A number that is calculated to summarise a group of numbers. The most commonly used average is the mean, the sum of the numbers divided by however many numbers there are in the group. The median is the middle value in a group of numbers ranked in order of size.
Perennials: cutting back. Perennials are not demanding plants, but trimming them after flowering finishes in autumn helps improve their appearance and flowering. However, you can leave some stems over winter to provide homes and food for wildlife, and then trim back in spring.utting back herbaceous perennials during autumn restores order and tidiness to the garden. However, this removes potential winter interest, in the form of height and structure, plus food and habitat sources for wildlife.
The term  perennial  refers to herbaceous (âgreenâ) plants since woody plants, such as trees, are perennial by definition. Unlike annuals, perennials tend to bloom for just a short time â one to three weeks â each year. Examples of popular perennial flowers include tulips, asters, black-eyed susans and lilies.hatâs because some of them are annual plants and some are perennial plants. Annual plants are plants with a life cycle that lasts only one year. They grow from seed, bloom, produce seeds and die in one growing season. They then need to be replanted each spring.