There is a moment in late May — blink and you’ll miss it — when the hedgerows along Hertfordshire’s country lanes seem to exhale. Great flat-topped clouds of creamy white blossom appear almost overnight, filling the warm air with a scent that is floral, honeyed, and unmistakably, gloriously English. That’s elder. And if you’re not already on first-name terms with it, now is the time to be introduced.