A “Gardening” post here in the Critters and Gardening MRB.

Spring arrives and suddenly our gardens and any patch of ground really are ablaze with blossoms. How do the plants know?

We are talking about  angiosperms, flowering plants not conifers.


Quote:

most plants measure day length using a green pigment called phytochrome (literally plant colour). This exists in two forms, one of which is active in triggering plant metabolism. This phytochrome system enables plants to measure, with remarkable accuracy, both day length (also known as photoperiod) and the night length. The ratio of the two forms allows plants to measure time like a biological clock. Photoperiod is a very accurate and reliable measure of time and season and so plants nearly always get their flowering times in spring right. In some plants there is an extra feature that can affect flowering, where the plants produce an inhibitor (abscisic acid) before winter that keeps them dormant. Abscisic acid is cold-sensitive. So when spring comes, the inhibitor level is low. This, combined with photoperiod, helps initiate flowering. The two mechanisms combined are a very reliable and consistent trigger for flowering.

https://theconversation.com/how-do-flowers-know-its-spring-a-botanist-explains-264782

Spring has lots of water generally and hungry insects that have also noticed that winter has given way to spring so it is a good time for a plant to blossom and spread pollen.

The abscisic acid is the dormancy hormone that keeps deciduous trees bare in winter and to flower and grow starting in spring. The more abscisic acid the more “cold hours” the plant or tree needs before coming back to life. Certain apple trees really require a long cold period before they will blossom, too long a period for where I am tho 99.9% of apple trees will get enough cold hours here.

Cold hours exclude hours of frost and below zero temperatures, funnily enough!

I find that typically plans grow and bloom when it gets warm. the day length theory finds a problem as plants will bloom early if it is warm early and late if it remains cold longer. This is even consistent with daylight times varying.

That would be the Abscisic acid, having declined over winter it cannot stop plants blooming in warm, sunny conditions.

Dnarever wrote on Sep 10^th, 2025 at 3:50pm:

I find that typically plans grow and bloom when it gets warm. the day length theory finds a problem as plants will bloom early if it is warm early and late if it remains cold longer. This is even consistent with daylight times varying.

Plants are living entities.
If it is a cold morning, I want to stay in bed longer too.
If it is warm early it is not as conducive to sleeping.

Darkness entices sleeping, does not enforce it