You Only Bloom Once … Or Twice … Or More

by | May 20, 2019 | Feature, Native Plants

Mexican hat in various stages of blooming. PHOTO Wildflower Center

 
High in the White Mountains of California lives a very old tree. Now over 5,000 years old, this Great Basin bristlecone pine has a long story to tell. Meanwhile, Eurasian thale cress goes by the nickname “fruit fly of the plant world” for its ability to complete its entire life cycle in a few short weeks. Disparate life histories such as these are just one of the many things that make plants so fascinating.

Avid gardeners are well aware of the various life cycles of certain plant species. In planning our gardens, we know that the annual plants we place in our beds are temporary, while some of our perennial plants could easily outlive us. In some instances, plants that would otherwise be perennial in their native range are treated as annuals, succumbing each year to the freezing temperatures of winter.

In the wild, plants have a variety of strategies for enduring diverse environmental conditions. How long they take to complete a life cycle is just one of these strategies.

Plants that complete their cycle within a year’s time are considered annuals. They are also monocarpic, meaning they flower once and then die. Winter annuals germinate in the fall, then flower and set seed the following spring; stiff greenthread (Thelesperma filifolium) is one example. Summer annuals, such as common sunflowers (Helianthus annuus), sprout during warm months and have matured by the time cool weather returns.

Annual plants are commonly found in areas that experience regular disturbance (such is the case with bluebonnets and Hill Country pastures) or where environmental conditions are unpredictable. A significant portion of our common weeds are annuals, as are numerous desert wildflowers. The extensive seed bank they maintain in the soil allows them to persist long term. By staggering the years in which their seeds sprout, annuals ensure the survival of their species — a strategy ecologists call “bet hedging.”

Most plants are perennials, going on to live year after year and — in most cases — producing seeds numerous times throughout their lives. During times of the year when conditions are not conducive to growth, such as the frigid months of winter or the hot, dry days of summer, perennial plants can go dormant. Their foliage may die back while they wait things out as roots or modified stem structures underground. The return of favorable conditions signals them to resume growth.

Some plant species are oddballs. Biennial plants, such as standing cypress (Ipomopsis rubra), spend their first year as a rosette of leaves, gathering nutrients in their roots. The following year they send up a towering flower stalk, set seed, and then die. Agave americana is known as the century plant because it was once thought to take 100 years to flower; 10 to 20 years is actually typical (though one plant in a Michigan botanic garden bloomed at age 80). It is considered a monocarpic perennial, because it only flowers once; however, it also reproduces vegetatively, sending out “pups” via underground shoots. Upon the death of the original plant, these offshoots go on living and are clonal copies of their mother.

Many perennial plant species have ways to persist long term without having to rely on seeds. Just like the agave mentioned above, such plants produce offspring asexually through structures like rhizomes, stolons, tubers or bulbs. Life spans vary among perennial species, but part of their strategy for surviving through unpredictability is to live for many years, hunkering down when conditions get rough. Their seed production varies from year to year. If conditions aren’t great one year, they have the following year to try again.

Certain species defy description. Depending on where you find them within their range they can be annual, biennial or perennial. Black-eyed Susan (Rudbeckia hirta) is one such plant. In fact, throughout evolutionary history numerous annual species have evolved from perennials, while other annuals diverged into perennials, adapting to the environment in which they found themselves.

We can take a note from nature when we choose plants for our garden. A diverse selection of annuals mixed with a wide variety of shortand long-lived perennials will ensure that, like the plant species themselves, our garden will evolve with time, displaying for us the wide array of features that plants have to offer.
 
 

Gardening by Life Span:

Choose what to plant with longevity in mind

ANNUALS

PERENNIALS

TIME BANDITS
Prairie fleabane, Erigeron modestus
Often perennial, sometimes annual or biennial

Prairie verbena, Glandularia bipinnatifida
Often perennial, sometimes annual or biennial

Scarlet sage, Salvia coccinea
Can be either annual or perennial

Daniel Murphy is collections curator at Idaho Botanical Garden and founder of Awkward Botany, a weekly science blog. He specializes in native and waterwise plants and is working on a book about the world’s most hated plant group: weeds.