemisphere—plays a specific role.

The idea has gained traction as researchers examine how climate change is affecting the natural cues plants rely on.

The study highlights that at the local level, climate change affects different areas in different ways. In warmer southern regions, heat arrives before the solstice, making plants flower early. In cooler northern areas, it’s still too cold in June for plants to start reproducing.

While plants have evolved to use both temperature and daylength for growth, temperature may now be the more reliable signal, say the researchers. “That’s because it’s directly tied to the actual climate conditions plants are experiencing,” said Van der Meersch. “But it’s still unclear how temperature and daylength interact to shape plant responses.”

“Daylength doesn’t change from year to year. But with temperatures fluctuating more, plants may be having trouble adjusting to both signals,” says co-author Dr. Elizabeth Wolkovich, a plant ecologist and associate professor at UBC.

The costs of mistimed growth

When plants mistime key events—flowering too early or too late—it can have serious effects on ecosystems. Pollinators may arrive at fields only to find flowers have already come and gone. Crops like apples or cherries could bloom too early, only to be hit by a late frost, or ripen weeks before harvesters or markets are ready. Forests may miss their best growing window, reducing how much carbon they can absorb each year.

“These timing problems can affect food security and biodiversity. We need to understand the signals plants use for key events like flowering and fruit ripening,” added Dr. Wolkovich. “By tracking these signals, we can help ecosystems and agriculture adapt to climate change.”

Researchers are calling for better models to understand how plant growth affects the climate—to help farmers and land managers protect the resilience of ecosystems.

The study is published in Proceedings of the National Academy of Sciences.