How does cold affect plants?

When the cold arrives, life seems to stop. Some animals, such as migratory birds, leave, others hide or hibernate. And plants, fixed to the ground, also seem to stop: some stop growing, stop blooming or lose their leaves. But are they really at a standstill?

For many, surviving winter is quite a challenge. Since they cannot move or migrate, they have a system that allows them to “know” what time of year they are and thus “decide” the best time to germinate, grow, sprout and bloom. This system works like an internal biological clock that collects information from the environment to coordinate what is happening outside with their life cycle. Above all, it integrates two key signals, the hours of light , in technical terms called photoperiod , and temperature.

However, cold is not an enemy. For many plant species, cold is an essential signal, which must be given at the right time and with the right intensity, to complete their life cycle, and without it, many would not know if the time has come to grow, bloom or germinate.

As the days shorten and temperatures drop, many plants enter a resting phase known as dormancy . During this period, they stop growing and reduce their metabolism to a minimum to conserve energy and cope with adverse conditions, waiting for the right time to become active again.

Some plant species, especially those that live in temperate climates where the seasons are well defined, need to spend a certain period of cold time to activate flowering . This process is called vernalization . In this process, the level of cold, that is, the drop in temperature, is as important as its duration. Each species, and even each variety, has its specific needs. As, for example, in the case of cereals such as barley ( Hordeum vulgare ) which has a summer variety and a winter variety, or wheat ( Triticum sp. ) which is grown in both winter and spring varieties.

Some examples of plants that need cold to bloom are:

• Fruit trees: such as almond, cherry, plum, apple, chestnut, hazelnut or grapevine.
• Cereals: winter wheat and barley
• Vegetables: broccoli, cabbage and cauliflower, carrots or beets.
• Wildflowers: lavender ( Lavandula angustifolia ), tulips and some species of the genus Rubus , which include blackberries and raspberries.

Within the group of plants that vernalize, that is, that need this intense and accumulated cold impulse, there are two types. On the one hand, those that absolutely need this cold to bloom (obligate vernalization), and on the other, those that perceive the cold as an assistant that stimulates flowering (facultative vernalization), but that, without it, could also bloom. In addition, there are also plants that do not need cold and complete their cycle independently of it.

With climate change, winters are becoming increasingly irregular and milder, and for plants, this has consequences, because they can "get lost" and disrupt their life cycle.

If vernalizing plants do not accumulate the necessary cold, they may bloom with fewer flowers, bloom late and produce fewer fruits. In other cases, the opposite can happen, if they accumulate part of the necessary cold, and a warm period arrives early, they may bloom early, and if this happens, there is a risk that they will freeze and have spent energy unnecessarily when it was not necessary. However, they may not encounter species with which they have an ecological relationship, such as their pollinators or herbivores. All of this affects the survival and reproduction of the plants.

To understand how plants respond to cold and increasingly irregular winters, field observations are key. At CREAF, citizens can actively participate in data collection. With the RitmeNatura project, we follow phenology, that is, the key biological moments associated with the climate, such as budding, flowering, fruiting and leaf fall, thanks to citizen participation.

Citizen contributions allow us to detect anomalous situations, such as advanced flowering or second flowering.

This data, collected year after year and across the territory, helps us understand how climate change is altering natural rhythms. And the more observations over time, the easier it is to follow and understand nature's "clock" and detect when it goes out of sync.

Anyone can participate by observing the vegetation in their environment and noting when they sprout, bloom, bear fruit or lose their leaves.