Biennial flowers: what are they and how do you sow them?

As a gardener, you are probably familiar with perennials and annuals, but there is also a third group that sounds a little mysterious: biennials. Biennial plants may be less well known, but they have something magical about them. They require a little patience, but reward you with a sea of flowers at a time of year when you would otherwise have a kind of "gap" in your cutting garden. 

What are biennial plants?
Biennial plants mainly develop leaves and roots in their first year. It is only in their second year that they bloom profusely and reveal their most beautiful colours. Think of classics such as foxglove, Sweet William and honesty.

Dianthus Barbatus (Sweet William)

Why are they so special?

• The greatest advantage of biennials: They bloom after spring-flowering bulbs such as tulips, but before most other annuals are in full bloom. This creates a beautiful “bridge” between early spring and summer.

• Strong and reliable: Because they establish a strong foundation in their first year, they can withstand a lot and survive the winter.

• Sustainable: Many biennials self-seed, allowing you to enjoy their flowers for years to come.

Digitalis Apricot Beauty and Hesperis (dame’s rocket)

When and how do you sow biennial flowers?
The best time to sow biennial flowers is from July to October. During this period, the seedlings still have time to develop strong leaves and roots before winter. Biennial flowers need a period of cold – this cold stimulus in winter is often even necessary to trigger flowering in the second year. This is called “vernalization.”

1️⃣ Sow in a sunny spot in the open ground or in seed trays filled with light, well-draining potting soil.
2️⃣ Sow the seeds in each cell and do not cover them
3️⃣ Water until the seeds germinate and the first leaves appear.
4️⃣ Harden the plants off before winter. You don’t need to bring them indoors: the cold period is actually important for their development.
5️⃣ Transplant the young plants to their permanent spot in the garden in autumn. They will overwinter there undisturbed and prepare to bloom in their second year.

Extra tip:
Biennial flowers combine beautifully with perennials and annual summer bloomers. This way, you can create a garden full of flowers from early spring to late summer!

Do you still have questions, or would you like to know which plants are best suited to your garden? Let me know – I’d be happy to help!

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Best wishes,
Tuinkabouter Chrisje