Unlike annuals, which mature fully in one year, and biennials which take two years, perennials are plants that last for more than two years. Scientifically, the term refers to any woody-stemmed plant, but includes everything from tiny, low-growing thyme varieties to towering maple trees. Perennial plants overwinter by storing food either in their trunks or roots. Unlike annuals, they do not need to attempt to self-sow each summer.

Some perennial plants may only last three years, or will benefit from being lifted and separated at that time by root division.