Discover the hidden world of trees

Two school students in a park, examining the leaves on a treee

Investigate the health and ecosystems of your local trees.

In this activity, your students will make observations of the species that call trees home, both friend and foe. It’s easy to do, supports scientific skill building and can be completed in around 15 minutes. 

Running the activity

  • Select a tree or trees on or near your school grounds.
  • Use the four checklist questions and detailed information found below to guide the class in spotting any common pests or diseases, as well as healthy features of tree ecosystems. 
  • You can take photos of your findings and share them with us using #ExploreUrbanNature on Twitter.

If you find any symptoms and want to go one step further, help the scientists who are trying to understand these problems by submitting an official record of your sighting to TreeAlert.

Remember: look but don’t touch! We must always be careful to avoid spreading tree diseases from one tree to another, and to avoid coming into contact with anything that might be harmful to humans. For a handy guide on keeping things clean if you’ve been in the outdoors and around trees, see DEFRA's Keep it clean campaign.


Tree checklist questions

This section contains extra information to support identifying each of the features in the quiz above. You can also download this guidance as a document (PDF 1.2MB).

Why trees?

Trees are a vital component of ecosystems across the globe. In urban areas they can provide important habitat for other species, help to clean the air we breathe, reduce noise pollution, take up water to prevent flooding and play a role in mitigating the negative impacts of climate change. Trees also directly improve human health, as being under a tree canopy lowers stress and boosts self-esteem, attention span and the immune system. 

When you take your students outside try to notice any trees and other plants, you might be surprised by how many there are when you really start to look.

Many species of plant, animal, bacteria and fungi live on trees and do no harm. Unfortunately, trees can also be damaged by many different pests and diseases, making our urban ecosystems more vulnerable. Scientists across the UK record where this is happening, so that we can better protect our urban trees and ecosystems into the future

1) Healthy habitat. Many species call trees home and don’t do serious harm. Most of these can live on both healthy and unhealthy trees and are important components of hugely diverse tree ecosystems in both. See if you can discover any of these friendly characters in the hidden world of your tree!

  • Lichen. Seen as brown or yellow to blue-green coverings on trunks and branches. In cities where pollution is high, lichen is most commonly crustose – flat against the trunk.
  • Slime mould. These growths are usually found on or around the base of trees and are most often a bright yellow or orange colour, they look bad but are actually harmless to the tree.
  • Insects. Hundreds of insect species could be living on the tree at any time, from beetles and true bugs to butterflies and crickets. They might be hard to see at first but wait a while and look carefully at all parts of the tree for any signs of movement! See the NHM species guide to identify any you find in more detail.
  • Snails and slugs. You might not see the snail itself but look out for shiny trails on the bark too. Some gardeners might think of these as pests but they’re mostly harmless to trees.
  • Spiders. Likely to be small and well camouflaged, so look for a web first.
  • Fungi. Many fungal species help trees become more inviting homes for other creatures by rotting down tree parts that die from natural causes (eg wind). Other fungi live in symbiosis with tree roots, helping them stay healthy. Most of each fungus is hidden inside the tree or soil, but sometimes they make a bracket or mushroom on the trunk, branches or coming up from buried roots of the tree.

2) Leaf loss. Deciduous tree leaves change colour and drop normally every year, but other types of colour change in the leaves can be a sign of health problems for the tree.

  • Healthy leaf colour change – Normal leaf loss in autumn begins with a change to red or yellow over the whole tree, and at the same time as nearby trees as the weather cools, then leaves transition to brown and eventually drop off.
  • Unhealthy brown leaves – A good check is comparing to previous years or to trees of the same species in the same area. If a tree is losing leaves earlier something is wrong. Single brown leaves or branches that aren’t broken but the leaves have died, or leaves looking patchy green to brown, are more likely to be signs of an unhealthy tree. This could indicate environmental stress, bacterial infection or many other issues.

