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Gardening: biennials

The canny gardener is already looking ahead to 2015 and beyond; if that sounds like you, this is the month to sow biennials.

Dianthus barbatus - sweet William - is a charming biennial with variegated petals. Photograph: Shutterstock
Dianthus barbatus - sweet William - is a charming biennial with variegated petals. Photograph: Shutterstock

Such plants take two years to grow and produce seed, so you need to be patient. In the first year, they produce foliage - usually a low rosette of leaves - and grow a fleshy tap root to store starches and sugars for the following year. After a period of dormancy during the winter, they grow strongly, and flower and set seed in this second summer.

You can treat many of these plants as annuals by buying flowering specimens and enjoying a few weeks of flowering. You then compost them and buy replacements the following year. Frankly, this is an expensive option. It makes more sense to grow the plants from seed and enjoy a fine display in the second year. Simply sow seed now in open ground to give the young plants time to establish before winter. Transfer the seedlings to their final position at the end of August, when the soil is still warm.

Which? Gardening trialled several verbascums in 2012-13 and found many didn't even survive the first winter. This is no great loss if your outlay was only a packet of seed. They conducted trials at Alnwick in Northumberland as well as Capel Manor in north London and, out of 24 varieties, recommend two as best buys.

"Gainsborough" survived the bitter winter and did well in its second and final year. It produced tall spikes of delightful pale lemon flowers with deeper yellow centres. This bee-friendly mullein sits well at the back of any border. The Which? team also reckoned "White Dominic" was a worthy plant. I'm not surprised bees were most attracted to the flowers' creamy-white outers and striking dark pink inners.

Other biennials such as wallflowers, foxgloves, sweet William, forget-me-nots and Canterbury bells are greatly underrated garden plants. So, let me sing their praises. I always include mathiola, stock, in my sowing programme. The delightfully scented Cinderella series boasts a fine range of colours: light, silvery and darker blues, lavender, pink, red and white.

Campanula medium, Canterbury bells, produce a mass of bell flowers. Deep blue is my favourite, but pink, white and purple are also attractive. In Victorian floriography - the language of flowers - campanulaceae represented gratitude, a feeling I have when looking at a blaze of these specimens. And I couldn't do without Dianthus barbatus, sweet William, with its distinctive patterns of variegated colour - tawny-white, pink-white-lavender and almost black.

Digitalis, foxglove, is another tall specimen plant. One of the most popular varieties is the subtle Digitalis purpurea f albiflora. Unusually, the inside of its immaculate white flowers are tinged with green.

Another two-year Which? trial assessed how well foxgloves worked as biennials. Given the severity of the 2012-13 winter, it's perhaps no surprise that most plants died. But for hardiness, D parviflora "Milk Chocolate" and D ferruginea "Gigantea", with its fat cream petals were best.

It's claimed that, as short-lived perennials, some foxgloves will last a few more years than the more traditional biennials. This was claimed for "Illumination Pink", which won Plant of the Year at Chelsea in 2012. But all five plants grown by Which? died after two years, so it's probably best to treat all foxgloves as biennials.

In the same way, erysimums (wallflowers) may survive more than two years. But they often suffer from virus attack, which weakens them and stunts growth. They also tend to become a bit leggy, so they're nearly always worth removing after flowering.

You may also enjoy a long flowering period - mine kept going for most of last year. E cheirii cultivars have yellow, orange and primrose flowers, but my favourites are "Blood Red" ,with dark, velvety red flowers, and mound-forming E asperum "Breddon". Its reddish-brown buds open to a rich yellow.

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