What is a Biennial Flower or Plant?

  • What is a Biennial Flower or Plant?

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

    Most gardeners are familiar with the difference between a perennial plant and an annual. 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 are plants that go from seed to seed within 1 season, but they do not die at the end of the season. Perennials are expected to live at least 3 years, optimistically much longer. Not all perennial plants are hardy enough to withstand extreme temperatures, so some...MORE perennial plants may not survive the winter in colder climates. That’s where the USDA plant hardiness zones factor in.


    What is a Biennial?


    Inbetween annuals and perennials is another plant catagory called biennials. These are plants that usually take two growing seasons to complete thier life cycle. During the first growing season the plant produces only foliage. In it's second year it will flower and set seed, often early in the season.


    Parsley is a biennial 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. The plants we buy in nurseries are usually in their second year and ready to flower. They then set seed and self-sow. If those seeds germinate the same year you planted the original flowers, they will be ready to bloom next season. You may think they are the same plants surviving year after year, but it is just a steady supply of new, self-sown plants.


    Some plants 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 3rd 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.


    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:


    Several vegetables are also biennial, like the parsley I mentioned above and most cole crops. With vegetables, being biennial is a great benefit to the gardener. If you don’t have to worry about plants bolting to see their first year, you can continue to harvest from them all season. When one of them does go to seed prematurely, it’s a disappoint. 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 in the vegetable garden.


    How to Fool Biennials into Flowering the First Year


    While plants that don’t flower is an advantage when growing vegetables, it can be frustrating if you are growing ornamental flowers. You can get around their 2-year cycle by starting seeds in the summer, instead of 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, 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.