What is shade?
Shade is the blocking of sunlight (in particular direct sunshine as opposed to diffuse light reflected from the sky) by any object, and also the shadow created by that object. To grow healthy plants in shady areas, it is important to match the degree of shade that a plant needs or will tolerate with available light. Unfortunately very few plants will thrive where shade is very dense, particularly when coupled with a dry impoverished soil.
What does shade do?
All green plants depend on sunlight to manufacture the sugars that go to providing energy for the plant and for making the organic molecules that make up plants. Shade leads to lower sugar production and reduced growth and flowering and ultimately, in plants ill adapted to shade, death.
Plants can ‘measure’ shade by using pigments that can detect far red and blue light and modify their growth habit in proportion to the degree of shade.
Plants can be considered as ‘shade avoiders’ and ‘shade toleraters’, to a greater or lesser degree. Sunlight is a mixture of wavelengths but the presence of shade alters the balance of colours which sophisticated systems within plants can measure and alter their growth according to the shade that they encounter.
Changes in red and blue light caused by the effects of shade in a garden trigger shade avoiders to elongate their stems and try to grow into more light above structures and bigger plants. Shade avoiders seldom make good plants for shady areas of the garden. A typical example is of most climbers, a plant form well adapted to clamber over obstacles to grow into the light. Although climbers are not well suited to shade there are some that can be used on east and north facing walls where there is little or no direct sunlight.
Amongst other changes, shade tolerators typically grow broader but thinner leaves with raised chlorophyll content when grown in shade making them better adapted to the conditions.