What Are Biennial Plants?

And Why It Is Important to Know

Image of forget-me-not flowers.
Forget-me-not flowers are an example of biennial plants. David Beaulieu

Biennial plants grow only vegetation in their first year. After overwintering in a dormant state, they flower in their second year (thus the "bi-" prefix), produce seed, then die. 

To reinforce this meaning with beginners, it is sometimes easiest to answer two related questions at the same time:

That is, learning the difference between biennials and these other plant groups will help you understand the definition better. Biennials can be seen as plants whose longevity status places them in-between annuals and perennials.

Do you confuse the terms "biennial" (as in biennial plants) and "biannual" (as in biannual elections)? The confusion is understandable. The latter adjective is defined as pertaining to something that happens once every two years; when dictionaries define it, they often list the former as a synonym, which can be misleading. An example of something that can be biannual is an event, such as Congressional elections in the United States of America.

When the subject is plants, however, "biennial" is the right word to use. This adjective (or noun) is defined as pertaining to a plant that lasts for two years. Notice the difference: The idea that something "lasts for two years" is very different from the idea that it "happens once every two years." Congressional elections occur every other year, but they are not around all the time (thank goodness) during the course of those two years, the way biennial plants are. If you remember that, you will never forget the difference between "biennial" and "biannual."

The Biology of Biennial Plants, and a List of Examples

Biennials complete their life cycle in a two-year period. What mission must a plant accomplish in its life cycle? It must reproduce. Biennial plants bloom and set seed in their second year (during their first year, they just look like a bunch of leaves). After that -- having accomplished their one mission in life -- they die. They simply have no reason to go on living.

The following are (or can be) biennial plants; click any link on the list to learn more about that particular plant:

  1. Black-eyed Susan
  2. Canterbury bells (Campanula medium)
  3. Dame's rocket
  4. Evening primrose
  5. Forget-me-not (Myosotis sylvatica; see picture)
  6. Foxglove
  7. Hollyhock
  8. Honesty, known botanically as  (or, more aptly, as Lunaria biennis)
  9. Pansies
  10. Queen Anne's lace
  11. Stock (Matthiola incarna)
  12. Sweet William (Dianthus barbatus)
  13. Wallflower (Erysimum cheiri)

If Only It Were That Easy

Note that the descriptions for certain plants will sometimes say, "may be a biennial or short-lived perennial." The lack of absolute clarity is unfortunate, but unavoidable. One of the entries on the list above (foxglove) is an example of a plant whose status is not entirely clear-cut; so do not be surprised to see such a plant listed as a perennial somewhere. So while these classifications are important to learn, be aware they are not always as nice and tidy as we would like.

In the case of poppy flowers, we must be very careful to specify exactly what type of poppy we mean:

  1. Oriental poppies (Papaver orientale) are perennials.
  2. Iceland poppies (Papaver nudicaule) can be biennial plants.
  3. Opium poppies (Papaver somniferum) are annuals.

Why Is It Important to Distinguish Biennial Plants From Others?

"I did not understand why my new plant was not flowering until a gardening friend informed me that it was a biennial and would not flower until its second year. I wish I had known earlier, because waiting for the flowers had me pulling my hair out." Have you ever heard (or made) an admission like this? It is very common. Gardeners lose sleep over this sort of thing all of the time.

Likewise, if you do not realize that a plant is a biennial, you might be waiting -- and waiting, and waiting -- for it to re-emerge in your garden in the spring of year three. If it is truly a biennial, you will be waiting forever (and experiencing unnecessary frustration), because it has already died (right on schedule). No amount of watering, fertilizing, etc. will bring it back. And if you buy the same type of plant again, the exact same thing will happen again.

That is why it is not merely an academic exercise to define something like "biennial plants." Sure, you can impress your friends by drawing a distinction between "biennial" and "biannual" (see above). But what is more important is understanding the biology that makes these plants biennial (rather than annual or perennial). If you understand the biology (in this case, the two-year reproductive cycle), you can save yourself some frustration and wasted energy.