Why doesn’t a tree have blue leaves?

Answer

Hi Dries,

To answer your question, let me first explain what colors are:

White light such as sunlight is a mixture of light rays with different wavelengths and we see these wavelengths as different colours. An object has a certain color because it reflects the light of that certain wavelength and absorbs the light of other colors. For example, white reflects all light colors, black absorbs all light colors and a red object only reflects red light and absorbs blue, green, yellow, etc.

Most leaves are green because they contain chlorophyll, which is a molecule that can “capture” light energy so that the plant can use this energy to make sugars, a process called photosynthesis. Leaf green absorbs red and blue light to use for photosynthesis and only reflects green. So trees are not blue because they need the blue light for photosynthesis.

Yet there are also trees with blue leaves, such as young Eucalyptus trees. The bluish color there is caused by a thick layer of wax that protects the leaves against drying out and bright sunlight, and that layer of wax already reflects a little light from all colors before the light reaches the chloroplasts on the inside of the leaf. Together with the green light reflected from the chloroplasts, that little reflected light from the wax layer gives the leaves a bluish color.