How to identify plant galls

Galls are abnormal growths caused when another organism interferes with a plant’s cells. We have illustrated 12 of the most curious and colourful galls to look for.

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All illustrations by Felicity Rose Cole

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1 Bean gall (above

Caused by the sawfly Pontania proxima. Forms on upper surfaces of willow leaves, especially crack willow and weeping willow.

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2 Spruce pineapple gall

Caused by the aphid Adelges abietis. Found on the twigs of spruce trees, often Norway spruce. Young aphids shelter inside the gall cavities.

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3 Fern gall 

Made by the fly Chirosia betuleti. Occurs on ferns, especially male fern, causing rolled-over, deformed frond tips. Fly larvae dig into the fern stem.

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4 Reed cigar gall

Caused by the fly Lipara lucens. A common, cigar-shaped gall on reed stems; conspicuous in late autumn. Each gall contains a single fly larva.

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5 Ergot

Caused by the fungus Claviceps purpurea. Occurs on seeds of grass, including rye and false oat. Toxic – was a scourge of medieval Europe.

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6 Tassel gall

Caused by the psyllid (jumping plant louse) Livia juncorum. Found on flowerheads of rushes. Common in damp places throughout UK.

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7 Ramshorn gall

Made by the gall wasp Andricus aries. Occurs on oak trees, turning the buds into long prongs. Mainly in south; first seen in UK in 1997, but spreading.

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8 Pea gall

Made by the gall wasp Cynips divisa. Occurs on undersides of oak leaves, falling in autumn. Wasps emerge in winter and lay their eggs in oak buds.

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9 Silk button spangle gall

Made by the gall wasp Neuroterus numismalis. Found on oak leaves, often in big numbers. Falls to ground in autumn; wasps emerge in spring.

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10 Witch’s broom

Has many causes, including insects, mites, fungi and viruses; usual trigger is the fungus Taphrina betulina. Occurs on birch and cherry trees.

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11 Nail gall

Caused by the mite Eriophyes tiliae. Very common on lime tree foliage, appearing in summer and persisting until leaf fall in autumn.

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12 Robin’s pincushion

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Made by the gall wasp Diplolepis rosae. Found on wild roses. The red, feathery mass contains many chambers, each with a wasp grub.