Weed experts, I gather, have great difficulty coming up with a scientific account of the term weed. In my own attempts to come to grips with the concept of the garden weed, I've encountered many different definitions: ‘a plant growing where we do not want it’; ‘a plant whose virtues are yet to be discovered’; ‘a plant growing out of place’; ‘a nuisance plant that interferes with human activities’; ‘a plant that you do not want’; ‘a plant you hate’. More precise definitions, it seems, are impossible – in fact, probably not practicable. The difficulty is that weeds are context specific. It depends entirely on location and on time whether something is classified as a weed or not.

Different soils clearly have different weeds. Some gardeners might spurn a plant that usurps and overgrows their garden. Others may admire that very same plant for its ability to thrive in the impoverished soil of their inner-city courtyard. Lantana, I recall when I was growing up, was much praised for its flowers and its capacity to flourish in neglected gardens.