Silver Oaks ∗ The Great Guide from A to Z (Grevillee)

Characteristics

• Scientific name: Grevillea robusta
• Family: Silver tree family (Proteaceae)
• Synonyms: Grevillee, Australian Silver Oak
• Origin: Australia
• Growth type: evergreen tree, shrub
• Growth height: 3 m to 5 m
• Leaf: doubly pinnate
• Flower: bizarre claw-shaped
• Fruit: follicle
• Toxicity: slightly toxic
• Hardiness: sensitive to frost
• Use: container plant

growth

the silver oaks (Grevillea robusta) is a fast-growing tree or shrub in the silvertree family (Proteaceae). In reference to its rainy habitats in eastern Australia, the evergreen deciduous tree is also called Australian silver oak and Grevillee. The German name Siloranges is an allusion to ornamental, fern-like pinnate leaves with a silvery sheen. Spectacularly shaped flowers intensify the extravagant appearance of adult silver oaks. These key growth data explain why Grevillea robusta is held in high regard in this country as a beautiful container plant all year round:

• growth type: Tree or shrub with evergreen pinnate leaves, claw-like inflorescences and dark brown follicles.
• growth habit: erect, slender-columnar, loosely branched.
• growth height in the habitat: 20m to 35m
• Growth height as a container plant: 3m to 5m
• root: Shallow-rooted with a high proportion of proteoid roots (dense clumps of tender roots to improve nutrient uptake in Australia's poor soil).
• Horticulturally interesting properties: easy to care for, sensitive to frost, tolerates pruning, hungry for sun, slightly poisonous, fast-growing.

Australian silver oaks in their natural habitat

Sheet

A silver oak wears a beautiful foliage of filigree leaves with these characteristics all year round:

• leaf shape: stalked, doubly pinnate.
• sheet size: 20 cm to 30 cm long.
• leaf color: evergreen, deep green to bronze on top, light green underneath, with silver-grey hairs.

Silver oak leaves contain tridecylresorcinol. This toxin can cause allergic reactions when exposed to direct skin contact with sunlight.

blossom

Cultivated as a container plant, a Grevillea keeps its bizarre flowers under wraps for the first few years. This is not contradicted by the fact that an adult silver oak surprises with these flowers in the bright winter garden:

• inflorescence: racemose, 8 cm to 15 cm long, composed of numerous tubular flowers.
• special feature: widely protruding stamens produce a claw-like appearance.
• flower color: golden yellow to orange-yellow.
• heyday: January to April.
• flower ecology: hermaphrodite

Pollinated flowers turn into leathery, dark brown follicles containing one or two seeds. Flowers, fruits and seeds contain toxic cyanide compounds. Consuming large amounts can cause nausea and vomiting.

Plant silver oaks

A ready-to-plant silver oak can be bought in local specialist shops and in online shops. Prices range from 24.95 euros for a 30 cm small specimen to 249.90 euros for a 3 m tall Grevillee tree. Room gardeners favor tub planting after vegetative or generative propagation. The requirements for the optimal location must also be met north of the Alps. Read here how to successfully multiply and skillfully plant a silver oak:

Propagation by cuttings

Grevillee cuttings root faster than cracklings. The following instructions explain step by step how to do it correctly:

1. Carefully tear off a two-year-old branch from the trunk or cut it off along with a tongue of bark.
2. Fill pot with coconut soil or lime-free potting soil over a drainage expanded clay.(€19.00 at Amazon*)
3. Defoliate cuttings except for the upper pair of leaves.
4. Drill a planting hole with a pricking stick and plant two-thirds of the cuttings.
5. Moisten the substrate with soft water, spray the cuttings.

The silver oak cuttings are cultivated on a bright window seat in a warm, humid microclimate under a transparent hood. Young leaves sprout as an indication of successful rooting.

Extra tip: If there is a thin layer of leaf compost between the potting soil and the drainage, give yourself timid silver oak cuttings have more trouble with rooting in order to access the tempting nutrient buffet reach.

Propagation by seed

the sowing of silver oak seeds is a time consuming project. It can take up to 12 months for the first Grevillee seedlings to appear. The following table provides information on all important framework conditions:

Two-leaved silver oak seedlings are fertilized every two weeks with a half-strength, phosphate-free liquid fertilizer.

planting tips

commercial potting soil is not suitable for Grevillea robusta. Corresponding to the lean, slightly acidic soil of Australia, a mixture of is recommended as a substrate rhododendron soil without peat, coconut soil as a peat substitute, lava granules and sand. Other planting tips get to the heart of important details:

• The still potted root ball is placed in a bucket with rainwater before planting.
• A 5 cm high expanded clay drainage on the bottom of the bucket prevents waterlogging.
• If the coaster is covered with expanded clay balls, accumulated water can evaporate and increase local humidity.
• The previous planting depth is retained and takes into account a watering edge.
• Thorough watering on the day of planting and in the period that follows promotes rapid rooting in the pot substrate.

