Plants and Shade

Sun exposure (or lack of it) can affect plants in a variety of ways. The total amount of light and quality (intensity, wavelength) affects amount of photosynthesis as well as triggering processes like flowering. Heat and water loss caused by sun are additional issues.

The amount of sun a given plant and tolerate (high or low) will depend on the species and will be a reflection on the habitat in which it evolved. Shady spots generally require plants that perform well in these conditions, but the type of shade must be considered.

Under a tree, a high, sparse canopy may cast light shade most of the day, while conditions under a low, dense canopy may be much darker. In the latter case, it may be possible to trim lower branches and thin the canopy to allow more light to penetrate. However, you will still have the problem of root competition to contend with. This will limit selection to plants which are tough enough to compete, plants of an epiphytic nature which require little or no soil (e.g. bromeliads, orchids) or plants in containers. The shelter of trees can be a good place to give indoor plants a holiday outdoors, or acclimatise newly purchased plants or propagate new ones.

Shade cast by a nearby building may be quite bright (especially if light is being reflected from other hard surfaces nearby) and amenable to growing a wide range of plants. However, at some times of the day when the sun has moved, the position may be be in full sun. Likewise a position on the south side of a house might be in full shade throughout winter, but when the sun moves higher and sets futher west or southwest, it might be exposed to some fierce sun in summer. Hard heat-radiating surfaces nearby will make the situation even worse.

Creating Shade

In an intensely hot and sunny climate like Queensland, some shade is generally welcome as it extends the range of plants that can be grown as well as making a more pleasant environment for the humans in the garden. Strategic planting of trees or construction of structures like pergolas or shadehouse can be used to provide the desired amount and placement of shade. See also Shadehouses

News

Roots and Shade

When plants aren't getting enough light, growth is reduced and elongated stems "stretch" towards the light source. We don't usually think about what's happening below ground, but new research has shown that excessive shade has an effect there, too. Roots of both tomato and Arabidopsis were found to be shorter and less developed when light was reduced. This appears to be a specific response associated with the activation of stress genes and possibly involving ethylene (the fruit-ripening gas). It enables the plants to direct more of their limited resources to growth of the light-harvesting parts. In the garden, shaded areas (under eaves, around trees) are often much drier than elsewhere. The inhibition of root growth due to shade could be making it even more difficult for plants to cope than simple observation of the soil would suggest. Home gardeners might also consider how spacing their vegetables too closely might stunt the roots due to mutual shading, even if soil conditions are very good. In this study, the researchers were primarily interested in how crop plants respond to overcrowding, with a view to engineering varieties that can be efficiently produced at higher densities. Source: Why roots don’t grow in the shade (October 2021)