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AGM plants have been through a rigorous trial and assessment programme. They are:
Biennials are some of our most valuable spring and summer flowers. Here's a selection of plantsman Graham Rice's favourites, all winners of the RHS Award of Garden Merit
Generally speaking, biennial seeds are sown in summer and the plants develop though the rest of the season; so that by the time winter sets in they will have made substantial plants. Then, in early spring, they’re off to a flying start and provide us with a colourful display.
That’s how true biennials behave, but there are also some annual and short-lived perennial plants that we can treat in the same way. My choice of plants to grow as biennials includes plants from both groups, all of which have been awarded the prestigious RHS Award of Garden Merit.
H1 - H7 indicated the new hardiness ratings
Full details of hardiness ratings (510kB pdf)
Seed of columbines can be sown in spring and, although plants may flower later that same summer, the plants are often small and the flowers less than impressive. They flower far more prolifically when sown in summer and have the chance to build up strength for a dazzling display the following year. ‘Florida’ has very large yellow and white bicoloured flowers that tend to look up rather than hang their heads. 70cm (28in). H5.
Although a short-lived perennial in its wild southern Chinese home, this is a plant at its best grown as a biennial. Its branched forget-me-not like sprays of clear sky blue flowers, held above soft greyish foliage, open for many weeks from a late summer sowing. Although the individual flowers are small, the overall display is delightful and they are lovely with tall tulips, in borders with perennials, and they also make long lasting cut flowers. 60cm (2ft). H3.
The polyanthus is an indispensable spring flower, sailing through soggy winters to bring us brilliant spring colours, sometimes even emerging to flower through the snow. Clusters of bright, deep scarlet flowers boast dramatically contrasting yellow centres and are held above the rosettes of deeply-veined foliage, well away from any mud splash. They flower prolifically for many weeks in spring. 'Rainbow Cream Shades' and 'Rainbow Blue Shades' have also been awarded AGMs. 25cm (10in). H5.
This is a classic case of a hardy annual that performs better when treated as a biennial. Sow in late summer and the plants start to bloom earlier the following season and flower far more prolifically than those sown in spring. The sky blue ‘Miss Jekyll’, which also boasts an AGM, is better known; but ‘Miss Jekyll Alba’ is a lovely thing - with pure white flowers featuring a neat, magenta-purple eye. 45cm (18in). H7.
Three different cultivars of the truly biennial honesty have been awarded AGMs, but 'Alba Variegata' is unique in its combination of features. From its bold overwintering rosette of rough, heart shaped leaves the upright stems carry pure white flowers, instead of the usual purple. These are followed by the familiar 2.5-5cm (1-2in) rounded silver seed pods which can be dried for the house. In addition, all the leaves are variegated – edged in white. 90cm (3ft). H7.
One of the most popular of all plants with bees, this intensely coloured form of our native biennial viper’s bugloss is usually grown as an annual, often reaching little more than 30cm (1ft) in height. It performs far more impressively if grown on its natural cycle, as a biennial. From a rough and rasping rosette, upright spikes of vivid blue flowers open over many weeks, developing pink tints as they age. 60cm (2ft). H7.
Over the centuries, our native foxglove (a true biennial) has yielded a huge range of different forms and colours; but probably the most appealing of them all is f. albiflora. It has two features which make it universally desirable. Firstly, it retains the elegance of the wild foxglove, with the flowers held in one-sided spikes that turn downward slightly at the tips. Secondly, its flowers are pure white with no spots and with no purple tints. Lovely. 1.2–1.8m (4–6ft). H7.
We always think of delphiniums as dependable border perennials, but those intended to be raised from seed are splendid when treated as biennials. It is important to realise that some seed-raised delphiniums are much better than others – and this where the AGM is so vital. 'Centurion Sky Blue' was stunning in the latest Wisley trial, with tall and slender, well-filled spikes in shades of sky blue and pale blue. 1.8m (6ft). H5.
Iceland poppies, P. nudicaule, are naturally perennial but they’re short-lived, and after a dramatic first season of flower often perform poorly the following year. As biennials, they’re stunning - and ‘Summer Breeze Orange’ is perhaps the most vivid of all. Developed in Norfolk in the 1980s, the yellow-centred vibrant orange flowers open in huge numbers over many weeks in summer. Although this was developed as an annual, as a biennial it’s spectacular. 35cm (14in). H7.
This short-lived perennial, always more effective when grown as a biennial, is famous for its ghostly silver-white flowerheads which combine so well both pastel shades or brighter and more brilliant colours. Its rosettes of bold, heart-shaped green foliage stand out through the winter, then in summer the branched heads with their jagged silver bracts surround silvery green cylinders of flowers. It will self-sow, too. 60-90cm (2-3ft). H7.
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Join the RHS today and get 12 months for the price of 9