How Plants Defend Themselves

A plant cannot run away

When a deer wants to eat a rabbit's lunch, the rabbit can hop away. But what can a plant do when an animal comes to munch it? A plant is rooted to the spot. It cannot run, hide or fight back with claws.

And yet plants are not helpless. Over millions of years, they have evolved a remarkable set of defences. Some are sharp and physical. Some are hidden and chemical. A few are downright sneaky. Let's look at how plants protect themselves.

Physical defences: armour and weapons

The most obvious plant defences are the ones you can see and feel.

Thorns, spines and prickles

A rose has prickles. A hawthorn has thorns. A cactus is covered in spines. These sharp points have one job: to make an animal think twice before taking a bite. A mouthful of spines hurts, so grazing animals learn to leave that plant alone.

These three are not quite the same thing. A thorn is a modified branch, a spine is a modified leaf, and a prickle is a sharp outgrowth of the plant's skin — but to a hungry animal, they all just say "ouch, keep away".

Stinging hairs

The stinging nettle has a clever trick. Its leaves are covered in tiny, hollow, glass-like hairs. When you brush against them, the tips snap off like tiny needles and inject irritating chemicals into your skin, causing a stinging rash. One touch is usually enough to teach an animal a lesson.

Tough armour

Many plants simply make themselves hard to eat. Thick bark protects a tree's living layers. Tough, leathery leaves are difficult to chew. Some grasses pack tiny grains of gritty silica into their leaves that wear down an animal's teeth.

Chemical defences: hidden weapons

Plants are also brilliant chemists. They make thousands of substances to defend themselves, and we cannot see most of them at all.

Bad tastes and poisons

Have you ever bitten something and spat it out because it was horribly bitter? That bitterness is often a defence. Plants fill their leaves with bitter or unpleasant chemicals so that animals stop eating after one nibble.

Some plants go further and make true poisons. Foxglove, deadly nightshade and the leaves of the rhubarb plant all contain chemicals that can make an animal very ill or even kill it. An animal that survives one bad experience will avoid that plant forever — and so will its young, who learn from their parents.

Irritating sap

Break the stem of some plants and a milky or sticky sap oozes out. In plants like spurges, this sap can burn skin and mouths, gumming up the jaws of any insect that tries to feed.

The sneaky tricks

Plant defences get even cleverer.

• Warning signals. When some plants are attacked by caterpillars, they release chemicals into the air. Neighbouring plants detect these signals and start boosting their own defences before the caterpillars even arrive — a kind of green early-warning system.
• Calling for bodyguards. Some plants under attack release scents that attract predators of the attacking insect, such as wasps that hunt caterpillars. The plant effectively calls in reinforcements.
• Bribery. Certain acacia trees grow hollow thorns and sugary nectar to house and feed stinging ants. In return, the ants attack any animal that touches the tree.

These defences are a great example of evolution and natural selection. Plants with better defences survived to make more seeds, slowly building up the armoury we see today. It is part of the endless contest between plants and the animals that eat them, which shapes whole food chains and ecosystems.

Why this matters to us

Plant defences are not just interesting — they are useful. Many of our medicines come from defensive plant chemicals, including aspirin (originally from willow bark) and powerful heart drugs from foxglove. The very chemicals plants use to fight off insects can, in tiny doses, help heal people. To meet some of the plants we rely on, read Plants We Use Every Day.

Observe it safely: a defence hunt

You can study plant defences without getting hurt.

1. Take a walk in a garden, park or hedgerow with an adult. Look, don't touch — and never taste anything.
2. Make a tally of the defences you spot: thorns, prickles, spines, stinging hairs, tough leathery leaves, thick bark.
3. For each plant, ask: which animals is this defence built to stop? Big thorns suit big grazers like deer; tiny stinging hairs suit insects and curious mammals.
4. Notice which plants have no obvious defences. How might they survive instead — by growing fast, hiding, or making lots of seeds?

You will never look at a rose bush the same way again.