It is the height of summer. At the time of writing, the sky is crystal clear and we can expect approximately 15 hours of daylight where I am in Southern England. During the summer it is tempting to give your houseplants a treat by standing them outside to get a dose of natural sunlight.  However, there are many dangers and you risk serious and permanent damage to your plants - even those that like bright light.

Even indoors, some plants really cannot cope with direct sunlight. Fragile leaves may be scorched, or at least discoloured.  This guide will help you to know which plants can cope with direct sunlight and which prefer diffused or shady situations.

Did you know…?

  • Double-glazed window glass removes between 65% and 95% of light?
  • Light levels at one metre from a window might only be 10% of the value at the window (and less than 1% of the light outside)?
  • The human eye is rubbish at judging light levels?
  • Houseplants are grown under relatively shady conditions to ensure that they are acclimatized to the low light conditions found indoors?

The facts of light

The first research project I carried out on interior plants over 25 years ago was on the real light requirements of houseplants in Europe.  So much of the data available at the time came from recommendations made by nursery growers in Florida, where indoor plants are grown under much higher light levels than those in Europe.  The published recommendations suggested that nothing would survive under the relatively gloomy conditions found indoors in high latitudes, even though experience showed that many plants could thrive, as long as they were properly looked after.

The reason for their survival under such low light conditions is the way that they were grown and acclimatized (acclimated) on nurseries before they were ready to be sold as houseplants.

It is worth remembering at this point that of the 256,000 species of flowering plants currently alive, and a few tens of thousands of ferns, only about 350 species are adaptable enough to reliably survive the artificial environment of the home, and with the exception of common ivy (Hedera helix), no UK native species will survive for long indoors at all.  You can read more about plants that do well in low light here.

The damage that you might cause

On a bright summer’s day in the UK, light levels outside can exceed 100,000 lux (lux is the unit of measurement of brightness, or light intensity where the light hits a surface, not at its source).  In a conservatory, at the same time, you might experience 5,000 - 10,000 lux. On the windowsill at a south-facing double-glazed window, at midday, 3,000 lux is quite likely.  However, it is unlikely that you will be able to perceive that the light outside is over 30 times brighter than the light inside.  Your eyes adapt almost instantly to changing light conditions by dilating or contracting your pupils.

When growing indoors, even on a south-facing windowsill, plants adapt very slowly to changing light conditions: the way they grow, the orientation of their leaves, the thickness of their leaf cuticles and waxy coatings, and the amount of chlorophyll they produce to harvest the available light all takes time.  Indoor plants are not exposed to UV radiation either, or the intense heat of direct sunlight, so when you expose foliage to direct sunlight, you risk permanent damage to the leaves.  They will bleach, scorch and wilt, and cells in the leaves will die.

Will any houseplants survive outside?

Some houseplants are able to cope with direct sunlight outside - some cacti and succulents have protective coatings of wax or hair that act as sunscreen.  However, even these plants are unused to such a significant change in the magnitude of light that, unless gradually acclimatized (over several months), they too might be damaged.  As a rule of thumb, if your plant was grown as an indoor plant, then keep it indoors.  If the same species (some hardy succulents, citrus, or other plants from a Mediterranean climate) were grown as outdoor plants on a nursery, then they will cope with outdoor conditions.

Can I put plants outside in a shady spot?

You might be able to put some of your conservatory plants outside if you can put them in a shady spot, but unless you have a particular reason to do so (maybe to add interest to an outdoor seating or dining area), I would not recommend it. You might have more success buying outdoor potted plants that have a similar appearance to some of your conservatory plants in order to carry the look from indoors to outside.  For example, if you have yuccas and dracaenas in the conservatory, consider cordylines or hardy yuccas outside. 

Plants that benefit from direct light

There are many indoor plants that do benefit from direct light through a sunny window or skylight or conservatory (read our guide on conservatory plants for more inspiration). These include most succulent species such as Echeveria species, Aloes and Crassulas as well as plants such as the Yucca (Yucca elephantipes) and palms like the footstool palm (Livistonia rotundifolia) or Areca palm (Dypsis lutescens).  

Ecehveria agavoides

Echeveria agavoides

Echeveria runyonii

Echeveria runyonii

Aristaloe aristata

Aristaloe aristata

Crassula ovata

Crassula ovata

Yucca elephantipes

Yucca elephantipes

Dypsis lutescens

Dypsis lutescens

Livistonia rotundifolia

Livistonia rotundifolia

Other plants that do well under direct bright light include colourful species like Croton ‘Mrs Iceton’ (Codiaeum variegatum ‘Mrs Iceton’) and the Yellow Coast dragon tree (Dracaena fragrans ‘Yellow Coast’).  Weeping figs (Ficus benjamina) also do well under direct light, especially variegated varieties such as ‘Gold King’.

Dracaena fragrans Yellow Coast

Dracaena fragrans 'Yellow Coast'

Codiaeum variegatum Mrs Iceton

Codiaeum variegatum 'Mrs Iceton'

Ficus benjamina Gold King

Ficus benjamina 'Gold King'

 

Plants that benefits from indirect light

Many of our plants will appreciate being close to a North facing window where the risk of scorching is minimized, but be careful if the windowsill is above a radiator, as the hot air can dry out the air and cause fern fronds to shed and other leaves to get a little crispy.

Many epiphytes will do well in these conditions.  These are plants that live on other plants high in the forest canopy which are able to cope with dappled light.  Good examples include the Mistletoe Cactus (Rhipsalis baccifera), Devil’s Ivy (Epipremnum aureum), Arrowhead vine (Syngonium podophyllum and species of Monstera (e.g. Monstera deliciosa and Monstera adansonii).

Rhipsalis baccifera

Rhipsalis baccifera

Epipremnum aureum

Syngonium podophyllum

Syngonium podophyllum

Monstera deliciosa

Monstera deliciosa

Plants with more lush foliage may need to be watered more frequently as the brighter light will encourage higher levels of photosynthesis and transpiration.  See our guide on watering for more advice on when, and how to water houseplants.

Plants that really need to be shaded

There are some plants that have their wild origins deep on the jungle floor.  These plants rarely, if ever, are exposed to direct light in the natural habitats, and their leaves are adapted to deal with shady conditions and excess moisture in the air and soil.  Their leaves are thin and delicate and can be very easily damaged by intense light and heat.  Examples include species in the maranta family (such as various Calathea species and Ctenanthe burle-marxii), as well as the Elephant’s Ear (Alocasia ‘Poly’).

Calathea ornata Peacock

Calathea ornata 'Peacock'

Ctenanthe burle-marxii

Ctenanthe burle-marxii

Alocasia 'Poly'

Alocasia 'Poly'

Other plants that really can’t cope with intense direct light include ferns, such as the Boston fern (Nephrolepis exaltata), Antenna fern (Doryopteris caudata) and Fishtail fern (Microsorum punctatum).

Nephrolepsis exaltata

Nephrolepsis exaltata

Doryopteris caudata

Doryopteris caudata

Microsorum punctatum

Microsorum punctatum

 

By Kenneth Freeman

 

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