Window-box butterflies

RSPB Scotland's Paul Walton shares how you can bring butterflies to your window and experience their life cycle at home.

Window-box butterflies

For those of us living in cities and without our own gardens, containers and window-boxes are taking on even more significance during lockdown. Here’s a way that you can bring fundamental ecological processes right to your window ledge, in real time. It turns a ‘common garden pest’ into a wildlife spectacle and allows small kids – or anyone, really - to experience the life cycle of an extraordinary insect unfolding right in front of their noses. It’s something we have done many times in our Glasgow terrace flat - and every time it is, honestly, totally magical.

black and whiter just hatched butterflies

Spoiler: Just hatched small whites

Take a fair-sized window-box, at this time of year, fill with peat-free compost, and plant Brassicas in it – seed, or young plants if possible. Purple cabbage works very well and adds that incredible colour too. Keep the density of young plants quite high, higher than you would if you were growing cabbages to eat. Now, I must apologise - but please just forget any thoughts of harvesting a crop from this exercise!

When the seedlings come up, all you need do is water and feed the plants regularly. Just let them grow - but watch out for white butterflies. Here in Glasgow’s Southside, the large white, and the small white, are the species we see most. Together these are often known as ‘cabbage whites’. These species generally have two generations each year. I think we usually ‘catch’ the second one. The wings of the males mostly all white, apart from dark grey tips. The females have a few spots on the upper wings and here, in the inner city, they evidently find window-box cabbages irresistible.

If you are lucky and keep a close watch you might see – and even more importantly, the kids might see – the females gently laying their tiny eggs on the underside of the leaves - bright yellow and in batches of about a dozen in the large white, and whitish, laid singly, in the small white.

Keep watering, feeding and watching. After a couple of weeks, the eggs start to darken in colour – this is when you know they are nearly ready to hatch. You can watch the tiny caterpillars emerge and start to eat the leaves – by now, of course, growing bigger.

Keep watering and feeding, and watch the caterpillars grow - at a truly staggering rate! You are now watching a ‘growth-race’ between the leaves and the green caterpillars – stripy in the large white, less so but brighter in the small. The cabbage plants are at risk of being completely consumed – and that is, in fact, exactly what you are looking for. We find that one fair sized window box can cope until the caterpillars are looking fat and feeling sleepy, and the cabbage leaves have been more or less stripped (goodbye, crop!). This it takes about 3-4 weeks.

And then we leave the window slightly open. White butterfly caterpillars like to pupate on vertical surfaces. The caterpillars crawl inside, climb up the sides of the window frame, and pupate, making their incredibly delicate, pale chrysalises on the white window frame.

black and white print of butterfly

Small white in window ready to be let out

Then all that’s needed is to make sure someone is in the house when the chrysalises hatch into adult butterflies a few weeks later. You can watch their wings expand, dry and harden just after hatching, and see the adult butterflies attracted to the light of the window. And then you open the window wide and watch the lovely white snowflakes flutter off - to find your neighbours’ cabbages! Your window-box is now ready for some new plants - and the beautiful chrysalis shells are left behind on your window frame.

The timing is very roughly May for the egg laying, June for the caterpillars, and adults hatching round the end July.

Voila. The cycle of life plays out, right in front of you, on your Glesga windaledge.