Lichens in a Garden and What They Tell You

These remarkable organisms offer useful clues about ecosystem health.

blue tit perched on a lichen-covered branch

Images from BarbAnna / Getty Images

Lichens are fascinating. Not really plants at all, though they frequently look like plants, these are crusty or moss-like composite organisms that grow on a huge range of surfaces and in many different environments. They are made up of fungi and algae or cyanobacteria that have formed a mutualistic relationship.

In a garden, you may see lichen growing on a garden wall, on roofs, on the surface of the soil, on dead wood, or on living trees and shrubs. Many people look at these lichens and consider them to be a problem. But these fascinating organisms can actually be very useful to have around. We can learn lessons from them that apply to our gardens, and they can tell us a lot about the environment where we live.

Symbiosis: A Functioning Ecosystem

Lichens are made up of fungi and different bacteria that work together, often forming symbiotic relationships. They are sometimes considered to be something called a "holobiont", forming a miniature, relatively self-contained ecosystem with well-balanced beneficial interaction, or symbiosis.

The concept of a holobiont comes from the concept of holism, which suggests that we should look holistically at systems in their entirety, focusing on the interconnections between various components rather than on individual parts. Looking at lichens can help teach us to think in this way, which is important in creating larger ecosystems within the broader context of our gardens.

Looking at the way that organisms in lichen work together can show us how to form holistic systems in our gardens with as many beneficial interactions between species as possible.

Other Lessons From Lichen

Learning more about lichens can also allow us to recognize that we ourselves are composite organisms, hosting and reliant upon a wide variety of microbial life. Through looking at lichens, we can also learn to use and value the natural process of change and succession in ecological systems.

Lichens are important pioneers in ecological succession. They are the first living things to colonize areas of bare rock or areas denuded of soil by a disaster. They thrive in conditions where higher plants typically cannot grow and pave the way for plants to grow and soil cover to develop.

Understanding ecological succession can help us repair and restore, and develop healthy, functioning ecosystems much more quickly than nature can manage on its own. Through looking at lichens and their role in the world's nitrogen cycle, we learn more about the nitrogen cycle in our own gardens, and the role that we as gardeners can play in it.

Natural Yields and Historic Skills

Lichens help us to learn about how we can make full use of resources in the natural world around us to meet our own needs.

Lichens have, historically, sometimes been used as edible yields. For example, Iceland moss (Cetraria islandica) was an important food source for people in Northern Europe, and edible horsehair lichen (Bryoria fremontii) was regularly eaten by native peoples in North America.

Other animals also eat lichens, including reindeer, squirrels, snails, and a number of Lepidoptera (butterflies) and other insects.

Most lichens are not nutritious, but many do have other uses. Delving into these may help us to rediscover some historic, traditional skills, such as natural dye-making, for example. A number of lichens have been used to make natural dyes.

You might have heard of a litmus test, used as a pH indicator. Litmus is a dye that comes from the lichen Roccella tinctoria. Here in Scotland, lichens are used to make traditional dyes for Harris tweed.

Lichens are also being investigated for various potential medicinal benefits. 

Iceland lichen closeup
Iceland moss (Cetraria islandica).

Dieter Hopf / Getty Images

Ecosystem Indicators

Which lichens are present in our gardens can tell us a lot about the conditions that are present. In particular, they are known to be useful indicators of air quality.

Lichens get their nutrients from the air and cannot filter what they absorb, so they are commonly intolerant of air pollution. This means that we can see how polluted air is by looking at the lichens present in our gardens. As a general rule, the smaller and less varied the lichen in an area, the more pollution there is in the air.

Lichens can, in particular, tell us about the presence of nitrogen dioxide and sulphur dioxide. When it comes to nitrogen dioxide, some lichens will thrive in its presence, while others will die. Crustier lichens tend to be hardier than hairy or leafy-looking types. Where there is a lot of nitrogen, crustier lichens will thrive. So, seeing these can be a sign that there is a lot of nitrogen dioxide around from traffic or farms.

Sulphur dioxide pollution comes from coal burning and industry. Where there is a lot of this pollutant in the air, lichens will often die. If you have hairy lichens growing where you live, then this is unlikely to be a problem.

But is lichen an indicator of problems when it grows on trees? Does it pose a threat to the tree or show that something is wrong? No, lichen does not harm a tree. There is absolutely no need to get rid of it, and trying to do so may do more harm than good.

Dead or dying trees may be the perfect habitat for lichens, due to the more open canopy letting light through and the moist conditions. But lichen can also often grow on perfectly healthy trees, so the lichen itself does not indicate a potential issue.

Don't be tempted to try to eradicate lichens from trees or other locations in your garden. Just learn from the lessons it has to teach us, and be aware of what it has to tell us about the air quality where we live.