Before digging in the back yard, a good gardener needs to know how long a plant will stick around the garden, so understanding a few plant life cycle basics – annuals, perennials, biennials – is essential. But it shouldn’t end there. Today’s savvy gardeners should also expand their plant life cycle know-how to be able to characterize winter annuals and tender perennials, as well as long- and short-lived perennials.
Peonies are true perennials that can live for over 100 years.
Photo Credit: Jessie Keith
Dahlia is a tender perennial, and its tubers can be dug and overwintered indoors.
Photo Credit: Jessie Keith
African marigolds are true annuals that can bloom all growing season if deadheaded.
Photo Credit: Jessie Keith
A plant’s life cycle is simply the time and conditions needed to take a plant from germination to reproduction and finally death. Some plants can complete their life cycle in a matter of weeks, while others can take years – even decades. Mouseear cress (Arabidopsis thaliana), for example, only takes eight weeks to complete its life cycle, while American century plant (Agave americana) can take as long as 100 years before it blooms and dies. It’s all pre-programmed in a given species’ genetics.
If you’re still a little unsure about basic plant terminology and the life expectancy of certain types of plants, read on…
An annual is simply a plant that can complete its full life cycle – from germination to death – in one growing season. Some do this quickly, only living through spring, for example. Others might stick around the garden and bloom all spring, summer and fall before they kick it. Still others, called winter annuals, germinate in the fall or winter, remain as seedlings throughout the cold season and finally bloom in spring before dying. (So it pays to know what kind of annual you’re dealing with.)
Most folks like annuals because they are floriferous. They brighten up mixed borders with nonstop color and are a must for containers. Of particular interest are the ones that thrive and bloom all growing season, like the sun-loving narrowleaf zinnia (Zinnia angustifolia), sweet alyssum (Lobularia maritima) and marigold (Tagetes.).
Another important thing about annuals is they often germinate quickly and easily, and they set a lot of seed. Consequently, they tend to be easy to grow from seed and commonly reseed themselves in the garden if they’re allowed.
Very few garden plants are true biennials. “Bi-” means “two,” so as you can probably guess, a biennial generally completes its life cycle in two years. True biennials are vegetative in their first year of growth (meaning they don’t flower). That’s when they store lots of energy in their root systems. In the second year, they use up all this energy to flower and set seed. Even the truest biennials can be manipulated to live more than two years if their flowers are removed from the plants before they set seed.
Biennials are less common in the garden, but there are a few to mention. Money plant (Lunaria annua) is an interesting biennial that blooms in late spring of its second year. It sets interesting seed pod that are used in dry floral arrangements. The tall floral spikes of hollyhocks (Alcea rosea) also appear in the plant’s second year, but the plant dies after seed set. Purple foxglove (Digitalis purpurea) is commonly referred to as a biennial, but it actually behaves more like a short-lived perennial. In fact, foxglove can manage to stick around in the garden for as long as four years. While beautiful, the plant is highly toxic, so it’s important not to plant it where children or pets might accidentally ingest it.
The general definition of a perennial is a plant that lives for three or more years. The problem with this limited description is that some plants, like peonies, can live for over 100 years, while others, like some Agastache species, tend to only live for about four. Moreover, some perennials, like Monarda, bloom in the first year from seed, while others, like Baptisia, can take four years before they bloom from seed at all. Basically, there are no hard or set rules for the generic term “perennial,” so you should get to know a little more about a particular perennial’s life cycle before you plant it in your garden.
Perennials will be in your garden for the long haul, so think of them as the staple plant for herbaceous beds and borders. A key goal for perennial garden design is to produce continued bloom and interest. This means planting with perennials requires more careful planning and thought because many of these guys bloom only for set times in the growing season. For example, daffodils bloom in early-, mid- and late spring (depending on the cultivar); Baptisia blooms in the early summer; coneflowers (Echinacea.) bloom in midsummer; asters bloom in fall; Coreopsis verticillata blooms all season long with deadheading; and blue fescue looks great year-round. Put these sun-lovers in the same border, and you’ll have continual seasonal interest.
The Tender Perennial
These are tropical perennials or shrubs that we grow in the garden and treat as annuals in northern regions. The great thing about these plants is that they bloom profusely and flower their first year from seed. As a consequence, they make the most superior bedding and container plants and should be planted wherever continuous color is desired.
A few common, super sun-loving tender perennials for the garden include the fragrant blue-flowered heliotrope (Heliotropium arborescens); the colorful, heat-tolerant lantana (Catharanthus roseus). All of these actually grow as tropical shrubs in their native habitats. An all-time favorite tender perennial for shade gardens is the common impatien (Impatiens walleriana), which grows as an herbaceous perennial in its native tropical habitat
Some tender perennials are commonly overwintered because they have fleshy roots or rhizomes that can be dug up in the fall and placed in a cool garage until springtime. Many people do this with dahlias, cannas and elephant ears (Colocasia esculenta). But truly, all tender perennials can be overwintered in pots indoors or in a greenhouse (if you have the time and energy to care for them).
Knowing plant life cycle basics is absolutely an essential. This simple knowledge opens the door to great garden planning and can help ensure you’ll have the right color in the right spots all year long.