Algae 

A green film or powdery deposit is typical of algae on greenhouse glazing and floors. They are most likely to be found in damp, shady areas.

Liverworts 

Liverworts found growing in greenhouses tend to have green, flattened, plate-like growths. They thrive in damp, shady conditions with poor air circulation, so often grow in corners and under benches.

Liverworts form low, spreading mats in damp, shady areas 

Mosses 

Mosses that grow along the bars supporting greenhouse glazing tend to form cushion-like growths. A range of moss species may also colonise stone flooring (especially the gaps between paving) and wooden structures, such as doors and benches.

Mosses will colonise areas where moisture gathers, such as this lower section of glass

Although algae, liverworts and mosses are an important part of your garden's biodiversity, and can be kept and encouraged in other areas, it is generally advisable to remove them from greenhouses because they can:

• Reduce light levels – growths on glass or polycarbonate glazing, both inside and out, can reduce the amount of light reaching your plants. This is not such an issue in summer but can be detrimental to plants during winter months when sunlight is in short supply.  
• Make floors slippery – growths of algae on greenhouse paths increase the chance of slips and falls. Liverworts and mosses can also become slippery when wet. As a safety measure, it is advisable to prevent or manage their growth on areas that are walked on.  
• Spread to compost – the enclosed, humid environment within a greenhouse makes it possible for algae, liverworts and mosses to spread to compost in seed trays and plant containers, potentially hindering germination and plant growth. See our page on Algae, liverworts and mosses on borders and containers for information on how to stop this becoming a problem.

Remove existing growths  

• Dislodge growths from between glazing panels with a thin, flat blade or a flexible plastic plant label. 
• Brush floors with a stiff broom to remove growths.   
• Clean your greenhouse thoroughly at least once each year, removing all staging and equipment and getting right into the corners of the frame. See our page on greenhouse cleaning for more information.  

Discourage further growths  

• Increase ventilation, where possible, as better air flow reduces humidity and helps surfaces to dry out. 
• Water carefully to avoid puddles of standing water. Where plants are on slatted benches or directly on the floor, consider placing them on trays or saucers to collect and contain excess water that drains through pots. Empty these regularly so pots don’t sit in water for too long.  
• Install drainage if you are planning an additional or replacement greenhouse. Slope the greenhouse floor slightly towards a drain to prevent standing water.   

© Shutterstock

Algae can be brushed or wiped off glass or polycarbonate roofs

Regular cleaning prevents build-ups of algae, liverworts and mosses 

Using a cleaning product 

Growths of algae, liverworts and mosses in greenhouses can be managed with regular cleaning and by making conditions unfavourable for their growth. Warm water is generally sufficient to keep surfaces clean, but cleaning products for greenhouses are available. Garden centres and large retailers have trained staff who can advise on suitable products for your needs.