It's bad news when these appear in your garden. This is the spore-producing shoot of field horsetail Equisetum arvense, about to liberate thousands of spores which have a unique adaptation to aerial invasion, which you can see by clicking here.

Once it's established this plant is well-nigh impossible to eradicate. Its underground stems (known as 'devil's guts') run deep and fragment easily, regenerating new plants just when you think you've dig it all out. Still, it could be worse; back in the  Carboniferous, 300 million years ago, when the coal measures were laid down, these grew 10 metres tall.