Days are getting longer and temperatures are rising: Spring is a miraculous time of rejuvenation that brings new life as it sweeps across the land.

Before Mother Nature can offer up scenes like this one and those on the pages that follow, a gigantic movement must take place from south to north across the Northern Hemisphere. It releases more energy than the world’s population consumes in an entire year and reactivates more creatures than the total number of human beings who have ever lived.

It’s time for a decision—do it now, or hold off until later. Acting prematurely could be deadly if a cold snap comes. But highly responsive sensors provide a wealth of information for making the right call, tracking the length of nights, the temperatures, and trace elements in the air and soil. Is it worth the risk?

In this particular year, February 28 is a warm day, and the crocus decides the risk is worth taking. It rises out of the cold ground, spreads its leaves, and opens its purple flower to the sun in a visible sign of spring. The vernal equinox—one of the two days a year when the day and night are the same length—is still three weeks away, but Mother Nature has signaled that it’s time to get going again. In the weeks to come quadrillions of leaves will be unfurled and hungry earthworms will make their way to the soil surface as birds engage in ear-splitting turf wars. All of this will take place in what was, until recently, a barren landscape.

In the spring a steady progression takes place from south to north across the Northern Hemisphere. The British Science Association estimates that spring moves north in Britain at about 2 miles per hour. The rule of thumb in the U.S. suggests that spring moves north by 13 miles per day. The truth probably lies somewhere in between and depends on the year and specific location and conditions (like whether there are any mountains in the way, for example). But though the coming of spring may seem spontaneous and capricious, it is the product of a finely tuned system that scientists continue to decipher more and more.

Back in the 1920s botanists thought the amount of daylight each day was the key to the process of dormancy and reawakening they called photoperiodism. Consequently they labeled plants as “long-day,” “short-day,” or “day-neutral.” But it turns out it’s the total length of uninterrupted darkness that actually matters. During nighttime hours plants produce phytochrome, a photosensitive protein that controls the onset of dormancy in the fall and the first burst of buds opening in the spring. It can be upset by even a small amount of electric light—which can be verified by observing the behavior of a tree standing under a street light: It will be on a different timetable than other similar trees that are located a short distance away.

Plants constantly compare external light conditions with the stored target values that have been passed on over countless generations. People who cultivate plants like chrysanthemums or poinsettias are well aware of the phenomenon. The former are “shortday plants” that flower only if they receive enough hours of darkness. Greenhouses where mums are grown used to leave the lights on at night to prevent them from flowering too early, but then it was discovered that a short burst of light during the night would have the same effect. In poinsettias, the dark makes them turn red (or pink or white). To change coloration, they require a minimum of 12 hours of continuous darkness. Any amount of light during this “nighttime” period will prevent coloration, but they still need plenty of light during the day so they absorb enough energy to flower. Trees aren’t generally grown under artificial light and, once planted, they can’t take to their heels in search of better conditions. Therefore they’ve learned to adapt to signs that indicate changes in the weather conditions. Evolution has “taught” them that if they let themselves be fooled by a few warm days in January and produce leaves and flowers when there aren’t any insects to pollinate them, they’re likely to suffer serious damage when the next period of frost arrives, which it almost always does. Thus plants have adapted to react less

to temperature (though they are highly sensitive to it) and the brightness of light and more to the length of the nights. In general, once the nights have become shorter than the days (after the vernal equinox around March 20), spring has arrived. At Bonn University, biologist František Baluška has conducted research into the possible ways trees communicate with one another by way of their roots. He believes each tip in a root system acquires information at least partly independently. If the information is processed in interaction with other roots, the whole system might then solve a problem in a way a lone root could not. If his hypothesis holds up, it would serve as evidence that trees possess collective intelligence, akin to how those “wisdom of the crowd” estimates can fairly accurately gauge the number of marbles in a jar or the weight of livestock at a county fair. So if they are discussing the matter among themselves, why shouldn’t the trees know when it’s time to leaf out?