How plants use light to grow

Feeling the sun on your face is one of life's simple pleasures, but for plants, catching rays is vital for survival. Here you can find out how plants use light to fuel their growth, how they're adapted to different light levels, and how you can make sure your plants are getting the right amount. 

Plants basking in late summer sun
Plants basking in late summer sun

Quick facts

  • Plants make their own food by photosynthesis – harnessing the energy in sunlight and using it to create simple sugars
  • Plants that are growing rapidly, flowering or fruiting need lots of energy, and therefore plenty of sunshine 
  • When plants don’t get enough light, they can’t produce the food they need to function, so you may see weak, pale, spindly growth and fewer flowers and fruit

How plants make their own food

Even the most shade-tolerant plants need some light to thrive, and this is because plants use sunlight to make their own food, in a process called photosynthesis. 

During photosynthesis, plants harness the energy in sunlight and use it to fuse water (absorbed from the soil) and carbon dioxide (absorbed from the air) to form simple sugars, releasing oxygen as a by-product.

The process of photosynthesis

The sugars (such as glucose) produced by photosynthesis are moved around a plant inside phloem vessels and used to release energy for growth and repair – this process is known as cellular respiration.

In ideal conditions, leaves produce more sugars than are needed straight away. The surplus is converted to starch and stored for future use, either in granules in stems and roots or in specialised storage organs such as bulbs and tubers.

This is important to bear in mind when growing bulbs and pruning woody plants:

  • Bulbs contain lots of starch to fuel new growth after a dormant period. To allow this food store to be fully replenished, it’s important to keep their foliage exposed to light until it dies down naturally. Cutting off or tying-up daffodil leaves after flowering may look neater, but will usually result in poor or no flowers the following year.  
  • Woody plants store starch in older, ripened wood. Pruning spring-flowering shrubs immediately after flowering allows plenty of time for new growth to ripen, so you are rewarded with a good display the following spring.  
Daffodil foliage left to die down
Plump buds on a cherry tree

The importance of colour

The chloroplasts in leaves contain light-absorbing pigments and these capture different wavelengths of light. Blue light stimulates growth, while red light is important for flower production, and both are absorbed by the green pigment chlorophyll. 

Light quality and quantity

The amount and intensity of light reaching leaves affects the rate of photosynthesis and overall growth.

The strength of light (intensity) a plant receives changes with the seasons, as sunlight is much weaker in winter than it is in summer.

Low, weak, winter sun over borders at RHS Garden Wisley

Aspect also makes a difference, with a north- or east-facing position getting significantly fewer hours of direct sun than a south- or west-facing one. 

If you grow houseplants, it’s a good idea to move them with the seasons. In summer, for example, a south-facing windowsill provides too much strong, direct sunlight for most plants, while in winter a north-facing one usually provides too little.

Aspect makes a big difference to the amount of light a garden receives

Getting to know which areas of your garden receive sun, and for how much of the day, both in summer and winter, helps you know where best to position sun-loving or shade-loving plants.

How energy requirements vary

Plants need more energy at certain times in their lifecycle: 

Your next steps

Now you know more about how plants use light, put this into practice to help your plants thrive:

  • Look out for weak, pale or spindly growth – these can be signs that light levels are too low. Prune back any overhanging branches to let in more light or move light-starved plants to a sunnier spot
  • Position plants with white-variegated leaves in a bright spot, as they contain less chlorophyll. Also see our guide to variegated plants
  • Get to know the light in your garden – which areas receive sun and for how long – before adding new plants. See our guide to shade gardening and inspiration for sunny locations
  • Move houseplants to south- or west-facing windowsills in winter, when sunlight is weaker and the days are shorter, to ensure they get enough light. See our video guide to houseplants and light
  • Make sure fast-growing, flowering and fruiting plants have plenty of light, water and good ventilation so they can photosynthesise effectively

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The Royal Horticultural Society is the UK’s leading gardening charity. We aim to enrich everyone’s life through plants, and make the UK a greener and more beautiful place.