This is probably not what you were expecting, if you know the charm. The usual translation is ‘lambs cress’, or hairy bittercress – cardamine hirsuta. I’ve even seen watercress. There are other candidates too, which we’ll discuss, but this is the one I’ve settled on. First, the translation:

This plant is called ‘thunder’ * it thrives upon stone.

It halts a fever * it is powerful against aching.

The word in inverted commas is a translation of ‘stune’ and the nearest I can get to it is ‘stunaþ’ – ‘it crashes’. Geoffrey Grigson says that the Cumbrian name for houseleek is ‘thunder plant’ and that, for me, is the clincher. Again, there is actual botany here. This plant ‘thrives on stone’ – unlike the hairy bittercress, which likes its soil wet, but will grow anywhere, as I know to my cost. Grigson says it could not have been native, but has been planted in Britain from very early times, presumably because it was so valuable medicinally, unlike hairy bittercress, which is edible, but hasn’t much recorded in the way of medicinal uses.  As well as being good for fevers and rheumatism, houseleek is, like aloe vera, good for burns, and had a reputation for protection from thunder, which is why it was so often planted on roofs.

There are alternatives – the stonecrop

which does thrive on stone, but is not so well-regarded medicinally. Culpepper says it’s more likely to provoke a fever than cure it. And there is biting stonecrop, (sedum acre) which is small and yellow and related to the houseleek. This does have a medicinal history, especially in Scotland, and even Culpepper thought it was ‘so harmless you can scarce use it amiss’, but mostly it was used when you couldn’t get houseleek, so that’s the one I’ve gone with.