A galling time of year

Knobbly knoppers, silky buttons and fluffy pincushions; it must be gall time...

Have you ever seen a growth on a plant stem or leaf and wondered what might have caused it? I was very little when I discovered that some of these growths had 'maggots' living in them and it took me ages before I'd sit under a tree again; I was terrified of a leaf landing on my head and a maggot escaping into my hair. Thankfully, this worry didn't last too long and now i find myself quite fascinated by the miniature worlds that happen in a gall.

Galls are abnormal growth forms that are caused by an organism other than the host plant. This organism triggers the plant to react to its presence, enlarging or proliferating its cells or tissues to provide protection and nutrition for the gall causer. Some galls can provide homes for several species other than the original gall-causer - some of these parasitise the gall-causer and some eat the gall tissue, others move in once the gall is empty and use it for food and shelter then.

Robin's pincushion - 'fluffy' red gall on rose stem - with several 7-spot ladybirds hiding amongst it and the surrounding green leaves by Wendy Carter — Robin's pincushion with 7-spot ladybirds by Wendy Carter

Galls usually affect the leaves but they're also found on stems and buds as well as other parts. Look at plants and trees in hedges, woodlands, your local greenspaces and even your own gardens - you might be amazed at what you can spot. Galls and their makers are incredibly diverse and often specialise in a particular species - knopper galls are found on oaks, robin's pincushions on roses, for example. Daisies, thistles and their relatives (Asteraceae) have over 100 species of gall associated with them whereas roses, brambles and relatives (Rosaceae) have about 80 species. Oak trees top the charts with about 50 species of gall occuring on them and because oaks support such a wealth of wildlife, over half of these form a different type of gall at different stages of lifecycle on different parts of the tree!

Red growth coming out of the top of a green alder cone at the end of a twig by Rosemary Winnall — Alder tongue gall by Rosemary Winnall

Whoever told me that galls contained lots of tiny maggots, they were only partly right. There are a whole range of gall-causers from mites and moths, wasps and sawflies to fungi and bacteria. The protagonist can be difficult to identify but many galls are themselves distinctive and give us a clue to what's hiding beneath. Some, such as the alder tongue gall, are obvious, weird and wonderful whereas others, such as the silk button gall, are much more surreptitious and you might have to turn over a few leaves before spotting them.

Silk button galls - small golden discs with dimples in the middle and a silken texture - on the underneath of an oak leaf

Silk button galls by Rosemary Winnall

Alder tongue gall by Rosemary Winnall

Artichoke gall, growth on oak bud that resembles an artichoke, with flat spangle galls on the underneath of the surrounding oak leaves

Artichoke gall with spangle galls on leaves by Wendy Carter

Abnormal growth of a knopper gall on an oak tree - knobbly green uneven growth of an acorn - with normal growth of acorns on the same branch (photo by Wendy Carter)

Knopper galls and acorns by Wendy Carter

Red pustule galls - small dark red growths growing out of the top of a field maple leaf

Red pustule galls on field maple leaf by Rosemary Winnall

Flat circular pancake-like cream-coloured discs with tiny tufts of reddish hairs, clustered together on an oak leaf

Spangle galls by Brian Eversham

Robin's pincushion with 7-spot ladybirds by Wendy Carter

If you've keen to learn more about identifying galls, you could take a look at this great photo ID guide by the team at Naturespot, which is a great place to start playing 'snap' with something that you've found. A photo with a camera that has a green spot means that it's relatively easy to identify but if you see a camera with an amber or red spot, it means that further investigation might be necessary.