I trust that few of us have enjoyed an ale flavoured with gruit. There are some ales in production which still use it, but it is a rare drink made for connoisseurs, but had you lived in Saxon England, gruit is what you would have got. So what is it?
Gruit [grut, gruyt] is a herbal mixture. There is no set recipe, and it is likely that the brewers used whatever was available to them at different times of year and in a variety of places. We need to be clear, beer was made in private homes, often by the wife of the household, and much depended upon what herbs were available to her. It was also brewed in institutions,such as monasteries, and in the large monasteries there were always herb gardens, so the task of the monastic herbalist was not always to make medicines, but to grow the herbs to flavour the monks' beer. If you look at Strabo's garden, the plot tended by Walafrid Strabo, a mediaeval abbot, there is artemisia, which is a herb widely used for flavouring ale, and Strabo was not growing that for nothing. In some large monasteries there would have been a special garden tended for the purpose of providing flavourants for the ale.
So what was in gruit? It was water flavoured with several or all of the following herbs:sweet gale [myrica or bog myrtle] , artemisia, yarrow [Achillea millefolium] ground ivy [hedera] , horehound [marrubium] and/or heather. The recipe involved soaking the herbs in warm water for several hours, depending upon the herb, until the water was flavoured to the brewer's satisfaction. It was then added to the must, the beer/ale mixture and drunk. Certain herbs have advantages as ale flavourants. Artemisia, the genus that includes wormwood, is the bitterest herb known to humans, and other members of this genus are bitter as well. The Saxons knew artemisia as mugwort. wort being a Saxon term for plant, the wort which goes in your mugs!
Heather is special. Its flowers make a delicate flavouring, but you need a pint of them and they have to be soaked overnight for twenty four hours, so I believe. I can imagine a misbehaving monk being given a penance of collecting a few pints of heather flowers. They make a lovely drink, but picking them is a long and arduous task. Of course the penance might involve going to pick bog myrtle, a muddy and chilly task at times.