6 April 2025

Coltsfoot (Tussilago farfara) has been blooming since last month but when I visited Schenley Park on Friday under overcast skies and threatening rain, all the flowers were slammed shut.

Many daisy-like flowers such as coltsfoot respond to lower light levels at dusk by closing their petals, then opening them as light increases after dawn. It’s a daily exercise.

Their movement is non-directional with respect to the stimulus — i.e. the petals are not flinching away — and has nothing to do with growth of the plant so it’s a nastic movement. When it occurs in response to lower light levels it is called nyctinasty.

Nyctinasty from Greek: nyct = night + nastos = pressed together.

In the morning the petals start to open again.

After most are open you can see that flowers in shadow unfurl a bit later.

Watch as daisies and speedwell open and shut in response to light.

Every day the petals get a workout.