Why do plants flower in the spring?

Many plants flower in the spring so that they can take advantage of the good weather to reproduce and still have time in the summer to make seeds.

Not all plants flower in the spring. At any given time of year, there will be some flowers in bloom. But spring is the most popular time, because plants that flower then tend to receive a better balance of resources for their different needs.

The plant’s ultimate aim is to reproduce, but for this to happen, it must go through a series of steps: fertilization (which is where the flower comes in), seed production, and seed dispersal (which means spreading the seeds to places where they can grow). These jobs are much easier when conditions are good—in other words, when there is plenty of sunlight and water, the temperature is high, and the weather isn’t too erratic. In the temperate regions of the world (such as Europe, most of North America, Japan, Argentina, and most of Russia and China), the best weather is in summer, and plants time their activities to take advantage of the season.

If a plant tries to flower in winter, it will struggle to build enough energy for that purpose (which is clearly an energy-intensive endeavor); there won’t be many insects around to help pollinate the flower (see below); and bad weather, such as high winds, rain, and frost, will damage or destroy the delicate flower. If a plant waits until summer, it will have much more energy available for flowering, but it may not leave itself enough time to grow seeds and spread them around before winter arrives again. It seems like the best time for most plants to flower is spring: The weather is becoming more temperate, there is ample sunlight to provide energy, and there are plenty of insects around for the business of pollination. The plant then has the whole summer to take advantage of good conditions to grow a healthy, successful seed.

Matchmaking

Plants use flowers to attract insects because they need help with pollination. Pollination is the plant equivalent of sexual reproduction, in which male and female gametes merge to achieve fertilization. Because plants (which are mostly hermaphroditic, meaning they contain the equivalent of both female and male sex organs) can’t pick up and wander around looking for a mate, they rely on insects for fertilization. The insect is lured in by the flower, lands on it, and picks up pollen, which it then carries off to fertilize a different flower. Some plants are capable of self-pollination, but this can actually decrease the vigor and viability of the plant, and limit the diversity of its progeny. However, self-pollinating plants can flourish in environments where pollinators are not readily available or exist in small numbers.

Flowers may offer special treats to attract the insects, such as nectar. Honeybees collect nectar from flowers and take it back to their hives to convert into honey, which they consume during the winter when there are no flowers available. It takes a large quantity of nectar to make honey; for just a single pound of honey, bees have to fly about 55,000 miles and tap two million flowers. But the labor is well worth it; an ounce of honey contains enough energy for a bee to fly around the world.

You might be wondering, how do plants know when it’s the right time to flower? Some plants seem to measure the temperature before flowering; others rely on the number of hours of daylight. Many species of plants need a cold shock before they will flower, such as the winter frost. There is some evidence that climate change is actually changing the timing of flowering for many plants. According to a study in the journal Science, the average first flowering date of 385 British plant species has advanced by 4.5 days over the last 10 years, and one in seven species is flowering more than 2 weeks earlier than usual.