zinnias grown by NASA at Kennedy Space Centre
leaf in sunlight

Light & Plants - the science

This week, we've gone science - and we ask:
  • Why do plants need light?
  • Does everything need light to grow?
  • Longer light or stronger light - which is better?
  • Can plants get too much light?
  • Which places get most light?
  • What colour light helps plants to grow?

Why do plants need light?

liquidambar leaves Cast your mind back to school science lessons and a process called photo-synthesis.

Plants take in light and carbon dioxide via their leaves. Using water (via their roots) they convert that light energy into sugars, enzymes, and chlorophyll that they use to grow. They breathe out excess energy as oxygen.

What about plants that aren't green?

  • Chlorophyll makes plants green, and helps them absorb more sunlight.
  • Carotenoids give leaves orange-yellow colour, and help chlorophyll do its job.
  • Anthocyanins, which create purple-red leaves, act as ‘sunscreen’ when plants are stressed from too much light.

Does everything need light to grow?

phototropism in bulb At one time it was thought photosynthesis could only happen in visible light. But new ocean bacteria have been found that can photosynthesise in infra-red light, which means that however dark it gets, they can continue to grow. Pretty amazing!

When it comes to plants however, it's a different story (for now) - they need the kind of light we can see.


How do plants know where to grow?

Seedlings navigate by moving away from gravity. Once they reach the soil surface, they grow towards the strongest source of light, in a process called phototropism.

You'll notice this if you grow plants on a windowsill - they'll need a turn every now and then to straighten them up.



Longer light v Stronger light

For some plants it is important how long they have daylight for - chrysanthemums and poinsettias, for instance, need extra hours of light when grown commercially, to make sure they flower all year round.

Several herbs only start to flower once days get shorter, telling the plant that the summer is over and they'd better start making seed.

Other plants care how much light they can get each day and, for them, a few hours of strong sun is as good as a full day of dappled shade.

Then there are plants which will only flower once they reach a certain age. Vegetables like tomatoes and capsicum fall into this type.


Etiolation - Too Little Light

etiolation in seedlings This is a fancy word for growing "leggy" - plants that don't get the light they need will grow tall, pale and weak.

You can see this in action if you discover a forgotten potato at the back of the pantry covered in long white shoots.

Plants can usually absorb and use as much light as you can give them. What will affect them is how hot that light is - most plants stop growing when temperatures get too high.

Grass will stop growing at temperatures over 32C. Lettuce will "bolt" or send up a flowering shoot once it gets too hot - shading plants to reduce daylight hours can slow this process.

Too Much Light

Sometimes plants do get too much light, if they're grown outside their native range or a shade plant is grown in sun. This shows in bleached pale leaves.

Full-sun plants usually need at least 25,000 lux - or 6 to 8 hours of direct light each day. Sunlight intensity on a bright summer day can reach 100,000 lux.

For comparison, indoor light levels are around 100 lux. Plants that thrive in shade, or indoors, can flourish with just 2 hours of daily full sunlight.

Plants often adjust their leaves, to reduce or increase the light reaching the surface - you can see this on many gums which have a "weeping" look as the leaves hang down.

The lithops succulents (above) have tiny "windows" which they open in winter and close during summer, to regulate the light reaching the centre of the plant. (Image: yellowcloud)


The Sunny Country

sunlight through magnolia Nearly all of Australia enjoys lots of sunshine. Australian cities get around 3000 hours of sunshine a year (twice as much as the average northern European city, fyi).

Perth leads the sunshine stakes, thanks to its dry climate. Inland towns also have lower humidity, and so less cloud cover and more sunlight.

Coastal areas, especially in the south, have more humidity, more cloud cover, and less sunlight than inland places all year round.

During the October-April wet season, there is even more cloud in subtropical regions because of the monsoon, while southern Australia gets drier, less cloudy conditions with more sunshine.

During the May-September dry season there is an increase in sunshine in the subtropics; while southern towns receive frontal systems which bring rain and cloud, and less sunshine. Melbourne gets just over 2000 hours of sunshine a year (alternating daily with rain, fog, and snow...)
christmas cactus

North v South

North-facing sites have the most intense light - five times that of a south-facing site in the same location.

The same goes for northern versus southern towns in winter - you can see the difference in light levels across the country in the chart above. In summer, UV levels are much more equal nationwide.

Horticulturalist James Beattie says

"South-facing aspects are one of the most challenging spots for gardeners in the southern hemisphere as they get no sun in winter, but face summer's unbridled baking onslaught."


One Light Two Light Red Light Blue Light

zinnia growing at NASA Kennedy Space Centre Red Light is more abundant in summer, which is handy as it helps to regulate flowering and fruiting.

At the other end of the spectrum to blue, red has a long wavelength that plants need more of, to utilise.

Blue Light is more abundant in winter. It has a short wavelength and lots of energy which plants can readily use; blue makes leafy green growth and shorter bushier plants.

It's also ideal for seedlings which emerge stronger and sturdier.

Here you can see zinnia plants growing under Red, Green, and Blue LED lights, at NASA's Kennedy Space Centre and the International Space Station.

Changing the wavelength, and so the colour, of light is not the only way us humans can control when plants flower.

Our blog post on Plants & Light explains more about the effects of shorter days and longer nights during winter on plant flowering and growth.