<em>Grevillea</em>: planting, pruning and caring for

Description and botany

Grevillea genus comprises nearly 250 species of trees or bushes, sometimes creeping, with very variable evergreen foliage. It belongs to the family Proteaceae, abundant in South Africa and including genera Protea, Banksia, Hakea, Telopea, Leucodendron. Grevilleas are mainly native to Australia with a few specimens in New Caledonia and Papua New Guinea. They are pioneer plants, with rapid growth, growing in wooded areas, wet forests and clearings. They tolerate fairly poor dry soils but dislike calcareous conditions, and require full sun to flower. Size ranges from 30 cm to 35 m in height (G. robusta) with habits from erect tree-like, dense bushy to creeping.

The leaves have a lamina with reduced surface, either needle-shaped as in Grevillea juniperina or rosmarinifolia, slightly broader with revolute margins in G. olivacea (olive-leaved), fern-frond-like finely divided (bipinnate) as in Grevillea robusta (15 to 30 cm long) or ‘Robyn Gordon’, or short succulent leaves as in Grevillea lanigera ‘Mount Tamboritha’. Foliage colour ranges from light green to dark green through silvery tones. In all cases, the leaf shows adaptations to drought whether by a waxy layer on the surface, presence of hairs on the underside, succulence, etc.

Grevillea oleoides – botanical illustration

Grevilleas generally produce abundant, very exotic flowering due to their arachnoid form, with reduced tepals rolled like snail shells opening to reveal 4 stamens at their tip and 1 long protruding pistil curved like a pin before opening. Terminal inflorescences form, in some species such as robusta, unilateral spikes reminiscent of a toothbrush, others are cylindrical and others globose. Flowers sometimes appear directly on the trunk (cauliflory). Flowering displays a whole palette of hues: red, pink, orange, yellow, white. The flowering is very nectariferous for birds and insects, which is a boon as it lasts quite long, often starting in winter followed by an explosion of flowers in spring and then a resurgence in autumn. Bushes have very rapid growth and reach adult size within 3 years. Flowering occurs from the second or third year after sowing.

Botanical species produce a lignified and hard fruit that opens at ripeness. Cultivated Grevillea produces relatively few fruits compared with the number of flowers and hybrids are often sterile.

The wood of the tree species Grevillea robusta is valued for furniture making.

Warning: some species from the ‘Robyn Gordon’ group in particular cause allergic reactions on contact in sensitive people.

Grevillea juniperina

Main varieties of Grevillea

Varieties with needle leaves

Broad-leaved varieties

Variety with creeping habit

Grevillea rosmarinifolia

Probably most widespread Grevillea thanks to good hardiness to -15°C and ease of cultivation. Its narrow bright-green leaves are studded with frilly bright-red inflorescences for many weeks from February to May. Resistant to disease and drought.

• Flowering time March to July
• Height at maturity 2 m

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Grevillea Canberra Gem

Hybrid between juniperina and rosmarinifolia, Grevillea 'Canberra Gem' has a dense habit up to 2 m in all directions and sharp, light-green needle-like leaves scattered with red flowers from March to July. Hardy to -10/-12°C.

• Flowering time April to August
• Height at maturity 2 m

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Grevillea johnsonii

It bears long, fine leaves reminiscent of pine needles in dark green. Delicate flowering occurs from late winter to late spring as erect terminal clusters composed of curious bright-pink, cream-washed flowers rolled up like snail shells and topped with long pistils. An excellent bush for borders or hedging in coastal gardens (hardy to -11°C) or for container culture.

• Flowering time April to August
• Height at maturity 3 m

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Grevillea rosmarinifolia Rosa Jenkinsii

Compact variety ideal for containers, with very ornamental light-green foliage and a beautiful bright-pink spring flowering. Resistant to disease, drought and cold, hardy to -10°C in perfectly drained soil; perfect for coastal sites, in a flowering hedge or planted singly.

• Flowering time April to October
• Height at maturity 1,30 m

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Grevillea rhyolitica

Rare Grevillea species presenting lanceolate leaves. Its slightly trailing inflorescences with coral-red tubular flowers somewhat evoke those of the firetree (Embothrium). Compact rounded habit well suited to container culture. Not very hardy (-5°C).

• Flowering time March to October
• Height at maturity 2 m

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Grevillea lanigera Mount Tamboritha

Creeping bush forming pretty groundcover with succulent, leathery grey-green foliage with a slightly wooly aspect that can spread to 1.50 m across. Its pink and cream flowering from late winter to spring brings a real touch of softness to borders and rockeries at a time of year when flowering shrubs are still scarce. Hardy to -10°C in acidic to neutral, well-drained soil.

• Flowering time March to June
• Height at maturity 15 cm

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Planting

Where to plant Grevillea?

