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RHS Journals

The Garden
February 2005

Fast and flamboyant

Architectural plants need not be expensive, imported, aged specimens. Phil Clayton salutes the versatile annuals, biennials and perennials that can be easily grown from seed for similarly bold effects

Images: Tim Sandall

One of the strongest trends in planting design over the last decade has been the rise in the use of ‘architectural’ plants. Referred to almost everywhere in gardening today, these plants have caught people’s imagination, even non-gardeners, and now books and entire nurseries are devoted to them.

Architectural plants are an indistinct group, collected together purely on their aesthetic qualities. They are usually impressive individuals with bold outlines and strong forms, and many provide year-round impact. They are eye-catching and often exotic-looking, although companion planting is also important - if you use too many architectural plants together, then their drama can be lost.

Despite the fashionable new name, the use of these plants is not a recent idea; and though to many the epithet may be ‘designerspeak’, bold and sculptural plants have a genuinely important role to play in the garden, creating excitement and injecting poise to plantings.

Garden designers have championed the use of these plants; the thousands of palms, phormiums and bamboos sold every year pay testament to that. Large individuals are preferred, usually with a hefty price tag, but expensive, often slow-growing plants are not the only ones with architectural appeal - some can be grown easily and cheaply from seed for impact the same year.

Specimen annuals

Recently a plant of Nicandra physalodes (shoo-fly), similar to its cultivar ‘Violacea’, seeded itself into a crack in the pavement close to where I live, causing a stir with horticulturally minded passers-by. Few could identify it, but many admired its striking exotic looks and bell-shaped mauve flowers held on upright black stems. It made a superb architectural plant, reaching about 1m (39in) before the frosts. Shoo-fly relishes a sunny spot and is easy to grow. It is a true annual, completing its life cycle in a single year, and so by nature
is quick-growing. Indeed, many annuals develop into large and striking specimens, while other speedy growers cultivated in the UK as annuals are actually perennials, but too tender for our winters.

Striking annual Zea mays ‘Quadricolor’ has leaves and cobs streaked pink, white and yellow (see below) and makes an impressive sight filling a gap in a mixed border, here planted with pink Hydrangea and Crinum x powellii

Surprisingly seldom seen now that other grasses have become so popular is Zea mays ‘Quadricolor’ (ornamental maize). While this easy-to-grow plant is actually a kind of sweetcorn, it has foliage variegated with pink, yellow and white, creating almost a rainbow effect. The feathery male flowers appear at the top of upright stems 2m (6.5ft) high, and by the end of summer ornamental cobs appear lower down. Plants are best sited towards the back of an herbaceous border, more effective grouped than used singly. They are ideal at filling gaps left by earlier plants such as Crambe and Oriental poppies.

With its rigid, linear outline it creates a similar effect to a cordyline or phormium. As such, it associates well with frothy plants such as mauve-blue perennial Aster turbinellus or various thalictrums, or else position it in a more prominent spot and underplant with something low and shrubby such as Mirabilis jalapa with flowers in pink, mauve and yellow similar to the hues found in the leaves of the maize. Zea mays is quick to raise from seed sown in small pots in March at 18°C (64°F). Not all seedlings have strong variegation so only plant the brightest.

Somewhat different is Tithonia rotundifolia, an annual from Mexico rather like a giant zinnia, with strident orange flowers lasting from midsummer well into autumn, and bold rounded leaves. It will reach 3m (10ft) high and needs a warm, sunny, fertile spot, but there are smaller selections such as Tithonia rotundifolia ‘ Torch’ and ‘Goldfinger’. It looks good towards the back of a border, mixing nicely with grasses such as Miscanthus sinensis ‘Zebrinus’ or with kniphofias and heleniums for a hot combination, but watch for greenfly which can attack foliage. Sow seed at 13–18°C (55–64°F) in spring.

Atriplex hortensis var. rubra (purple orach) is grown for the rich purple colour of its foliage. It is an upright, narrow plant reaching 2m (6.5ft) and will seed around freely once established. Its flowers are insignificant, but where it pierces through lower plants, the intense colour and unusual form are effective. Use it as a scaled-down version of columnar conifer Cupressus sempervirens (Italian cypress).

Other annuals with architectural potential include Cleome hassleriana (spider flower) with mauve or white flowers; Amaranthus (love-lies-bleeding) with drooping red tassels of flowers; and sunflowers, of which there are many fine selections, some with double flowers, others with red or even white blooms. Annual architectural plants often have more impact planted in groups of three or five, rather than grown singly.

Perennial or annual?

Ricinus communis ‘Gibsonii’ has rich purple foliage that associates well with the strident blooms of Dahlia 'Bishop of Llandaff'

Of the tender perennials usually grown as annuals, many have excellent architectural qualities. Ricinus communis (castor oil plant) is one of the best known with huge deeply lobed leaves 30cm (12in) across, held on stout plants 1.5m (5ft) high. It has long been used as a ‘dot plant’ in bedding displays, but fits better into mixed borders, filling unsightly gaps. Ricinus communis ‘Impala’ has leaves of darkest purple and is best complemented with paler colours; it looks particularly good with tall variegated grass Miscanthus sinensis ‘Cosmopolitan’ or against plants with silvery leaves, such as shrubby Artemisia ‘Powis Castle’.

