March '26

I speak of nature’s awakening in spring. This is the moment when it is decided which tree will grow the tallest and which blossom the largest. Plants require solar energy to grow and reproduce, and green leaves are essential for photosynthesis. Yet maintaining them throughout the dark and cold winter would not pay off. It is far more economical to shed them and conserve strength for the spring awakening. And here lies the critical point.

A young tree may easily find that its neighbour sends out leaves earlier and, strengthened by sunlight, outgrows the latecomer. On the other hand, leaves deployed too soon may be destroyed by frost, wasting precious energy. In nature, spring is a moment of delicate decision-making.

Tens of millions of years of evolutionary calibration have made deciduous trees masters of this art. They know that temperature cannot be trusted. A week of warmth in mid-winter does not confuse them. Instead they rely on something better: colour. The decisive role is played by phytochrome — a sensor for red light.

The red part of the spectrum has the lowest frequency. A red photon deviates the least when passing through the atmosphere, which means it has the smallest probability of colliding with a gas molecule. A physicist would say that red light experiences the least scattering in the atmosphere. That makes it the most reliable indicator of day length, since red light is almost indifferent to the angle at which it enters the atmosphere or to the degree of cloud cover.