These exotic plants add flashes of brilliance to gardens and conservatories, says Miranda Kimberley.

G. juniperina – image: Flickr / Liz Jones
G. juniperina – image: Flickr / Liz Jones

Grevillea is one of those plants that has such exotic flowers it seems impossible it can be grown in our British climate. But get the conditions right and some of the varieties can be grown outside, adding a touch of brilliance to gardens, and those that are too tender can grace conservatories instead.

The genus is part of the Proteaceae family, which includes such beauties as the king Protea, Banksia and Leucadendron. All of these plants are distinguished by striking flowers or colourful bracts and Grevillea is no exception. Its flowers have been likened in shape to prawns and honeysuckle flowers, but in my opinion the coiled bottom petals resemble a small snail shell, from which protrude Grevillea's showy exserted styles. They are highly colourful and clustered in racemes that often form a cone-like structure. The flowers open in succession, so each flower head provides a long-lasting display.

There are more than 250 species of Grevillea, mostly from Australia. In habit they vary from sprawling ground cover plants to shrubs or tall trees. Like their flowers, their foliage can also be an attractive feature and is quite variable. G. robusta Award of Garden Merit (AGM), the silky oak, has ferny foliage, which has made it a popular houseplant. Many have foliage that looks remarkably like short fir tree needles, but there are larger-leaved types such as G. victoriae.

Grevillea are essentially classified as half to frost hardy. Some species can tolerate temperatures down to -5 degsC, or even -10 degsC, provided they are given the ideal conditions of an acid soil with good drainage and a sheltered position against a warm wall. The hardiest species include G. alpina, G. juniperina, G. longifolia, G. x semperflorens, G. thelmannia, G. rosmarinifolia AGM and G. victoriae. Cultivars G. 'Canberra Gem' AGM and G. 'Clearview David' are considered to be even tougher.

But many will not cope with the UK's colder climes and are best grown in pots and kept in the conservatory or greenhouse over winter. They should be in lime-free compost with added grit and placed in a position that receives bright light. Plants grown under glass may need restrictive pruning.

For cultivation outside, G. 'Canberra Gem' AGM and G. 'Clearview David' tolerate close clipping and can be used for hedging.

Grevillea should not be fed because like all members of the Protea family they are intolerant of phosphates, and even a high-nitrogen feed will contain enough phosphate to damage the plants. They only require occasional supplemental watering once established, if that.

What the specialists say

- Heather Godard-Key, owner, The Old Walled Garden Nursery, Warwickshire

"The genus Grevillea is hugely diverse, with many varieties being extremely flamboyant. We grow about 40 varieties, and there are hundreds more. I love them to bits. The key to their popularity would be their long flowering season. G. 'Canberra Gem' AGM can flower for 10 months in the right spot.

"There are only a few that are reliable in most gardens but many will do in some gardens. It depends on the garden. Having the right spot and conditions is key to their success - ie, shelter from strong winds, especially north and easterlies, and an acid soil with good drainage.

"I can recommend the deep pink G. 'Canberra Gem', the yellow G. juniperina f. sulphurea, G. rosmarinifolia (red) and G. victoriae (salmon red) for outside. Most of these need protection so will need to be grown in large pots and kept indoors for the winter."

- Paul Bonavia, owner and Grevillea National Collection holder, Exclusive Plants, Cornwall

"There are three Grevillea which are head and shoulders above the rest. The red-flowered G. alpina 'Olympic Flame', G. sulphurea with its yellow flowers, and the larger leaved G. victoriae. They are much more resistant to cold weather than all the others, if given adequate drainage.

"'Olympic Flame' is a five-star plant - it is amazingly cold-resistant. I've heard several reports that plants were not damaged this winter despite going down to -10 degsC. There's a large planting in the grounds of a hospital in Truro, that shows they can be used for landscaping. They grow big, but don't take well to being cut hard.

