A splash of sulphur to brighten the garden

There are several things that gladden my heart at this time of year, and most of them are yellow.

There are Marsh Marigold flowers, which are just starting to unfurl in my pond.

There is the sun when it chooses to come out, as it looks like doing in my garden today.

And I don’t even mind a splurge of daffodils, which I agree can be rather brash in many of our modern cultivars, but I adore our Wild Daffodil, rather daintier and paler, especially in the outer ring of petals. Look inside and you might find them dotted with little black pollen beetles.

However, here is this yellow that really gets me excited: the sulphur yellow of this most essential of spring butterflies. And it is the alternative name for sulphur that gives it its name – Brimstone. (The ‘brimstone’ of the phrase ‘fire and brimstone’ probably refers to the choking rotten-egg fumes of sulphur dioxide as emitted in the cauldronous crater of an active volcano).

The ones wafting through my garden this week wouldn't stop to be photographed, so I've had to dig into my archives to bring you a spring male Brimstone, here nectaring on English Bluebells in a local wood to me.

On any warm spring day, mainly from mid March through to May, male Brimstones start their merry wandering. Flying at about chest height, they travel through the countryside, following hedges and woodland rides and anything that approximates to that, including gardens.

Their mission is to find any of the cream-coloured females, which sit patiently down in the grass, probably emitting powerful pheromones to increase their chances of being found. They are laden with eggs, and therefore too heavy to fly far.

The number of miles that each male must fly is extraordinary, and they often seem to do so without ever needing to refuel on nectar. This is even more remarkable given that all the adults we see now will have hatched in late July and August, and won’t have fed since maybe September. They are running on empty; they are running on necessity. They must breed now or there will be no more Brimstones.

Once a female has mated and her eggs are fertilised, she now has a very specific treasure hunt ahead of her. Her caterpillars will only eat the leaves of two types of tree – nothing else. These are the Purging Buckthorn (on chalky, free-draining soils) and Alder Buckthorn (on acid or damp, clay soils). Here's a caterpillar on Alder Buckthorn in my garden last summer.

Female Brimstones must locate the buckthorns by taste. However, they do this not with their mouths but with their feet. (I started to imagine how great it would be if we could walk barefoot across a wildflower meadow and sense the different herbs with our soles, but I guess there would always be the risk of stepping in something untoward, so maybe it is just as well we can’t).

It means that there is one sure-fire way to help Brimstone butterflies: plant a buckthorn. The great thing is that they are small trees, and can be kept pruned, so they are suitable for even small gardens. They both bear berries and, what’s more, Alder Buckthorn in particular may only have small flowers in late spring but they are the most incredible lure for bees of all sorts of species. I presume that the tree must be pumping up copious amounts of liquid to keep the flowers topped up.

But there is a geographical caveat to this story. The two buckthorn tree species peter out as you get into northern England, so the Brimstone is, sadly, rare in Scotland and Northern Ireland.

So, if you live south of Hadrian’s Wall, enjoy the splash of odour-free suphur that will hopefully pass through your garden in the next few weeks. For those north of the border, may your Small Tortoiseshells and Peacock butterflies, which will also be emerging from hibernation, provide you with adequate compensation. And to everyone, give a thought to planting a buckthorn – even if the Brimstone isn’t in your area yet, there’s loads of value to be had from them. And maybe with climate change this is a species that will try to move northwards, so you may be helping its spread.