Is coated glass bad for indoor plants?

Phil Brown, European regulatory marketing manager at Pilkington UK, outlines what type of glass should be supplied in settings where indoor plant life is concerned.

“Will windows be bad for my indoor plants?” isn’t a question frequently put to installers or insulating glass unit (IGU) manufacturers. But it’s certainly an important concern when specifying glazing for orangeries and atria, or to home-owning plant enthusiasts.

We’ve recently fielded a few enquiries about the impact that the increased use of coated glass such as low-emissivity, solar control and strongly tinted glass has on plant growth. Coated glass isn’t necessarily bad for indoor plants. However, it’s important to remember that a healthy plant that flourishes behind glass is the result of numerous variables, many of which can be influenced by a coating. 

Variables include: the type of plant, the necessary wavelengths of light for plant growth, the percentage of daylight transmitted through the glazing, the internal room temperature, and humidity where the plant is located. 

Plants grow best behind glazing that has a neutral daylight transmittance, without a strong colour. 

A green plant doesn’t use all the light that is available for it to grow. Although natural sunlight appears white, it’s composed of separate wavelengths, each with its own colour: red, orange, yellow, green, blue, indigo and violet. Plants will use some wavelengths more than they use others.

Most of the important photochemical processes in a plant use the blue and red areas of the visible spectrum. Red light stimulates stem and leaf growth, while blue light regulates plant enzyme and respiratory processes and encourages low, stocky growth and dark green leaves. Providing plants with the right balance of red and blue light is key to successful indoor plant growth, therefore glass without a dominant colour should be specified if plant life is a concern.

The total amount of light received by a plant is obviously important for photosynthesis to take place. Some plant types grow best in shade with no direct solar radiation, such as Dracaena, while others prefer full sunlight like Aloe Vera. The hours of daylight, the window size and degree of shading, as well as the daylight transmission of the glass, will also determine the total amount of light available for plant growth.

In the Pilkington product family, Pilkington Optiwhite allows the most natural light through, with a 92% of daylight transmitted through 4mm single glazing. The glass maximises passive solar gain, reducing the need for heating during cold sunny days, which is important for those plants that need warmth too.

For orangeries and atria, solar control glass like Pilkington Suncool is becoming increasingly popular as it helps to create cooler, more comfortable spaces with less risk of overheating during warm weather. Additionally, a high level of thermal insulation can be provided during the winter. 

If orangeries are being used to grow plants, fruit or vegetables that require high light transmittance, but occupant comfort needs to be factored in, so a balance between light and heat may need to be achieved. A recent mid-performance addition to the family, Pilkington Suncool One 60/40 for example, only allows for 40% of solar radiant heat transmittance, but still allows 60% of light transmittance in an IGU, while maintaining a very neutral appearance. 

Specifying glass with a low UV transmittance, such as Pilkington Optilam, is desirable too, as excessive levels of UV radiation can be harmful to seedlings or younger indoor plants. However, some data suggests that the longer UV wavelengths can stimulate photosynthesis if visible light is also available.

Low-emissivity coatings have been found to have little effect on plant health. A study on its effect on plant growth concluded that low-emissivity glass doesn’t seriously influence yields, plant health or growth rates if the plants are simply kept near room temperature.

Our tool, Pilkington Spectrum, can take out some of the guess work if a specific amount of light transmittance, or heat insulation is required in an orangery or atrium. Spectrum users can use search parameters including U-value, g-value, light transmittance, and both external and internal light reflectance, to help find products and model IGUs that most closely match their customers’ requirements.

While it’s true that coated glass can influence the growth of indoor plants, there are tools available for fabricators to specify glazing that can provide the optimum conditions for healthy growth. Ultimately, glass with a neutral colour and high light transmittance should do the trick.