Most Grevillea do not withstand temperatures below −5°C. Their cultivation is therefore limited to Atlantic or Mediterranean coast. The hardiest species listed down to −12 to −15°C include Grevillea rosmarinifolia, as well as cultivars such as G. juniperina Camberra Gem tolerating down to −10/−12°C with occasional dips to −15°C, and lesser-grown species here such as G. australis, G. diminuta and G. victoriae.
Plant Grevillea in full sun, or with light shade if you are in the Midi to encourage generous flowering.
They prefer well-drained, low-nutrient, acidic to neutral soils except for species robusta which tolerates limestone.
Regarding winter hardiness, everything naturally depends on other environmental factors: nature of soil, degree of moisture, duration and frequency of frost periods, presence of protective elements near the plant, such as other plants or a wall, for example…

When to plant?

Plant Grevillea preferably in spring to avoid exposing it to sub-zero temperatures before it is established.

How to plant?

This plant establishes and grows very quickly if climate and soil are suitable, but it also grows very well in containers.

1. Immerse the bucket in a pail of water to thoroughly moisten it.
2. Dig a hole three times wider than the rootball and loosen soil around with fork tines.
3. Add a few handfuls of sand and gravel to ensure good drainage around the roots. In heavy soil, opt for planting on a raised bed or within a rockery.
4. Place the plant in the planting hole.
5. Replace soil and firm gently.
6. Water.

In containers, make a mix of potting compost, heather soil and sand and choose species with limited vigour such as rhyolitica or Jenkynsii. Potting compost should be very well-drained, while retaining sufficient water. A good mix can also approximate this composition: 60% pine bark, 20% coarse river sand, 10% fine river sand and 10% clay soil to retain water. pH should be below 7.

→ Find out more in our advice sheet: Growing Grevillea in a pot

Grevillea ‘Robyn Gordon’ in its setting

Pruning and care

Monitor watering during first summers to encourage establishment and vigour of the plant. However, avoid excess water; allow soil to dry by about 1 cm in depth between waterings. Avoid using hard water, which causes yellowing of leaves.

Cover the plant with horticultural fleece to protect from cold if needed but mainly to prevent excess water at relating to root level.

In poor soil, mulch base of Grevillea in autumn with debris to a depth of 8–10 cm (dead leaves, shredded branches, grass clippings). The plant thus produces a dense superficial root hairs capable of capturing maximum minerals and water contained in humus.

In a pot, fertilise with a low-phosphorus fertiliser (P) such as 18-2-10 so as not to reduce its flowering. Take care not to subject it to drought stress, which can kill it suddenly, while at the same time avoiding stagnant water in the saucer. During winter let compost dry to about half the height of the pot (a terracotta pot makes it easy to see moisture level). Repot in spring, especially in early years, then simply renew surface potting compost.

Pruning Grevillea

Reduce shoots by one-third of their length after flowering or during growth period if you wish to keep the plant more compact.

Possible diseases

Grevillea are rarely affected by disease except for shoots that sometimes dry out or become covered with black spots following fungal attack. Promptly prune dead shoots, open up base of bush (which must not be buried) and protect it from excess moisture. Apply a copper-based fungicidal treatment to spotted foliage.

Phytophtora cinnamonii, which affects many Australian and native plants, is active during wet winters by blocking sap-conducting vessels. When first warm weather arrives the bush can suddenly dry out partly or completely. Watering soil with a systemic fungicide such as Aliette helps reduce its impact.

Yellowing of leaves is often due to excess lime in soil or in irrigation water.

Propagation

Multiplication of Grevillea is quite complicated, both propagation by cuttings and sowing. You can try sowing after scarifying seeds or by separating suckers when plant is producing suckers.

Sowing

1. Scarify seeds with sandpaper or soak seeds in water for 24 h.
2. Sow them in a tray or cells filled with seed compost moistened the day before. Cover with a thin layer of vermiculite and place in indirect light.
3. Place in a mini-greenhouse maintained between 10 and 15 °C, indirectly lit by sun until seeds germinate.
4. Pot up into buckets as soon as you can handle seedlings without disturbing roots too much.

Uses and associations

In an exotic or Mediterranean garden on neutral to acidic soil, it can be included in a large rockery with upright, architectural plants such as Agave, Beschorneria, Aloe, Cordyline australis, Canary bugloss (Echium pininana), palms, combined with bold forms such as Euphorbia mellifera, Protea, evergreen ceanothus, Melianthus major, Euryops, Callistemon and Leptospermum. The creeping form ‘Mount Tamboritha’ can carpet foreground rocks while other Grevillea species are better placed at back of border or at top of bank.

An example of a combination: Ceanothus ‘Skylark’, Grevillea ‘Robyn Gordon’, Leptospermum scoparium ‘Martini’, Euphorbia mellifera (photo H.U. Küenle)

Grevillea generally form tangled masses of rather spiny branches that can be used to create impenetrable hedges, flowering for a very long period, within 3–4 years. This full-sun, robust species, tolerant of sea spray and preferring fairly poor soil, grows perfectly by the sea.

In containers, pair it with Mediterranean plants such as Cassia florinbunda, Cape Plumbago, Leptospermum, Caesalpinia gilliesii, Sesbania punicea, palms and citrus trees, which should be brought into a cold greenhouse or a very bright, lightly heated conservatory over winter.