The bedrock of wildlife-friendly gardening is plants. They fuel the foodchains that ultimately lead to our towns and villages being full of birds, bats, butterflies, bees, and indeed many creatures that don't start with a 'B'.

The added value comes when you choose your plants well.

So last week I was thrilled (to the point of rushing to get the camera) to see that one of my purposeful plant choices had come to fruition.

My plant was Purging Buckthorn. Many of you I know will now have jumped well ahead of me and will know why I planted Purging Buckthorn - it is for the love of a certain green caterpillar. This one, in fact.

The caterpillar in question is a perfect match for the leaves of Purging Buckthorn, and several on my litle tree were lying down the midrib of the leaf on the upper surface, trying to play hard to see.

As with many minibeasts, it repays close views, because it turns out to bear the finest of polka dots.

But it is what it turns into that is most exciting:

...for Purging Buckthorn and its cousin, Alder Buckthorn, are the only foodplants of the Brimstone butterfly's caterpillar. No buckthorns; no Brimstones. (Which is where I should point out that, sadly, Brimstones aren't found across most of Scotland, and are very scarce in far northern England and Northern Ireland, just because that isn't the natural range of the buckthorns).

Both buckthorns are native to the UK, the Purging variety growing mainly on chalky soils, Alder Buckthorn on damper and more acid soils, and both develop into bushy little trees, no more than about 20 foot high, which can be pruned to keep them more manageable.

Both also bear berries; and Alder Buckthorn is the best tree for bumblebees I know - they can hum with the noise of them so many visit the tiny flowers.

Winter is the time to plant a buckthorn, when they can be bought cheaply as bare-rooted plants. So if you don't have one yet, pop it straight on the Christmas wishlist.

By today, all my caterpillars had disappeared, and I hope they are now safe in a chrysalis somewhere, ready for July when the summer emergence of adult Brimstones will start. They will then feed up before going into hibernation, and next spring my Purging Buckthorn will be waiting for those that survive - plus another two I now intend to plant!

If you want to think about other things you might like to grow that will produce you more butterflies, have a look at my activity webpage all about the subject here.