When you've taken the trouble to lay a high-quality stone path or driveway, any kind of dirt or discolouration can quickly upset the aesthetics. You can easily wash or scrub away mud and dirt, but some problems are harder to deal with. If you have a problem with lichens on your paths and driveway, find out what you need to do.
Lichens are natural organisms that occur across Australia. Lichens actually contain two living parts – a fungus and an algae or cyanobacterium. The non-fungal part of the organism (the photobiont) contains chlorophyll, which enables food production (carbohydrates) through photosynthesis. The fungal part of the organism (the mycobiont) cannot produce these carbohydrates and feeds off the photobiont instead.
Australia is home to around 3,000 species of lichen. You'll find them in almost any kind of environment, including tropical rainforests and deserts. Lichens will happily grow on natural surfaces like wood, soil and rock or on artificial substrates like bitumen, concrete and even glass.
You'll generally find lichens in one of four basic forms. Look for lichens that are:
- Crustose - looks like a crust that grows tight against the surface
- Squamulose - grows in tight clusters that look like small pebbles
- Foliose – appears more like a leaf, with flat sheets of tissue
- Fruticose – stands up in free-branching tubes more like a conventional plant
Lichens can vary in colour. Some species are silver-grey, but you can also find bright yellow or green lichens. Lichens are not normally poisonous. In fact, in some parts of the world, people use lichens for bread and even fermented alcoholic drinks.
Although some lichen can grow anywhere, you're more likely to find them in damp, shady places. The organisms do not normally damage the surface they grow on, but extensive lichen coverage can spoil the appearance of the path or driveway. Perhaps more importantly, wet lichens become slippery, which can cause a hazard.
Some materials are at higher risk than others. Lichens prefer a porous, sedimentary stone to grow on, so surfaces like sandstone are more likely to develop an infestation. Wet-cast cement products are also more prone to lichens, due to their relatively low cement content.
Some types of lichen are difficult to remove from concrete or stone surfaces. Brushing or scrubbing with water doesn't always get rid of the problem, and this type of abrasive action can also damage the path or driveway.
In most cases, you'll need to use chemicals to deal with the problem. Make sure you test a small area of the path or driveway first, as some products can cause discolouration. Household thin bleach (sodium hypochlorite) can help you deal with a small colonization.
- Clean the surface with a diesel-driven pressure water cleaner before applying the bleach solution.
- Add the bleach to a bucket of clean water (not the other way round) to avoid splashing neat bleach on your clothes or skin. Use 1 part bleach to 10 parts water.
- Wash down the surface with the diluted bleach.
- Rinse with the pressure washer.
Always wear goggles while using the pressure washer, and make sure you wash every part of the path or driveway. If you only treat areas where you can see lichen, you may create clean spots on the surface. You may need three or more treatments to deal with stubborn lichens. You can use a nail brush or scouring pad to get rid of the most stubborn visitors.
You should also make sure you have enough drainage around the path or driveway to avoid standing water. Excess water will encourage new lichens to grow. Dig out shallow channels along the edges and fill with coarse gravel to absorb excess water.
Several other products may work more effectively against stubborn lichen colonisations. Some proprietary patio cleaners have effective chemical ingredients that will deal with these organisms. Look for products with benzalkonium chloride, acetic acid and quaternary ammonium compounds. You can also get natural products that contain seaweed extracts. These products are more suitable for paths close to planted areas.
While some lichens can add a natural, weathered look to ornamental surfaces, excessive growth can create a slip hazard on paths and driveways. Deal with lichen growth as early as possible, as older, established colonies are often more difficult to deal with.