What Is a Biennial Flower or Plant?

Foxglove (Digitalis)
Foxglove (Digitalis). Photo: © Marie Iannotti

The distinction between annual and perennial plants is generally understood by most gardeners. Annual plants complete their entire life cycle in a single year, going from seed to plant to flower and back to seed, then dying off. Perennials, on the other hand,  are plants that go from seed to seed within one season but which do not die at the end of the season. Even this distinction is not quite as straightforward as it seems, though, because sometimes a plant that is perennial in warmer climates may be grown as an annual in colder climates where winter kills them off.

Or, some annual plants self-seed themselves so prolifically that they resprout every year and are virtually perennial in their habits. You may think they are the same plants surviving year after year, but it is just a steady supply of new, self-sown plants.

By some definitions, a perennial is a plant that you can expect to live at least three years, or in some cases much longer. As mentioned, though, not all perennial plants are hardy enough to withstand extreme temperatures, so some perennial plants may not survive the winter in colder climates. That’s where the USDA plant hardiness zones factor in.

What is a Biennial?

In between annuals and perennials is another plant category known as biennials, which can be regarded as short-lived perennials that usually take two growing seasons to complete thier life cycle. For the classic biennial, in its first growing season, the plant produces only foliage.

In its second year, it will flower and set seed, often early in the season.

Parsley, for example, is a biennial herb that often over-winters, even in colder climates. Although it’s nice to see last year’s parsley sending out new growth in the spring, don’t expect to be harvesting leaves from the plant.

It very quickly sends up a flower stalk and goes to seed. At that point, leafy growth slows and the flavor and tenderness of the leaves is diminished.

What are Some Commonly Grown Biennials?

Flowers: Many popular flowers are biennial, although very often it goes without notice, because the plants we buy in nurseries are usually in their second year and ready to flower. And many of these then set seed and self-sow themselves. If you watch closely, you may notice that sometimes these self-sown volunteers do not produce flowers in their first year, though, but instead fully mature in their second year. Foxglove, for example, is a biennial that readily self-seeds, and if you allow the seedlings to come back their second year, flowers will be produced. 

With some biennials, if they set seed early enough in the season, you may get flowers in the first growing season. Other plants, though, will not set seed until the fall and/or germinate until the spring. In that case, they can skip a year of blooming between the first year when you planted the original plants and the third year when the new seedlings are ready to flower. But once you’ve had your plants in the garden for a couple of years, you will that steady supply of new seedlings always coming in.

Be aware that plants may be available in different varieties, some of which are annual, some biennial, and some perennial. A close examination of the subspecies may be necessary to determine which you have.

Some of the most popular biennial flowers include:

  • Black-eyed Susan (Rudbeckia hirta)
  • California poppy (Eschscholozia)
  • Canterbury Bells (Campanula medium)
  • Forget-me-not (Myosotis)
  • Foxglove (Digitalis)
  • Hollyhock (Alcea)
  • Honesty (Lunaria)
  • Pansy (Viola wittrockiana)
  • Poppy (Papaver)
  • Stock (Matthiola incana)
  • Sweet William (Dianthus barbatus)
  • Wallflower (Erysimum cheiri)

Vegetables: Although it is sometimes not recognized, quite a number of vegetables are biennials, too. The reason it is not noticed is that when you purchase seedlings at the garden center, these are usually plants already in their second year, and as a result, they behave like annuals, producing their fruit and then dying the same season you plant them.

It is when you plant vegetables from seeds that their true biennial nature becomes apparent. For example, when you plant onions from bulb sets, these bulbs are already second-year plants. On the other hand, if you plant tiny onion seeds in the garden, it requires two full years before you have edible onion bulbs. 

Among the biennial vegetables are the parsley mentioned above, as well as most cole crops. With vegetables, being biennial can a great benefit to the gardener. If you don’t have to worry about plants bolting (flowering and setting seed) in their first year, you can continue to harvest from them all season. When one of them does go to seed prematurely, it’s a disappointment. Broccoli, for example, often behaves more like an annual plant when summers are extremely hot. It can mistake the fluctuating spring temperatures as a change of season and think it has already gone through its first summer and winter. But most of these vegetables will not flower the year you plant them as seeds in the vegetable garden.

How to Fool Biennials into Flowering the First Year

Non-flowering in their first year is an advantage when growing vegetables, but it can be frustrating if you are growing ornamental flowers. You can get around the two-year cycle of biennial ornamentals by starting seeds in the summer instead of the spring, and putting the plants outdoors in the fall. The plants will then go through the winter season and be ready to bloom their first full year in the garden.

How to Save Seed of Biennials

The one drawback to biennial vegetables that don’t flower until their second year is that it can be difficult to save seed. It’s not a problem with hardy plants, like parsley and Angelica, but most of the cole crops cannot live through a hard winter without protection. If you want to save seeds of these plants, you will either have to mulch heavily or dig the plants and store them elsewhere.

For the details on how to do this, take a look at Saving Seed of Biennial Vegetables.