"The larger-leaved ones have been disappointing over the past two winters in terms of hardiness. We are reviewing whether we should continue the National Collection. 'Clearview David', which can survive down to -5 degsC, was one variety we lost a large batch of, though they were in a sheltered position."

In Practice

- John Winterson, deputy buyer, RHS Plant Centres

"Although we sell a range of Grevillea there are just a few that sell well. The top seller by a long shot is G. 'Canberra Gem' AGM.

"The award is well deserved. It is easy to grow and is a prolific flowerer with pinkish-red flowers. Our next best sellers are G. williamsonii, G. alpina 'Olympic Flame', G. 'Clearview David' and G. lanigera 'Mount Tamboritha'.

"Grevillea often start flowering very early in the year, bringing an exotic feel to the garden. They get two stars for hardiness in the RHS Encyclopaedia, which is why we recommend planting them in a sheltered spot. Most sales we have are from our Plant Centre at Wisley as opposed to Harlow Carr, our North Yorkshire site, where it is much colder in winter. In these colder areas Grevillea is best grown as a conservatory plant, but many would get quite large for this purpose."

Species and varieties

- G. alpina 'Olympic Flame' is a vigorous bushy evergreen shrub that has sharply-pointed bright green leaves and bears racemes of bi-coloured pink-red and cream flowers during the spring and into the summer. Reported to cope down to -10 degsC if given good drainage conditions. Height and spread: 1-2m.

- G. 'Canberra Gem' AGM (H3-4) is a vigorous medium-sized evergreen shrub of rounded habit, with linear, sharply-pointed leaves and deep pink flowers from late winter to late summer. Its hardiness is intermediate between H3 and H4. Ultimate height and spread: 2.5-4m.

- G. 'Clearview David' is a fast-growing, evergreen shrub with dark green leaves with silky undersides. Produces racemes of large red flowers throughout the year. Hardy in sheltered gardens. Height: 2m.

- G. juniperina is a hardy form that can be grown in the shrubs border or as a specimen. Has dense needle-like foliage and red flowers from late spring to mid summer and often blooms again in the autumn.

- G. juniperina f. sulphurea is another of the hardier forms. It has light yellow flowers massed along the stems in spring and bright green, needle-like leaves. Height: 2m. Spread: 3m.

- G. lanigera 'Mount Tamboritha' is a prostrate, dwarf form with an attractive bushy habit and pinkish red and cream flowers. Frost hardy down to -5 degsC. Height: 60cm-1m. Spread 90cm-1.5m.

- G. 'Pink Lady' is a low-growing shrub with fine prickly foliage. The pale pink flowers are in terminal spidery clusters during winter and spring. Height: 30cm. Spread: 1.5m.

- G. robusta AGM (H1+3), the Silky Oak, will become a 30m tall tree in warmer climates but is grown as a houseplant in the UK, with its young fern-like foliage.

- G. rosmarinifolia AGM (H3) is a frost-hardy form withstanding down to -5 degsC. It has dark-green needle-like leaves and crimson flowers. Height: 50cm-3m. Spread: 1-5m.

- G. rosmarinifolia 'Jenkinsii' is a rounded, evergreen shrub with stiff, narrow, bright-green leaves with silky hairs beneath. It bears short, dense clusters of pink and cream tubular flowers from late winter to summer.

- Grevillea (approx equal to) semperflorens is a cross between G. thelemanniana and G. juniperina f. sulphurea. It is a shrub with pendulous branches and gorgeous apricot and pink flowers. Maximum height: 2m.

- G. victoriae has flat, grey leaves and clusters of orange-red flowers between September and March. Becomes a medium-sized shrub and is said to cope down to -18 degsC. Ultimate height: 1.5m.

- G. williamsonii is a dense shrub with horizontal branches of grey-green foliage and yellow flowers that age to pink in short toothbrush-like clusters during spring and summer. Height and spread: 1m.

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