How do plants defend themselves?

With an arsenal that is chemical, physical and — genuinely — communicative, because a plant cannot run away and must therefore solve the problem where it stands.

Physical defences: thorns, spines and prickles; tough waxy cuticles; silica in grass leaves, which wears down the teeth of grazers and drove the evolution of high-crowned teeth in horses; stinging hairs delivering irritants; and sticky resins that trap small insects.

Chemical defences, which are the main event. Plants produce a vast range of compounds that deter, poison or impair feeding — and a striking number of things humans value are among them. Caffeine, nicotine, capsaicin, tannins, mustard oils, cyanogenic compounds in almonds and cassava, and most medicinal alkaloids are plant defence chemistry. We have domesticated the poisons of plants and call them flavour.

Induced defences. Many compounds are produced only after attack, because producing them constantly is expensive. Damage triggers a signalling cascade that ramps up production within hours — which is why previously damaged leaves are less palatable.

Communication, which is real and well documented. Damaged plants release volatile organic compounds into the air. Neighbouring plants — and other parts of the same plant — detect these and pre-emptively raise their own defences. Some volatiles specifically attract predators and parasitoids of the attacking insect, effectively calling for help: a plant attacked by caterpillars can emit compounds that attract parasitic wasps.

Underground signalling through fungal networks connecting root systems has been demonstrated, though the popular "wood wide web" framing has been criticised by researchers as running ahead of the evidence.

Indirect defences also include housing and feeding ant colonies that attack herbivores.

What follows for gardeners: stressed plants defend less effectively, so healthy plants resist pests better than sprayed sick ones; diverse planting makes host-finding harder for specialists; and many pest-resistant varieties work through defence chemistry — occasionally the same compounds that make a plant unpalatable to us.