Updated: Feb 7, 2020
Perennials, annuals, biennials... and what the heck is an ephemeral? And, why should you care?
You've likely heard these terms before, or you have seen them on the tags at the local big box store plants or at your local nursery. Perennials, annuals and biennials are all words that tell us how long a plant will live. These terms should not confuse you too much, because they simply tell us if the plant will come back each year or if they will die after it goes to seed. Well, there's a bit more to it, so let's discuss.
There are many types of plants that will thrive year after year, depending on the climate in which you live. You may recognize the climate as the zone in which the plant is recommended. To understand zones, visit this post. In short, where we live - how cold it gets in the winter, how hot in the summer, and how wet or dry it is - determines what zone in which we live. Without having to understand all the details, you can easily find your zone by visiting the US Department of Agriculture site.
In Colorado, we find ourselves somewhere between 4a and 6a, with some exceptions. In the front range area, we mostly fall between 5a and 5b with a few caveats. For example, here in Parker, I have learned that plants in my front yard, which faces north and doesn't see the sun from late October through mid March and is covered in snow most of that time, should be labeled for either zone 5a or 4b. The plants in my back yard, which faces due south and gets a ton of our intense sunshine, can be labeled up to zone 5b. These microclimates are not exactly something we have to know to get started; rather, gardening is a never ending experiment to guide us. Also, with more intense weather due to increased climate change, these zones may well require a more fine-tuned approach in the future.
Plant hardiness zones are determined by the average annual minimum winter temperatures. Plants in zone 5a will live through the winter if the winter temperatures do not go below -20 degrees (F) (-29.9 degrees Celsius for everyone else on the planet). Each zone (13) are divided into 10 degree segments, which are then more defined by sub-zones of 5 degrees Fahrenheit (respectively). These zones are the main categories in which you will see plants labeled at the stores or on seed packets. However, they are not the only way to determine if a plant will survive in your area. Plants can be further categorized by moisture, humidity, heat, soil types, wind, or drainage. But we'll talk about those later.
Perennials: Plants that live through the winter
Perennials are plants that will live for many years in your plant hardiness zone. These plants typically live three+ years, with winter dormancy between growing seasons. They grow, they go dormant (and "die back") in the winter, and then re-sprout in the spring. Perennial plants die back down to the ground each winter, but the roots are well adapted to continue to hold through winter underground. They then regenerate from root systems or buds when the temperatures rise.
Still, a plant may be considered a perennial in Florida but not in Montana: the cold hardiness of a plant does not alone define a perennial plant - you must first consider the zone in which you live.
Most perennials will loose their leaves in the winter, and all other signs of life will fade down to the ground. Some people, like myself, leave these dried up stems and leaves in place all winter long to provide homes for pollinators like mason bees and for food and refuge for birds. I often provide a little extra protection and food for them by piling dried leaves over the base of the plant in the fall.
Types of perennials:
Herbaceous perennials - develop overwintering woody tissue only at the base of shoots or have underground storage structures from which new stems are produced. Examples include peonies, coneflower (top left), columbines, larkspur, and hardy mums.
Spring ephemerals - have a relatively short growing season but return the next season from underground storage organs such as bleeding heart (top right) or trillium.
Woody perennials - develop overwintering tissue along woody stems and in buds. Think about shrubs like roses (bottom left), burning bush, lilac, or dogwood.
Evergreen perennials - most often (but not always) retain their leaves throughout the year despite cold temperatures. Some leaf shed may occur, but most of the leaves or needles stay green even in the winter, such as pine (bottom right), spruce, or holly.
Annuals: Plants only live for one season
Annuals start and complete their life cycle within a single growing season. This means that annuals can be started from seed, grow, and then set seed and die completely. They do not regenerate each season. It is important to note that annuals have growing seasons that can be any of the four seasons - but they only come back from new seeds.
Types of annuals:
Summer annuals - these annuals germinate from seed in the spring and complete flowering and seed set by fall. The plant then dies - mainly due to cold temperatures. Their growing season is from spring to fall. Summer annuals can include both flowers and vegetables/fruits: peas, tomatoes, marigolds, cosmos, and petunias. Some summer annuals are cool season or warm season, meaning these plants grow and flower in either early spring or late fall (cool season) or only in the warmer months of summer (warm season). Lettuce is an example of a cool season annual. Sunflowers are considered warm season annuals.
Winter annuals - these plants germinate from seed in the fall, with flowering and seed development the following spring, followed by plant death. Their season is from fall to summer. Winter annuals include annual bluegrass, chickweed, or winter wheat.
Biennials: Plants live and die within two growing seasons
Biennials germinate from seed during the growing season and develop a storage root system during the first growing season, and will only produce leaves. The second season for these plants produces flowers and seeds followed by death. Biennial plants often self seed. Biennials include hollyhocks, foxglove, and forget-me-not.
It's Your Choice - But Know Before You Plant!
When you are choosing plants, it's important to understand what you are buying and how long it will live. Many annuals are easy to start from seed, and starting plants this way can save you a lot of money. Annuals are best for container gardening or vegetable gardening. When selecting plants for your landscape, you really need to understand the difference between annuals and perennials. By choosing the latter, you'll be investing in your property - not in your local nursery! Choosing long-living perennials for your landscape - especially choosing those that are best suited for your climate, soil, and location - will not only save you money on plants each year, but they often survive with much less fuss over the years. You'll get a beautiful landscape that the pollinators and birds will love, as well as your neighbors for years to come.