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Biennial Plants

Biennial plants complete their lifecycles over two years. They begin growing in their first year, flower and set seed and then die in the second. A number of common crops grown in vegetable gardens call into this category, as do a number of self-seeding flowers commonly included in perennial planting schemes.

Understanding the lifecycles of different plants is important in order to work out which ones to choose, where to plant them, how to care for them correctly and how you might propagate them to obtain new plants should you choose to do so.

When learning more about the lifecycle of plants, you will soon find that there are three different categories that plants can fall into depending on their lifecycle: perennial, biennial and annual.

Many are most familiar with annual plants, which you will encounter when growing crops in a typical vegetable garden. And most will also be familiar with at least some perennial plants, that remain in a garden over a number of years (trees, shrubs and other plants that remain in a garden over more than a couple of growing seasons).

Biennial plants, however, may be less familiar to those without much gardening or growing experience. Some might overlook the true lifecycles of these plants because they can often be treated as annuals, or self-seed and therefore look more like perennials within a system.

What Are Biennial Plants?

Biennial plants are plants that usually complete their lifecycles over two years. Annuals will flower, set seed and die in a single season. And perennials will live for longer. But biennial plants grow foliage in their first season and then flower, set seed and die in the next. These plants are typically found in temperate climates.

The Properties & Lifecycles of Biennial Plants

Biennial plants typically go through their primary growth during their first year. After seeds germinate, leaves, stems and roots develop and the plants typically remain short and relatively close to the ground. Often, rosettes of leaves are formed.

After growing through their first season, biennial plants typically enter a period of dormancy for the colder months. This period of cold, which as vernalization, is often required before these plants will flower and set seed.

The following spring or summer, biennial plants will typically 'bolt'. Their stems will lengthen and the plant will flower, and will then produce fruits and seeds before they then die and complete their life cycles.

Some biennial plants can, however, take a little longer before they flower, produce seed and complete their lifecycles. So while we tend to think of biennials as two-year plants, they can often take 3 or more years to fully mature in the wild. Many biennials will only flower and set seed once their rosettes or leaves reach a certain size.

What sets a biennial plant apart from perennial ones, however, even when it lives over more than two years, is that it will flower only once in its lifecycle, while a perennial plant can flower more than once over multiple years.

Choosing Biennial Plants

Biennial plants are often chosen for an annual vegetable garden, they are found in herb gardens, and are chosen for their ornamental appeal, and as flowering additions to a pollinator-friendly garden.

In a vegetable garden, some of the most common vegetables that we grow are technically biennial plants, including:

  • Alliums (such as many onions and leeks)
  • Beets/ Beetroots & chard
  • Brassicas (a number of different cabbage family plants including certain kales, collards and cabbages, kohlrabi, Brussels sprouts, and turnips...)
  • Carrots
  • Celery
  • Parsnips
  • Salsify

However, things can become somewhat confusing because often, plants will 'bolt' and rush to complete their lifecycles, flowering in their first year, after just a matter of months. Or they are harvested before they flower. So these are often grown as annuals even though they are technically biennial plants.

These common garden crops are among the most commonly grown biennial plants and are certainly options that many gardeners will consider.

Either within an 'annual' vegetable garden or in a dedicated herb garden, there are also biennial plants commonly grown as culinary and/or medicinal herbs. Parsley is one of the most common and best-known examples.

Beyond these common edible options, of course, there are also plenty of other biennial plants that might be considered for a garden.

I would always recommend seeking out biennial plants that are native to your location. While biennials are not plants, like perennials, that will remain in your garden longer-term, those that are suited to the growing conditions that you can provide will often self-seed readily, and can therefore be wonderful choices for a low-maintenance, wildlife-friendly scheme.

Some biennials that I embrace in my own garden include foxgloves (Digitalis purpurea), teasels (Dispacus ssp.), Viper's bugloss (Echium vulgare), feverfew, sweet rocket, and Angelica ssp., to name a few examples.

Some short-lived perennials are also best treated as biennials in an ornamental garden, as their flowering tends to diminish after their first period in bloom. But true biennials can also be great additions and should not be overlooked.

Though biennial plants are not as numerous as annuals and perennials, you are sure to find options ideally suited to your own individual garden.

When choosing biennial plants, as when choosing any plants for your garden, it is important to think about the needs of the plants in question, and the conditions you can provide in terms of sunlight, shade, wind, water and soil. Match up the two to find the right plants for the right places.

Where to Grow Biennial Plants

As mentioned above, common biennial plants include crops commonly grown in vegetable gardens, culinary and medicinal herbs, and flowering plants for ornamental planting schemes and wildlife-friendly planting schemes that draw in pollinators and other beneficial insects.

You might grow biennial plants, therefore, in:

  • Vegetable plots, raised beds or containers in a kitchen garden.
  • Dedicated herb gardens.
  • Flower beds and borders.
  • Forest gardens or other predominantly perennial planting schemes (especially those biennial plants that self-seed readily where you live).
  • A rewilding or natural scheme such as a native woodland for example. (When appropriate native plants are selected.)

Propagating Biennial Plants

As mentioned above, many biennial plants are effective self-seeders that will propagate naturally and pop up year after year in your garden.

If you would like to propagate the plants then you can grow them from seed – either seed that you purchase, or seed that you collect from your own plants.

Seeds of biennial crops are typically sown in the spring or summer months. This gives the plants plenty of time to mature during their first growing season, so that they can flower and set seed during the next.

One important thing to remember when starting a garden with biennial plants is that, even where the plants will successfully self-seed, you will need to sow seeds over two seasons in order to have the flowers in bloom every year.

Remember, biennials typically have a two year lifecycle. So if you sow only one year, you will have flowers only on every second year.

Getting Biennial Vegetable Crops to Flower and Set Seed

If you wish to propagate biennial vegetable crops then there are further specific considerations when it comes to getting these crops to the point where they flower and produce seeds.

Of course, many common crops are harvested in their first year, before the plants flower and set seed. But there are several reasons why you might wish to grow on these biennial plants so that they do flower and set seed in their second season.

You might wish to get biennial crops to flower and set seed in order to:

  • Have the flowers to attract pollinators and other beneficial insects to your garden during a time after tree blossoms are done, but before most annual flowering plants are in bloom.
  • Collect the seeds for resowing. (Which can save you from the need to purchase new seeds for your garden each year, and allow you to become a more self-sufficient gardener.)

To get biennial vegetables to flower and set seed, you will of course need to be able to get them successfully through the winter months.

In some locations, certain biennial vegetables can make it through the winter months without too much difficulty. These hardier plants can remain in the ground or in the vegetable garden during the coldest part of the year, overwintering successfully before flowering and setting seed the following growing season.

With certain plants, however, and in colder areas, plants may require some additional protection in order to survive through the winter months. You might grow them inside a polytunnel or other greenhouse structure, for example, or cover growing areas using row covers or cloches. Roots can also be protected with the addition of a deep organic mulch.

Crops can also be lifted in autumn/fall and stored in appropriate conditions over the winter months. Ideal temperatures for storage are between 0 and 7 degrees C. Roots should be in moist potting compost, sawdust or sand in a cool, dark spot that remains frost-free. Specimens can also be stored in a refrigerator until spring.

After a few months of cold but not freezing temperatures, the plants can be planted back out into the garden and they should quickly begin to flower and set seed to complete their lifecycles.