Oaks in flower

So who’d noticed that oaks have catkins?  Maybe the fact that they emerge more or less at the same time as the leaves distracts us.  Like many trees, oaks are wind pollinated, so don’t need showy flowers to attract insects – dangling catkins are perfect for dispersing clouds of lightweight pollen over long distances. In fact tree pollen, in general, is one of the biggest allergens for hay fever sufferers for this reason.

Oak catkins

Oak catkins with flowers not quite open on May 9th

The male flowers in these catkins are about as simple as they can be – just a cluster of stamens and anthers which, when they split open (dehisce), reveal a brown interior.

Oak catkins with brown, dehisced anthers, May 21st

Oak’s female flowers are even less conspicuous, located at the tips of branches. My tree has a girth of just 75 cm, so is only about 30 years old, and oaks don’t produce acorns until they are at least 40 years old so I wondered whether that meant the tree wouldn’t produce female flowers yet.  However, it seems that acorn production must fail further down the line as, at the tip of some branches, I can see minute pink flowers, just a couple of mm across.

Female oak flowers

Much more obvious than the flowers, though, are the galls which stud the leaves and branches, looking like under-ripe redcurrants.  Thanks to Sue Antrobus, I now know that these are indeed currant galls, produced by the gall wasp Neuroterus quercusbaccarum.  They are produced on both the stamens of the male flowers and on leaves, depending on where the wasp lays her eggs.

Over the last couple of weeks the oak leaves have lost their initial reddish hue and are now a beautiful fresh green.  The initial redness was due to anthocyanin pigments, which are thought to make the tender early leaves just that little bit less appealing to herbivores – they don’t seem to deter gall wasps though!


Oak leaf, May 21st


  1. Fascinating, Heather! I must at more attention to my local oaks (though they are very old so I shall need binoculars😁) Just a quick question – are all catkins wind pollinated? I’ve often seen bees on willow catkins but I guess they’re just opportunistically harvesting the pollen….

    • I’ve also seen lots of bees on willow but I’m guessing they are mainly just feeding – interesting question though – I wonder how much pollen they do transfer ‘accidentally’? I suppose it must depend on whether they visit the female flowers too…

      • Hi Heather. Co believe that. On Saturday I felt worse walking through the woods next to Junior school. Back home I had chronic colds throughout the year. I know now it was due to ever flowering fruit trees. Mum thought I was allergic to dust🤔. I was on daily antihistamine which made me constantly sleepy without concentration. Since being in England, it’s this time of the year. I feel better today but my head is light and painful eyes. Staying off today. Xx

  2. […] Most of the male catkins have dried up but they remain on the odd branch. I was going to write a blog about the flowering of oak, but my friend Heather Kelly has just published an excellent one – here is a link. […]

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