What is a Biennial Plant?

Vibrant red-stalk swiss chard in a garden

You’ve heard of annuals and perennials, but don’t let biennials pass you by. While annuals can be a flash in the pan and perennials take an eternity to start blooming, biennial plants take two years to complete their life cycle, occupying the sweet spot between fast growth and hardiness. 

Delicious vegetables like beets, carrots, Swiss chard, and kale are biennials, as well as cheerful flowers like sweet William and forget-me-not. So, let’s dive into what a biennial is, examples of biennials to grow, and how to care for them.

What is a biennial plant?

While annual plants complete their full life cycle in one year and perennials live for three years or more, biennial plants have a two-year lifespan. These flowering plants spend their first year preparing for the “big show” in their second year.

Year 1

In the first year, biennials establish the strong roots, foliage, and food storage structures they need for a successful bloom and harvest in the second year. Growth during the first season typically consists of a rosette of leaves surrounding a low-growing stem. 

Many biennial vegetables are planted in early spring and harvested in the summer of the first year, and some biennials like broccoli and Brussels sprouts may be sown in midsummer for a fall harvest. 

Over the winter, the plants die back, turn brown, and go dormant. Biennials require vernalization (prolonged exposure to cold temperatures) for a successful bloom the next year.

Year 2

When the second year rolls around, biennials put on the performance of their lifetime. Their stems elongate and they produce a profusion of flowers and fruits in summer. In the fall, biennials have their curtain call, releasing seeds and dying.

Note: Just because biennials die after two years doesn’t mean you have to start from scratch with fresh seeds. Many biennials are highly successful self-seeders, releasing fresh seeds in fall that germinate in spring to start the biennial cycle all over again. So, if you’re happy with a wilder, more natural aesthetic, you can let your biennials do the seeding work for you. 

Best biennials for your garden

Most biennials are either vegetables or flowers. Here are some of the tastiest and most beautiful biennials for your veggie or flower garden. 

Popular biennials for your vegetable garden: 

  • Beets
  • Broccoli
  • Brussels sprouts
  • Carrots
  • Celery
  • Collards
  • Dill
  • Fennel
  • Kale
  • Onions
  • Parsley
  • Rutabaga
  • Swiss chard

Popular biennials for your flower garden: 

  • Black-eyed Susans (can grow as biennials or short-lived perennials)
  • Canterbury bells
  • Evening primrose
  • Foxglove
  • Forget-me-not
  • Hollyhock
  • Lunaria (also known as “money plant”)
  • Scorpion grass
  • Stock
  • Sweet William

Benefits and disadvantages of biennials

Biennials are typically hardier than annuals and faster to bloom than perennials, making them a beloved “middle child” for gardeners.

Pros of biennials

Here’s why biennials are a savvy choice for your garden:

✓ More cold-tolerant and drought-tolerant than annuals

While biennials aren’t as hardy as perennials, they’re better equipped at handling adverse weather conditions than annuals. For example, Swiss chard is exceptionally drought-tolerant and beets grow well in nutrient-poor soil. 

✓ Excellent self-seeders

Most biennials readily reseed themselves, so you won’t have to worry about spreading new seeds every two years. Biennials will die in their second fall, but new seeds will start growing the next spring. Just be conscious of potential cross-pollination: Vegetables in the next year may look and taste different from your original vegetables. 

✓ Lower-maintenance than annuals

Biennials need more frequent watering and fertilizer applications than perennials, but they don’t need as much TLC as annuals. While most annuals require fertilization every week or two weeks, tough biennials like Swiss chard, beets, and turnips can thrive with just one or two applications of fertilizer per season.

✓ Many veggie options

If you’re growing a vegetable garden, chances are you’re growing biennials. From broccoli to carrots to onions, there’s a biennial that’ll give you a delicious, sustainable harvest, saving you from grocery runs and decreasing your carbon footprint.

✓ Native, eco-friendly plant choices

Native biennials like black-eyed Susan and evening primrose provide food and shelter to threatened pollinators like butterflies, bees, and birds. Since these biennials are specifically adapted to North America, they require very little maintenance and no harmful chemicals. 

Cons of biennials

A healthy garden should be composed of a majority of perennials, supplemented by a mix of biennials and annuals. Here’s why you shouldn’t solely grow biennials in your yard. 

✗ Not as hardy as perennials

Most biennials are native to the Mediterranean region and grow best in temperatures between 50 and 70 degrees Fahrenheit. So, if you live in the Deep South or far up North, biennials can be tricky to grow. 

✗ Fewer size options

Because biennials only live for two years, they don’t grow extremely high or wide. While some intrepid black-eyed Susans may grow as tall as 6 feet, you won’t find any biennial trees or hedges at your garden center. 

✗ Not as many native species as perennial plants

While you can find native biennials, there aren’t nearly as many varieties as there are native perennials. For an eco-friendly xeriscape (low-water landscape), rain garden, or pollinator garden, you’ll need a majority of native perennials.  

✗ Unpredictable life cycles

Some biennials may not survive their first winter, while others may bolt (experience their full life cycle in just one growing season), and still others may live longer than three years. It can be disappointing when a plant fails to produce fruit or rushes through its growing season too quickly for you to harvest mature fruits. 

