How we define weeds impacts our attitude towards them.
For instance, weeds may be defined based on human desire - plants that grow where they are not wanted

OR perhaps a weed is any non-native plant - ie. any plant that colonises and persists in an ecosystem in which it did not previously exist

OR based on management logistics - a weed is any plant that requires some form of action to reduce its effect on the economy, the environment, human health and amenity.

We define weeds as competitive plants which are not locally endemic and have a detrimental impact on ecological and human assets.

There is no such thing as a bad plant - context is everything!