How do plants know when it's time to grow?

What happens if it's unusually warm but daylight hours are still short?

When it's warm but hours of daylight are short, or vice versa, all sorts of strange things happen. Plants get confused as the warmth tells them to start growing, but the lack of daylight tells them to wait.

The warmth speeds up the plant's metabolism, telling it to grow, but there aren't enough hours of sunlight for the plant to photosynthesise and make energy for growth, so stems grow long and thin. This is called etiolation. Tall, thin stems can be weak and prone to falling over.

We see this with our bean in a jar grown in the dark, without light, the bean is pale and tall.

There's also the risk of an early frost damaging new buds if a plant starts to grow too early.

The time of year when there are fewer than 10 hours of sunlight is called the Persephone Period. Even if the temperature is warm, plants will still grow slowly or not at all during this time.

This is why a false spring is dangerous for plants. The warm weather tricks them into starting to grow, leaving their fragile new tissues vulnerable to drops in temperature and frosts.

Why do some flowers bloom when it's still cold?

Flowers that you see right at the start of spring, like tulips, use energy stored in their bulbs to start growing before there's too much competition from other plants and trees.