How Do Plants Know It’s Spring (While We’re Still Being Fooled!)

Ah, springtime in Canada! That time of year when the weather goes up and down: 20 °C (68?) for three days and snow the following week. I had barely taken out my patio chairs in mid-March, just to enjoy the warmth and the sun, when I had to quickly bring them in when it dropped back below zero with snow the following week. My hopes for spring have now cooled down – literally!

But what about the plants? They don’t look at the weather forecast on their smartphones and have no way of knowing that a winter episode will follow the scandalous 20°C mentioned above! Is this madcap spring a challenge for our landscaping? Fortunately, plants have a much more sophisticated system than our thermometers and calendars to tell if it’s “really” spring!

In fact, plants have much more precise receptors than our temperamental senses when it comes to detecting light or temperature. Their method of recognizing spring combines several signals that are much more reliable than what our human perception can detect.

Sensitive Light Sensors

The photoperiod – or the length of the day – is their first clue. Have you noticed that the sun is lower on the horizon and cooler in winter? But it gradually rises again as the earth tilts towards it, and in March, we can feel its warmth on our faces. The time it spends in the sky and also its “strength” can be detected by plants.

This apparent “power” quite simply comes from the fact that the rays strike the earth more and more perpendicularly and therefore a greater concentration of rays is found on a given perimeter.

As a picture is worth a thousand words, here it is:

Phytochromes are responsible for receiving these light signals. What makes them special? They can detect light even through the snow! Light easily passes through a thin layer of snow, allowing plants to “feel” changes in light, even when an unfortunate shower surprises us in March.

These proteins function as molecular switches: they change shape according to the wavelength of the light. Red light activates one form, infrared light activates another. Programmed by millions of years of evolution, plants are thus able to understand the most subtle nuances of light. A stroke of luck: whatever the weather, the sun and the celestial movements are stable from year to year, so it is an excellent way of knowing that it is time to start waking up in order to possibly bloom at the right moment.

The Week That Doesn’t Exist

You know that I love historical anecdotes! Here is one that I adore: in 1582, Pope Gregory XIII proclaimed that Thursday, October 4, 1582 would be followed by Friday, October 15. Boom! 10 days of history that never existed!

But why is that? Time, with its hours and dates, was “created” by humans. However, it does not fully reflect reality. For example, the earth does not rotate on its axis every 24 hours. Rather, it does so every few… 23 h 56 min 4 s! So, we have about 4 minutes “too many” every day.

Similarly, the earth does not take 365 days to go around the sun, but 365.25 days. And that explains why there are leap years every four years! This was not applied when the calendar was ‘created’. Our dear Gregory therefore removed 10 days from the calendar to set the record straight (in terms of the sun) and once again make the equinoxes coincide with public holidays and gardening calendars. Because, of course, plants continue to follow the sun.

Cold, a Necessary Evil

Light is not their only indicator. Soil temperature plays a crucial role in this spring awakening. The roots and seeds are equipped with temperature sensors that precisely measure variations. It is not only the air temperature that counts, but also the soil temperature – a much more stable and reliable parameter.

For example, if we think of flower bulbs, they need to go through a period of cold before they can bloom. This is vernalization. Otherwise, your tulips might decide to bloom again in August, exhaust their energy, fail to rebuild reserves and be unable to bloom again after winter.

Other plants, such as the pawpaw, a North American fruit tree, need their seeds to be exposed to low temperatures for several weeks in order to germinate. This ensures, once again, that they do not germinate in the middle of Indian summer. It is, in a way, a safety mechanism to germinate at the right time, when water is available in the soil, and the warm season will be long enough for optimal growth.

Heat (According to Plants!)

Not all plants need vernalization, but all are sensitive to temperature. For these species, the warming of the soil is a crucial signal of spring. Temperature variations trigger an underground awakening long before the aerial parts show signs of growth.

The best example is maple water. The sap rises in the trunk sometimes as early as the end of February. However, the buds do not open until April or May!

The warming of the soil activates a whole biochemical process. The enzymes start working again, the circulation of the sap accelerates and the energy reserves accumulated during the winter are gradually mobilized. And we don’t see any of this!

Each species has its own temperature threshold. Peas and lettuce begin to germinate as soon as the soil is around 4°C, 40?, (the famous “as soon as the soil is malleable” on seed packets!), while tomatoes wait until the soil temperature exceeds 15 or 20°C (59 or 68?). These thresholds vary considerably depending on the species, reflecting their geographical origins, their evolutionary adaptations and the selection made by humans.

Spring Awakening and Climate Change

As might be expected, climate change is gradually altering the signals to which plants are accustomed. Milder or harsher winters, more frequent and greater temperature variations, not to mention completely disrupted rainfall, all create new adaptation challenges. Some plants are already showing signs of disruption: buds that are opening prematurely, staggered flowering cycles, etc.

But there is no need to worry: evolution has given plants a remarkable capacity to adapt. Their complex detection mechanisms enable them to gradually adjust to these changes. Yes, some individuals will probably bloom too early, or will be caught in a late frost, but this is not a cause for concern. This is not the first global warming: some species adapt and others settle in to replace those that have not succeeded. It is not a perfect process, but rather a constant negotiation with a changing environment. In a word, it is what we call Evolution with a capital E!

In short, I hope that by the time you read these lines, I will have brought out my patio chairs! And for good, this time! But I can’t be really sure… I’m not a plant after all!