Annuals vs. perennials: what the difference means for your garden

What is the difference between annuals and perennials?

Annuals and perennials differ mainly in how long they live. Oregon State University Extension defines an annual as a plant that goes from seed to seed in one year or growing season, and during that time it grows, matures, blooms, produces seed and dies, as zinnias do. Oregon State also separates summer annuals, which complete their life cycle in spring and summer, from winter annuals, which grow mostly through fall and winter. In Florida, University of Florida IFAS Extension groups annuals into warm-season and cool-season types for the same reason.

A perennial lives for more than two years, according to Oregon State. Herbaceous perennials have soft, nonwoody stems that generally die back to the ground each winter, and new stems grow from the plant's crown each spring, and trees and shrubs make up the woody perennials. University of Vermont Extension adds that some perennials, such as moss phlox, stay evergreen, while others, such as hostas, die back to ground level and survive through their roots. In practice, you plant annuals again each year, and a perennial that suits your climate returns on its own. Our flowers and plants section covers individual plants in more detail.

What is a biennial?

Oregon State University Extension explains that a biennial, which falls between the two groups, needs all or part of two years to complete its life cycle. In the first season it produces leaves and food storage organs, and after the winter it produces flowers, fruit and seed in its second season before it dies. Oregon State lists Swiss chard, carrots, beets, sweet William and parsley as biennials, and University of Vermont Extension names foxglove and hollyhocks among the flowering ones.

Oregon State also notes that environmental stress such as drought, temperature swings or exposure to cold can push a biennial from germination to seed in a single growing season, a response gardeners call bolting. In a flower bed, knowing that foxglove or hollyhocks bloom in their second year helps you plan for a season of leaves before the flowers arrive.

What is a tender perennial, and why is it sold as an annual?

University of Maryland Extension describes hardy perennials as plants that withstand freezing winter temperatures, and tender perennials as plants of tropical origin that need frost-free conditions to survive the winter. University of Vermont Extension explains that tender perennials thrive only in warmer climates and are often sold and treated as annuals in colder ones. In a climate without frost, the same plant can keep growing from one year to the next.

This is why the same plant can be labeled an annual in one state and a perennial in another. University of Florida IFAS Extension notes that Florida's longer growing season changes how some plants behave, and that a plant may move from one category to another depending on where you are in the state. If you want to keep a tender perennial from one year to the next in a cold region, University of Vermont Extension says you can overwinter it indoors and return it to the garden in spring.

How do USDA hardiness zones decide whether a plant comes back?

The USDA Plant Hardiness Zone Map is the standard gardeners use to decide which perennial plants are most likely to thrive at a location. It is based on the average annual extreme minimum winter temperature and is shown as 10°F zones and 5°F half zones. University of Maine Cooperative Extension explains that each zone is divided into an "a" and a "b" half. For example, Zone 4a has an average winter low of -30 to -25°F and Zone 4b has an average low of -25 to -20°F.

A perennial labeled hardy to your zone or a colder one has the best chance of returning each spring. For more confidence, Maine Extension suggests buying a species rated one-half or even one full zone colder than your site. It also lists what the map leaves out, including summer heat, the length of the growing season, snow depth, wind, and soil drainage and fertility. Snow cover strongly affects the winter survival of herbaceous perennials, so the map is a less reliable guide for them than for trees and shrubs.

How do cost and care compare?

University of Vermont Extension notes that perennials usually cost more up front and work as a long-term investment because they come back each year, while annuals must be bought again every season. Vermont Extension also says annuals demand more labor and frequent watering and have shallower root systems than established perennials. University of Florida IFAS Extension adds that annuals need substantial nutrients and water and benefit from deadheading and pruning, and that perennials, once established, typically need less water and fertilizer, although some need rejuvenation every few years.

Before you choose either type, University of Maryland Extension recommends looking at the conditions of the site itself: how much light it receives, how well the soil drains, which wildlife visits, and how much space the plants will have. It also suggests thinking about what you want the bed to do, whether that is attracting pollinators, supporting wildlife, providing seasonal color or managing stormwater, and then choosing plants with varied leaf textures, growth habits and flowers. The same questions apply to annuals, which you can change from year to year as you learn what grows well in each spot.

Which blooms longer, annuals or perennials?

University of Vermont Extension observes that annuals tend to have a longer bloom time than perennials, and it suggests combining the two types to extend flowering through the season. Vermont Extension also suggests using annuals to fill the gaps between young perennials in a new bed while the perennials grow into their space. When you plan light for a mixed bed, University of Florida IFAS Extension notes that most perennials prefer at least four hours of sun a day, so check each plant tag for its needs before you group them.

Bulbs add another kind of seasonal bloom to a mixed bed. University of Florida IFAS Extension describes bulbs as a specialized group of perennial plants grown for flowers that appear year after year, and it uses the term broadly to include corms, rhizomes, tubers and tuberous roots. Many of them die back during a seasonal dormancy, although some, such as the gingers grown in South Florida, keep their foliage all year.

How do you care for perennials over the years?

University of Maryland Extension says you can plant perennials whenever they are available and the ground allows it, although spring-flowering types transplant best in late summer or early fall and fall bloomers transplant best in spring. Maryland Extension notes that most perennials left in the same place for more than three years are likely to be overcrowded, with dead centers, and it recommends dividing spring and summer bloomers after they flower and fall bloomers in early spring. Deadheading spent flowers encourages more bloom, pinching produces bushier plants, and thinning stems improves air circulation. Maryland Extension also suggests leaving some standing plant material through winter to shelter wildlife and pollinators. If you are also growing vegetables, our guide on when to plant tomatoes covers one of the most popular warm-season annuals, and the garden section gathers more seasonal tasks.