People Shade vs Plant Shade
People and plants require different amounts of shade. Whilst people do need some of the Vitamin D that comes from exposure to light, we do not need the energy from sunlight for photosynthesis, as many species of plant, algae, and bacteria require.
Further, people have more flexibility when it comes to choosing the times of day they would like to be in the sun - just get up and go outside, or go back inside as required.
Generally, we say that people shade should be a minimum of 90% UV protection, and many of the commercial shade fabrics available nowadays offer up to 98% protection. Waterproof shade fabrics offer 100% UV block.
However, most plants require some levels of light during the day for photosynthesis purposes, and as such 90% and greater is generally too much shade. But all plants are different, and even the same species can differ in its requirements from one location to another.
We can, however, generalise somewhat, so here are a couple of pointers for your plant shading requirements:
Summer vegetables like full sun, EXCEPT for the hottest summer days in Australia (and particularly in the drier climates of SA and WA) when a light, 30% shade can benefit.
Plants such as gardenia, camelia, and azalea enjoy growth under medium shade, approximately 70%. They can also benefit from being located under a north facing pergola, so they have a little bit of exposure to the sun during the cooler months (when the southern hemisphere sun is lower in the northern sky)
Delicate flowering plants such as orchids incite a great debate amongst their passionate and faithful group of devoted growers. The outcome is that 50% or 70% shade cloth (depending on the exact location and exposure) - in colour WHITE - is the best option.
This also raises the issue of colour - and this is an important issue. Many believe that plants will not properly photosynthesise if the light that they receive is distorted. Thus white light should pass through either white fabric (zero distortion), black fabric (zero distortion), or green fabric (green distortion only) before being received by the plant. This is why only those 3 colours are generally available in the lighter shade fabrics.
All in all, it's a complicated subject, and I recommend you get some good advice prior to selecting the shade you require.
** For those of you wishing to learn more about photosynthesis and how the process works, I suggest you CLICK HERE for the full wikipedia article.