What are the strange lumps and growths on my plant?

Galls are one of the odder things in a garden: the plant grows the structure itself, in response to chemicals from an insect, mite or bacterium, and provides the organism with food and shelter. They look like disease and are, in the great majority of cases, of no consequence at all.

Step by step

1. Recognise the common insect galls on oak.Marble galls, hard brown spheres on twigs; knopper galls, sticky ridged distortions on acorns; oak apples, spongy and pale; spangle galls, small flat discs on the undersides of leaves in autumn. Every one of them is caused by a tiny gall wasp, and none does the tree any harm.
2. Recognise the common galls on other plants.Robin's pincushion, a red mossy ball on wild roses, made by a gall wasp. Nail galls, red spikes standing up from lime leaves, made by a mite. Bright red pouch galls on currant leaves, made by aphids. Azalea gall, thick fleshy pale swellings on azalea leaves and flowers, which is a fungus. Witches' brooms, dense twiggy clusters on birch and conifers.
3. Leave nearly all of them alone.The plant is not being damaged in any meaningful way, the organism inside is part of the garden's ecology, and there is no treatment worth applying. Pick off azalea galls and bin them if they are unsightly, and pick off currant pouch galls if you prefer, but nothing needs doing.
4. Check the base of the stem for crown gall.A rough, irregular, woody or corky swelling at or just below soil level, or occasionally on a root or a stem, caused by a soil bacterium entering through a wound. On a mature woody plant it is usually tolerated with no visible effect. On a young plant it can restrict growth, and there is no cure — remove and destroy badly affected young plants and avoid replanting susceptible things in that spot.
5. Recognise a graft union, which is not a problem.Almost every rose, fruit tree and many ornamental trees have a visible swollen knuckle where the variety was grafted onto a rootstock. It is meant to be there. Growth appearing from below it is rootstock suckering, which should be removed at its point of origin.
6. Recognise a burr, which is also not a problem.A rounded woody bulge on a trunk, often covered in small shoots, caused by a mass of dormant buds. Burrs are prized in timber and are not a disease.
7. Recognise root nodules and leave them.Small round nodules on the roots of peas, beans, clover and lupins are nitrogen-fixing bacteria and are wholly beneficial. Cut the tops off spent pea and bean plants and leave the roots in the ground.
8. Know the one to identify carefully on brassicas.Distorted, swollen, club-shaped roots on cabbages, sprouts, wallflowers or stocks, with the plant wilting in hot weather, is clubroot. That is a serious soil disease, not a gall — it persists in the soil for many years and dictates where you can grow brassicas.
9. Check blackcurrants for big bud.Buds that swell to round balls over winter instead of staying pointed contain gall mites, which also spread reversion virus. Pick off and bin swollen buds in winter; a badly affected bush that has stopped cropping should be replaced with certified stock.

Tips

• Galls are a good sign of a garden with plenty of invertebrate life in it. Most gardeners who learn to recognise them stop worrying about them entirely.
• If you want to know what made a gall, cut one open — the larva or mite is usually visible, and the shape of the chamber is often as diagnostic as the outside.
• Witches' brooms on birch look like large birds' nests and cause no harm. On conifers they occasionally produce useful dwarf cultivars when propagated.
• Crown gall is more likely on plants propagated by grafting or budding, and on ground that has grown fruit or roses for a long time.

Common mistakes

• Spraying insecticide at galls — The organism is inside a structure the plant built around it. Nothing sprayed on the surface reaches it, and there is no benefit even if it did.
• Cutting a graft union off a rose or fruit tree because it looks like a growth — It is the join between variety and rootstock. Removing it removes the plant.
• Removing root nodules from peas and beans — They are nitrogen-fixing bacteria doing free work. Leave the roots in the soil when clearing the crop.
• Treating clubroot as a gall — It is a serious soil-borne disease that commits the ground for years. It needs identifying correctly and the affected material must never be composted.

Questions people ask