Ricinus communis ‘Gibsonii’ (above) is almost as dark; ‘Carmencita’ (right) has bronze foliage with red veins, while ‘Zanzibariensis’ has huge leaves with white veins. The spiny seed capsules of Ricinus are attractive, but beware: all parts of the plant are poisonous. Seeds germinate quickly at temperatures of 20°C (68°F) or higher; plant seedlings in early summer. Plants may be lifted in autumn and kept inside, making larger, more spectacular specimens the following year.

You may feel flowers are also important. Nicotiana sylvestris (left) is an impressive tobacco with large floppy foliage, slightly sticky to the touch. The mounds of growth reaching 2m (6.5ft) high are topped in midsummer by long-lasting heads of white flowers that are scented at night. It is excellent in a border, associating with many plants, especially those with a more open, delicate appearance such as annual cosmos or Verbena bonariensis. However N. sylvestris can be used as a stand-alone specimen plant once it has started to bloom, as the foliage is held to the ground. Sow seed in March-April at 18°C (64°F).

Other plants grown as annuals that bloom and have architectural qualities include Leonotis leonurus with orange flowers held in whorls around 2m (6.5ft) stems, and Datura inoxia ‘Evening Fragrance’ (right) which produces huge, silvery trumpet flowers, that open in late afternoon and have a delicious perfume, on a handsome plant furnished with soft, velvety grey, heart-shaped leaves. Both grow well in large pots, allowing them to be moved about to strategic effect, but they cannot be planted out until all risk of frost has passed.

Brilliant biennials

Biennials are neglected by gardeners; many feel growing a plant from seed, yet seeing little reward until the following year, after when the plant will ultimately die, is a waste of time. Though in truth biennials often need longer to build up steam than annuals, such is the brilliance of their display that many fall into our ‘architectural’ category.

Alcea rosea (hollyhock) is usually thought of as a biennial, with tall spires of open, bell shaped flowers in usually pastel shades. It has long been used by cottage gardeners to add structure and impact to plantings, and gives an indication of what may be achieved with architectural biennials. Verbascum bombyciferum is of similar stature but altogether more refined. It needs a well-drained, poor soil in full sun to give its best. In the first year the plant makes a wonderful rosette of felty, bright silver leaves, eye-catching in itself, but in the year following it produces a branched spire of bright yellow flowers up to 2.4m (8ft) tall. Seedlings spring up here and there, and this random effect can throw up interesting plant combinations. It is memorable at Chatsworth, Derbyshire, where it grows in gravel paths by the house and holds its own well amid all the grandeur.

If rust is the Achilles heel of hollyhock, that of Verbascum is mullein moth, caterpillars of which can destroy plants in days.

Also silver in leaf is Onopordum acanthium, a statuesque and handsome thistle. In the first year, rosettes of spiny leaves are formed, while in the second the angular and branched flowering stem can soar to 3m (10ft); purple flowerheads popular with bees are produced in summer. It is superb towering at the back of a border, but the plant is well furnished with foliage down to its feet, so it can be used in the open where its full impact is seen. It enjoys full sun and a fertile, well-drained soil. Plants seed around when happy.

There are other biennials with similarly architectural flowering stems such as angelicas including purple Angelica gigas and green A. archangelica, both of which resemble colossal cow parsleys and make spectacular plants flowering in early- to midsummer. For sun in the mildest gardens, Echium pininana, a true horticultural skyscraper, forms rosettes of leaves the first year and rocketing spires of blue flowers 4m (13ft) tall in the second.

Not all architectural biennials are monsters. Eryngium giganteum (Miss Willmott’s ghost) despite its name is a well-behaved sea holly with stiffly upright stems, spiny silver leaves and heads of blue flowers. The whole plant rather resembles a candelabra, reaching 1m (39in). It seeds freely and associates with many plants such as alliums, blue grass Elymus canadensis, and white rosebay willowherb. Some biennials which grow wild in Britain are worth considering, such as Dipsacus fullonum (teasel) with its prickly flowerheads to 1.5m (5ft) that are effective as winter skeletons, although it can be invasive. Bizarre-looking Euphorbia lathyris (mole plant) has upright stems, striped leaves held at right angles, and in the second year, bright green flowers. It seeds freely but is easily controlled.

Fast-growing monsters

Perennials that establish quickly from seed also provide architectural elements in the garden. Cynara cardunculus (cardoon) is a popular back-of-border plant and grows easily from seed, although it is likely to flower only after the second year. The silver leaves form an impressive basal clump, before a tall flower spike erupts, with large thistle-like flowers. It needs plants around it at this stage, as the lower leaves become tatty; grow it behind substantial perennials such as pale yellow Helianthus ‘Lemon Queen’ or even in a shrub border for drama. Sow seed in spring in pots of seed compost kept in a cold frame.

If you can overwinter plants under glass, Ensete ventricosum (Abyssinian banana) is surprisingly easy from seed, sown in spring at 18-21°C (64-70°F). An impressive plant is quickly formed, which if kept frost free will develop into a spectacular foliage plant for the following year, with a short ‘trunk’ and fleshy leaves up to 2m (6.5ft) long. Give it sun, moisture and rich soil for big leaves. Many other perennials quickly make effective plants from seed; Acanthus, surely the original architectural plant immortalised in stone by the Greeks and Romans, Euphorbia characias with heads of lime-green flowers in spring, pink-flowered but tender Geranium maderense, and huge, scarlet Lobelia tupa to name a few.

Surely the excitement and satisfaction of growing such imposing plants from seed, seeing them perform so quickly, is more pleasing to most gardeners than the benefits of an ‘instant’ garden?

For information on how to grow from seed click here

Phil Clayton is the Garden Writers’ Guild Horticultural Journalist of the Year

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