✗ Reseeding doesn’t guarantee evenness

Biennials generally self-seed, but that doesn’t mean they self-seed evenly. If you prefer a neat, linear garden aesthetic, you may want to collect seeds and hand-sow them or plant brand new seeds instead of letting the wind plant your seeds for you.

How to grow biennials

Biennials may be hardier than annuals, but they still need TLC to grow strong for two years. Here are some tips for planting and maintaining your biennials.

  • Work a 2- to 4-inch layer of compost into the soil. This is best done in fall before the spring planting to give compost time to decompose and improve the soil. 
  • Plant most biennials in early spring as soon as the soil is workable, to give the root system time to establish before the stress of the hot summer sun. Check your biennials’ specific planting needs before sowing: Some warmer-weather biennials like sweet William and dill should be planted in late spring to early summer, once the threat of frost has passed. 
  • Fertilize seeds or young plants with a fertilizer that fits their specific nitrogen, phosphorus, and potassium (N-P-K) needs. For example, celery benefits from a high-potassium fertilizer (like 4-4-8) whereas collards and other leafy greens require high-nitrogen fertilizer (15-0-0) to enhance leaf production.
  • Follow a fertilization schedule based on your biennial plants’ needs. Lunaria only needs one or two feedings of slow-release fertilizer per season, whereas onions need a high-nitrogen fertilizer every two to three weeks.
  • Spread a healthy layer of organic mulch around young plants to protect their roots, keep the soil moist, and prevent weeds.
  • Stake tall, top-heavy plants proactively, before they start to flop over. Stakes should be 6 inches shorter than the mature plant so they don’t detract from the plant’s natural aesthetic.
  • Weed your garden by hand once a week. 
  • Trim and prune plants to remove diseased leaves and scraggly stems.
  • Deadhead spent flowers of biennials like foxgloves and sweet William throughout the growing season to redirect the plant’s energy from producing seed heads to growing fresh blossoms. Allow the last flowers of fall to set seed for fresh seedlings in the spring.

How to water your biennials

Water daily for the first two weeks after planting. Then, transition to watering once a week in the cool spring weather and up to three times per week in the summer heat. Keep the soil moist but not soggy, as wet soil can cause root rot.

Biennials typically need 1 to 1.5 inches of water per week, either from natural rainfall or irrigation. Water them deeply to encourage long, strong roots. Frequent, shallow waterings encourage roots to stay close to the soil surface, which quickly leads to root dehydration when a drought hits. 

Water your biennials with a soaker hose or drip irrigation system, instead of sprinklers, to deliver water directly to the plant roots and minimize the amount of water lost to evaporation.  

FAQ about biennial plants

1. What is vernalization?

Vernalization is the process of exposing seeds or young plants to prolonged cold temperatures (like a chilly winter) to induce rapid flowering and seed production in spring. From the Latin root “vern,” it literally means “making spring.” 

Many biennials need to be vernalized in between their first and second season. Cold temperatures give plants a seasonal calendar, so they know when to stay dormant and when it’s safe to grow.

If you live in an area that is either too cold or too mild for successful vernalization, dig up your biennial plants in fall and store them somewhere cool (like a garage, root cellar, or shed) over the winter months to induce vernalization. Then, replant the biennials in spring. 

2. What is bolting? 

Bolting occurs when a plant mistakes a temperature fluctuation for a change of season and behaves like an annual rather than a biennial in response. Bolting is typically caused by a string of cold days in spring or a particularly hot summer. 

Bolting plants flower prematurely and experience their entire life cycle in one growing season (as little as three months), which can be disappointing for gardeners planning on a large harvest in the second year. For example, when spring temperatures rise too quickly, broccoli may rush through its “head” stage to flower production in a process called buttoning. That means you won’t have a chance to harvest and eat the broccoli. 

3. What pests should I watch out for in my vegetable garden? 

Cabbage worms and cabbage loopers are a major threat to biennials in the cabbage family such as Brussels sprouts, kale, collards, and broccoli. Check plants regularly and remove the worms by hand, or spray the plants weekly with Bacillus thuringiensis (Bt), which kills insect larvae without harming the plant. 

Another problem pest for biennial veggies is the flea beetle. Dust your plants with talcum powder to repel these leaf-chewing insects. 

Diamondback moth larvae, corn earworms, and cabbage aphids also can damage your veggies. Remove larvae by hand, or apply organic insecticide like neem oil. 

4. I don’t want to wait two years. Can I make my biennial flowers bloom  faster?

Yes! If you live in a cooler climate, you can “trick” certain biennial flowers like foxgloves and hollyhocks into flowering before the second summer. Sow the seeds in mid to late summer rather than spring. The cool fall and winter temperatures will induce flowering, and they will continue growing and flowering into spring. 

Build your best garden yet with biennials

Growing and harvesting biennials can be fun and fulfilling, but sweaty garden tasks like fertilizing, pruning, and cleanup can take a big bite out of your weekend — when the only bites you want to take are out of your fresh veggies. Call a local lawn care pro to keep your garden and lawn in tip-top condition, so you can fire up the grill and enjoy your harvest.

Main Photo Credit: MabelAmber | Pixabay

Maille Smith

Maille-Rose Smith is a freelance writer and actor based in New York. She graduated from the University of Virginia. She enjoys watching theatre, reading mysteries, and listening to psychology podcasts. She is an orchid enthusiast and always has a basil plant growing in her kitchen.