What is a Biennial?

What is a Biennial?

Biennials are easy-to-grow plants that grace gardens for two years. In the first year, after sowing, they develop into young plants and put down healthy root systems, before bursting into flower, setting seed and dying in the second year.


Most biennials are easy from seed and the two-year lifecycle makes them a cost-effective way to fill borders with a splash of colour. Because plants develop well in their first year, they’ll get off to a flying start in their second season, rewarding gardeners with an abundance of early flowers.


Many popular biennials bloom from spring until July, plugging the gap between spring bulbs coming to an end and summer bedding bursting into flower. Many are popular with lovers of cottage-style gardens and include old fashioned favourites such as foxgloves, Canterbury bells and Sweet Williams. As a general rule, biennials are sown in summer – often in June and July – so the magic of raising plants from seed doesn’t have to an end once annuals (plants that grow, flower, set seed and die in one season) have been sown under cover in spring. Another bonus is that some biennials will self-seed after flowering, so new plants will spring up in the garden without any further effort involved.


By growing biennials each year, you’ll benefit from a constant supply of young plants and spectacular displays of colour. From tall cottage garden classics to border fillers and fabulous flowers for cutting, there’s a biennial to suit every style of garden.


How to sow and grow biennials


Many popular biennials are commonly direct sown during summer using a spare patch of soil. This could be in a vegetable plot, a gap in a flower bed, or a specially prepared nursery bed (an area of soil, often at the end of a garden, that’s set aside to raise stock for next season). Good preparation holds the key to success.


Prepare the ground in advance, removing weeds and large stones, before levelling, gently firming down and raking the soil to a fine tilth. If you only have a small garden, or if there’s no space to prepare a seed bed to sow biennials, check the packet to see if seeds can be started in pots, trays or modules. Use a cane to make shallow drills (ridges) in the soil before sowing seed thinly.


Keep beds watered, and once seeds have germinated, they’ll need thinning out so they’re around 10cm apart, to provide young plants with more space to grow. As young biennials develop, they’ll need regular watering, especially during hot weather. The area should be kept weed-free, or weeds will compete with young plants for moisture and nutrients.


Biennials are sown at a time of the year when slugs and snails are often active, especially if it’s a wet summer. Gardeners will need to be on their guard against molluscs, as young growth is highly appealing to slugs and snails. By the end of summer or early autumn, plants can be transplanted (carefully dug up and moved) into their final growing positions. Consider where they should be planted, noting requirements such as sun or part-shade and select their new homes carefully before settling plants into the soil and watering well.


Six of the best biennials to grow from seed

Whether you’re looking for biennials for early season colour, cottage garden charm or sensational flowers for cutting, there’s a biennial that’ll go down a treat. Here’s our guide to tried-and-tested star performers in the garden.

Iceland poppy (Papaver nudicaule). Spectacular in summer borders and a worthy contender for cutting for the vase, Iceland poppies are easy-to-grow cottage garden superstars, rewarding with an abundance of papery flowers. Try sowing Unwins’ Poppy (Iceland) ‘Giant Coonara’, a hardy biennial that bears a vibrant mix of strong and subtly coloured flowers. Grow in full sun for best results.

Foxglove (Digitalis purpurea). Keen to plant for pollinators? You won’t go wrong with foxgloves. Bees adore the towering spires of cottage garden-style blooms, while these hardy woodland wonders infuse partially shaded areas with glorious displays of colour. Unwins’ Digitalis purpurea ‘Excelsior Mix’ (Excelsior Group according to RHS) is a magnet for beneficial insects, blooming from July until late summer in an eye-catching blend of shades.

Sweet William (Dianthus barbatus). Taking pride of place on the Royal Horticultural Society’s RHS Plants for Pollinators list, Sweet William is a must for flower beds, informal cottage displays and growing to cut for the vase. Unwins’ Sweet William ‘Early Summer Scented’ is early by name and early to flower by nature, creating a blaze of fragrant colour from April into early summer. It’s a hardy biennial, and plants thrive in a sunny location.

Canterbury bells (Campanula medium). For a blaze of colour that’s adored by gardeners and pollinators alike, Canterbury bells is unbeatable. The upright habit of Unwins’ Canterbury Bells ‘Cup and Saucer Mix’ adds height to borders, growing to 75cm (30in) and its bell-shaped, double flowers look sublime when cut for indoor displays. Ideal for part-shade, plants bloom from May to July.

Sweet Rocket (Hesperis matronalis). This rosette-forming hardy biennial is a must for informal cottage-style displays, getting the season off to a flying start when clusters of scented flowers lure pollinators from late spring into summer. Sweet Rocket ‘Enchantment’ from Unwins blooms in an array of shades, from deep-purple to pale lilac and white. This sun-lover fills borders with a fabulous sweet fragrance, which is at its finest in the evenings.

Wallflower (Erysimum). A classic springtime border filler: spikes of vibrantly coloured wallflowers set gardens ablaze with sweetly-scented flowers early in the season. Although wallflowers are technically short-lived perennials, they’re grown as biennials and can be composted once displays are past their prime. Wallflowers bear flower spikes in an array of shades, including yellow, red and orange, while plants are tough and demand little attention. Planting in an area that benefits from full sun is essential. Try 'Rubies & Pearls' for a wonderfully fragrant mix. 


Clockwise left to right: Giant CoonaraExcelsior MixEarly Summer ScentedCup and Saucer MixRubies & Pearls

Frequently Asked Questions

How can I protect young biennial seedlings from slug and snail damage? Organic slug pellets, beer traps and slug barrier granules may need to be used to protect young plants, especially during warm, damp weather.

Do I need to improve the soil before planting young biennials? As a general rule, soil improvement ahead of planting will pay dividends. Dig the area over using a garden fork and work some well-rotted organic manure or garden compost into the soil.

What’s the point of sowing biennials and waiting a year for flowers when annuals bloom profusely the same season? In their first season, biennials have plenty of time to bulk-up into vigorous plants, so they’ll pack immense flower power in their second year, while making superb border fillers.


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