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 helpful to deal with two related definitions at the same time:
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.
Don't confuse the terms "biennial" (as in biennial plants) and "biannual" (as in biannual elections). 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 is 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. 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 Botany of Biennial Plants, and Lists of Examples
Biennials complete their life cycle in a two-year period. The mission a plant must accomplish in its life cycle is to 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 flowering landscape plants are (or can be) biennial plants:
- Black-eyed Susan (Rudbeckia hirta)
- Canterbury bells (Campanula medium)
- Forget-me-not (Myosotis sylvatica); its relative, Italian bugloss (Anchusa azurea), is such a short-lived perennial that it is treated as a biennial
- Foxglove (Digitalis purpurea)
- Hollyhock (Alcea rosea)
- Honesty, also called "silver dollar plants," known botanically as Lunaria annua (or, more aptly, as Lunaria biennis)
- Pansies (Viola x wittrockiana)
- Stock (Matthiola incarna)
- Sweet William (Dianthus barbatus)
- Wallflower (Erysimum cheiri)
But many types of weeds are also biennials, including:
- Dame's rocket (Hesperis matronalis)
- Evening primrose (Oenothera biennis)
- Queen Anne's lace (Daucus carota)
- Bull thistle (Cirsium vulgare)
- Water hemlock (Cicuta maculata)
- Hibiscus trionum
If Only It Were That Easy
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. Foxglove is an example of a plant whose status is not entirely clear-cut; do not be surprised to see such a plant listed as a perennial in garden catalogs. So while these classifications are important to learn, be aware they are not always as neat 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:
- Oriental poppies (Papaver orientale) are perennials
- Iceland poppies (Papaver nudicaule) can be biennial plants
- Opium poppies (Papaver somniferum) are annuals
Why It Is Important to Distinguish Biennial Plants From Others
It is very common for inexperienced gardeners to lose sleep over a new plant not flowering. They wait and wait for a bloom to appear and get stressed out. Then a more experienced gardening friend comes along and solves the mystery: The plant in question is a biennial; it is not supposed to bloom in its first year, it will only flower in its second year.
Likewise, if you do not realize that a plant is a biennial, you might be waiting in vain 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." But what is more important is understanding the botany that makes these plants biennial (rather than annual or perennial). If you understand the botany (in this case, the two-year reproductive cycle), you can save yourself some frustration and wasted energy.