THERE is something special about winter walks, especially on cold crisp days when the ground is frozen, and the sky is blue. The sun never climbs high, casts long shadows over the landscape, picking out the contours and hinting at the underlying geology.

It is not just the landscape you notice more in winter, but also the structure of trees. Denuded of their foliage, secrets partially hidden in summer become apparent. A good example is a Downy Birch tree (Betula pubescens) across the road from our house on the edge of Burley Moor.

Viewed from a distance you might think you were looking at a collection of bird nests or possibly Mistletoe (although this would be very unusual; Mistletoe rarely chooses Birch as its host). As you approach you realise you are wrong on both accounts. What you are looking at are dense clumps of entangled twigs, giving this weird growth its common name - Witches’ Broom.

Keep your eyes open and you will find it is a widespread and common occurrence on Birch (both Silver and Downy), and to lesser extent on Hornbeam and Cherry, caused by a fungal infection.

The growth of lateral buds (the buds that normally make twigs and side shoots) lose control and cause multiple stems to form in a tangled and disorganised manner.

By themselves, witches’ brooms don’t have an impact on the long-term health of a tree and can be pruned if required on ornamental trees.

Brooms are actually a type of plant gall. Plant galls are abnormal growths or swellings induced by infection of the plant by fungi, bacteria, viruses, mites, or insects. There are many examples, ‘Oak Apples’ caused by a tiny oak gall wasp and ‘Robin’s Pincushion gall’ on Dog Roses caused by another small wasp. Folklore has it that the latter was caused by a woodland spirit, Robin Goodfellow or Puck. It is not surprising that people ascribed supernatural causes to some galls – they look strange, and their causes aren’t exactly obvious. Even today there are still unanswered scientific questions.

Some galls are symptoms of disease, while others appear to do little harm to their hosts, and some are beneficial. Nitrogen-fixing root nodules found on members of the pea family or legumes are a form of gall, caused by the symbiotic relationship with nitrogen-fixing soil bacteria.

Finally, until next time in March, I just need to pick myself one of those Witches’ Brooms, Harry Potter style, and fly up onto the moors – if only!