3) Suspicious symptoms on leaves. Lots of tree diseases and pests result in damage to the leaves of a tree, some of them are more damaging than others. See if you can spot any of these different tell-tale signs.

  • Leaf miners. These can be found on many tree species, are usually not a problem and can be considered part of a healthy tree ecosystem. One invasive species is specific to horse chestnut trees and can make them look very unsightly. Leaf miners are usually larvae of moths or flies. They leave tell-tale wriggly trails across healthy leaves as they chomp a tunnel between the upper and lower surface of the leaf. Hold the leaf up to the light and you might still be able to see the larva inside!
  • Galls. These are hard growths a tree makes when tricked by pest or fungal invasions. Most of the time this is considered harmless, for example they are often found on oak leaves and look like knobbly green acorns, green or pink marbles, or tiny buttons underneath leaves. However, some invasive species such as the oriental gall wasp, which causes galls on sweet chestnut trees, are carefully monitored by scientists in the U.K. as they can make the trees more at risk of catching more damaging diseases. These galls cause the leaf to bend as they grow, starting off green and hardening and turning brown over time. Except for in a few key places these are likely to be the most difficult symptom to spot! 
  • Powdery mildew. This fungal infection shows up as a white dusty coating on leaves that will curl up and die back in response.
  • Leaf rust. This looks similar to rust on the top or bottom surfaces of leaves but it is actually the result of fungal infection. It is common on poplar, willow and birch and silver birch trees in the autumn but can be found on other plant species too, for example roses and hollyhocks. Each rust fungus is species specific (only infects one type of tree), but they all result in characteristic orange or red rust-like colouring or coating. Some, like pear rust, also cause galls on the underside of leaves.

4) Suspicious symptoms on trunk and branches. There are also many problems that can be seen by inspecting the bark of the trunk or the branches of a tree.

  • Bleeding canker. A sign of several different underlying fungal or bacterial infections for different species of tree. Seen as dark sticky liquid in a patch or smeared down the bark. Common in horse chestnut trees and fruit trees.
  • Honey fungus. A general name for a collection of many similar species of fungus, they are honey coloured, have pale gills and grow on stalks in clumps low down on tree trunks or upwards from buried roots. They cause the roots and lower trunk to rot and will eventually kill the tree.
  • Brown rot. A common problem for fruit trees this shows as brown rotting patches on fruit that is still on the tree, which eventually produce brown fluffy fungal rings. Some of the infected fruit hang on the tree all through winter, protecting the fungus which comes out in the spring to infect again.

Helpful hints

This section contains a few hints to support the activity to run smoothly with your class.

Most checklist items are broad categories, for example: insects. Completing the activity with identification at this level should only take around 15 minutes. If you want to go into more depth with your students there are detailed ID guides linked below, and in combination with other tree related activities provided this can easily be built out into a full lesson.

Good options for trees to use are horse chestnut, sweet chestnut, oak, silver birch, sycamore and fruit trees. Although any tree where the branches are low enough for students to see some of the leaves up close will work.

You don’t need to know what species the tree is, but it can help because some pests are tree specific (eg horse chestnut leaf miners). There are automatic tree ID apps such as LeafSnap and iNaturalist, or online guides from the Museum on common UK urban trees and how to identify UK trees.

Dividing the class up to search carefully for different signs and symptoms could be a good strategy, or have groups investigate different trees as the competition allows more than one entry per teacher.

Getting up close to really search will be important, especially for things like spiders and insects, but students should be able to see any of the checklist items with the naked eye and should be encouraged not to touch, especially the disease symptoms.

Extra information

Identification Aids

You can find information on some of the species you might come across, or tree diseases, at the following sites:

The Royal Horticultural Society has information pages on:

NHM lesson extension activities



Citizen science and reporting projects

If you want to contribute to science with your findings, follow up with one of these projects:

Explore: Urban Nature is
developed in partnership with

With thanks to the Royal Horticultural Society

Explore: Urban Nature

We’re challenging students across the UK to better understand the nature in our towns and cities. For ages 9-14.

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