Having large numbers of plants which have become too big, and are hence spoiling the picturesqueness of a garden
If a garden or other place is overgrown, it is covered with a lot of untidy plants because it has not been looked after. We hurried on until we reached a courtyard overgrown with weeds
disapproval If you describe an adult as an overgrown child, you mean that their behaviour and attitudes are like those of a child, and that you dislike this. a bunch of overgrown kids