Biennial plants: everything you always wanted to know!

What is a biennial plant?

A biennial plant is characterised, in the strictest sense, by its life cycle that spreads over two years:

• in the first year following its sowing, it will develop its root system and leaves, which are often arranged in a rosette;
• in the second year, it flowers, produces seeds, and then dies.

Foxglove: the rosette of leaves in the first year and then its flowers in the second year

This is the case for Angelica, Purple Foxglove, Mullein, Money Plant, the Evening Primrose, as well as many vegetable and aromatic plants whose leaves (parsley), roots (carrots, parsnips) are consumed before they produce their flowers.

Why are some perennials referred to as biennials, and vice versa?

This lovely variety of primrose, very colourful, will lose its colours over time. That is why it is rather classified as a biennial.

The term biennial is widely used (if not overused!) in horticultural language to refer to many types of plants, which can sometimes lead to confusion.

Among these groups of plants, we are well acquainted with autumn biennials, represented by Large-flowered Pansies, Double Daisies, Colourful Hybrid Primroses, some Stock flowers… In fact, these are hybrid perennial plants whose only flaw is their tendency to exhaust quickly due to their prolific flowering, making them less durable. They are also classified as biennials because their vegetative cycle spans two calendar years (sowing at the end of summer and flowering in early spring the following year). In practice, they are cultivated as annual plants, placed in planters or pots and in seasonal beds. Although they do not retain their initial vigour, they can nevertheless be replanted in the garden to brighten up a neglected corner.

Conversely, some biennial plants are considered perennial because they self-seed easily as long as the exposure and soil suit them. This longevity in the garden gives the illusion that they are perennial.

Finally, there are also perennial plants grown as biennials because, not only do they only flower in the second year following their sowing (like many perennials), but their lifespan is short and their appearance deteriorates quickly over time. This includes, for example, Hollyhocks (Alcea rosea) or many foxgloves.

Where to grow biennials? Their advantages

Biennial plants find their place in both garden beds and in containers and pots around the house.

True biennials are primarily grown in the garden. For example, in this category, you can find:

• Purple foxgloves (Digitalis purpurea),
• Honesty (Lunaria annua),
• Many varieties of mullein such as common mullein (Verbascum thapsus, Verbascum bombyciferum),
• Common angelica (Angelica archangelica), as well as purple angelica (Angelica gigas),
• Teasel (Dispsacus sylvestris).

All these plants often share a tall stature and remarkable architectural appearance, which adds beautiful vertical lines to the beds, as well as a good ability to self-seed, which is particularly appreciated in natural gardens.

Angelica forms stunning umbel inflorescences, bearing many seeds at ripeness.

The biennials that adorn pots, containers, and also, in open ground, areas dedicated to seasonal flowering are mainly represented by:

• Pansies and horned violets,
• Daisies,
• Primroses,
• Forget-me-nots.

Planted in autumn, they bloom early in the season and provide a taste of spring while the garden is still asleep. In the garden or on the windowsill, we recommend pairing them with early flowering bulbs, as well as with perennial plants, particularly with evergreen grasses and those like heucheras, tiarellas and heucherellas, which offer beautiful coloured foliage all year round.

Pansies offer an unmatched “retro” charm.

When to plant, sow biennials?

Planting:

Biennial flowers intended for borders can be planted in spring or autumn. They will flower after experiencing a period of cold (vernalisation) corresponding to their first or second winter, depending on when they were sown.

In containers, biennials can be planted from September when they are in plug plants and until November when they are sold in buckets. To learn all about planting biennials in plug plants, discover our planting sheet: “Planting Biennials in Plug Plants”.

Sowing:

Similarly, “true” biennials like Digitalis can be sown in spring, late summer, or autumn:

• if you sow in spring (March, April), the plant will have time to form a lovely rosette of leaves and will flower the following year,
• if you sow in late summer or autumn (August to October), your plant may not be mature enough to flower the following year, so you will need to wait until the third year to enjoy its flowering,

Sowing biennials for containers and ephemeral flowering generally takes place in late summer, but the ideal period may vary depending on the species. For each variety in our wide range of flower seeds, you will find specific indications to successfully sow them.

Note that the germination of many of these plants is disrupted, or even inhibited, by temperatures above 18 °C, which makes their sowing sometimes tricky. This is why it is often easier to use young plants that are already started.

How to enjoy biennial plants for long?

In containers, we first advise you to care for the planting. Using a quality, well-draining potting mix for flowering plants that is rich in nutrients is one of the primary conditions for success. To prolong and encourage flowering, remember to regularly remove faded flowers which, by producing seeds, exhaust the plants. Also, and most importantly, water regularly, even in winter, as it is rare for rain to be sufficient to water the pots, and provide a liquid fertiliser as soon as the vegetation starts to grow again in spring.

In the garden, it is sometimes possible to extend the lifespan of biennials by a year: to do this, cut the flowers before they set seed. By not expending all its energy on producing offspring, your plant may reward you with a second flowering next year. However, the simplest approach is to grow them for two consecutive years and let them self-seed as they wish.

The Pope’s Money or Lunar: a biennial that elegantly contains its seeds