Midlands Dahlia Society Logo based on the original 1958 broach Midlands Dahlia Society
The Midlands Dahlia Society


Propagation 2011 - How much light is required?

To answer the question of 'what light to dahlias require to propagate?' I had to understand how plants grow.  Photosynthesis is the way plants synthesise food directly from Carbon Dioxide - CO2 using energy from light.  Water is also required, the whole cycle is called the Calvin Cycle - see diagram below.

The equation for photosynthesis is:
Carbon Dioxide + water + light energy ® carbohydrate + oxygen.

or in chemical symbols:
2n CO2 + 2n H2O + photons ® 2(CH2O)n + 2n O2 

 Plants absorb light primarily using the pigment chlorophyll.  There are two structures which are relevant: chlorophyll a which is universal in occurrence, and chlorophyll b which is found in most plants.  These have different peak absorbance frequencies which can be seen in the picture below.  Using diethyl ether, chlorophyll a has absorbance peaks at 430nm and 662nm, whilst chlorophyll b has peaks at 453nm and 653nm.

chlorophyll absorance curve