Location

The ideal location for a silver oak as a container plant simulates the general conditions of the Australian Coastal regions from Queensland in the southeast to New South Wales in the northeast with up to 1000 millimeters Precipitation per year:

• Sunny to shady location without hours of blazing midday sun.
• Minimum temperature: 10° Celsius
• High humidity of at least 50 percent.
• Ideal conditions in summer for rapid growth: very bright, humid, 18° to 25° Celsius.
• Ideal conditions in winter for flowering: sunny, humid, 10° to 15° Celsius.

From April to October, the Australian silver oak prefers to linger on the sunny balcony.

digression

Silver oaks in mini format

Maintain silver oaks

The Australian silver oak is easy to care for. Moderate watering, phosphate free Fertilize and a cool-temperate hibernation are the supporting pillars in the simple care program. Gloves and long-sleeved clothing protect against phototoxic skin irritation. This is how a silver oak is properly cultivated as a container plant:

Pour

• Keep the substrate constantly slightly moist from spring to autumn.
• Allow the surface of the soil to dry noticeably between waterings.
• From 18° Celsius, spray the leaves in the morning or evening with decalcified water.
• Water more sparingly in winter without letting the substrate dry out.
• Water silver oaks mainly with decalcified tap water or filtered rainwater.

Fertilize

• Australian silvertrees are fertilized with a phosphate-free liquid fertilizer (eg. B. Prote fertilizer from Flora Tuscany).
• Stir 2 grams of fertilizer powder into 1 liter of irrigation water every 10 days from March to September.
• Thoroughly water the silvery areas with the liquid fertilizer to supply the roots with nutrients right down to the bottom of the pot.

To cut

• Cut back Grevillee if necessary.
• The best time is in March/April.
• The robust cut tolerance allows a cut back by up to two thirds.
• Apply scissors just above an outward-facing leaf, bud, or bud Eye.

hibernate

Overwintering a silver oak has already been discussed several times. Read a compact summary here. How to overwinter Grevillea robusta correctly:

• Plant in pots from temperatures below 12° Celsius.
• Ideally sunny to very bright and cool-tempered hibernation at 10° to 15° Celsius.
• Recommended winter quarters: conservatory, glazed terrace, bright stairwell, unheated bedroom.
• Optionally, in heated living rooms, you can spend the winter under a daylight lamp as additional lighting.
• Winter care: water sparingly, do not allow to dry out, avoid waterlogging, do not fertilize, spray regularly.

repot

The fast-growing silver oak is repotted every one to two years. The best time is in spring. The previous bucket can be reused as long as two fingers fit between the root ball and the edge of the container. After repotting, a Grevillee is fertilized for the first time after six weeks at the earliest.

diseases and pests

The silver oak is mostly spared from an infestation by diseases and pests. It is primarily maintenance errors that cause headaches. Incorrect water supply, which leads to drought stress or waterlogging, is particularly problematic. The nutrient supply with a conventional fertilizer containing phosphate causes the lush green feathered leaves to turn yellow.

Popular Varieties

These multi-faceted cultivars compete with Grevillea robusta for the title of the most beautiful Australian silver oak:

• Robyn Gordon: compact Australian silver tree as a small shrub with red flowers from spring to autumn, height of growth up to 150 cm.
• Bank's Australian Silver Oaks: Small tree or shrub, always blooming with fiery red tubular flowers on 10 cm long inflorescences, growth height up to 300 cm.
• Johnson's Silver Oaks: Grevillea johnsonii scores with deeply cut pinnate leaves and red-orange clawed flowers from April, growth height up to 150 cm.
• Flaming Australian Silver Oaks: Grevillea rhyolitica grows waist-high, has needle-shaped leaves and pink flowers on elegantly overhanging branches, height 80 cm to 100 cm.

FAQ

Why should the silver oak be fertilized without phosphate?

Silver tree plants have adapted well to the poor, phosphate-poor soil of Australia with special roots. Conventional fertilizer causes an excess of phosphate. The uptake of other nutrients is inhibited or stopped completely, with negative consequences for overall growth. For this reason, a silver oak is fertilized without phosphate using a special fertilizer such as Protea liquid fertilizer from Green24, Protea fertilizer from Flora Tuscany or Engelhardt's phosphate-free garden fertilizer.

Is Grevillea robusta the only Australian silver oak?

Grevillea robusta is the best-known plant species within the extensive genus of silver oaks (Grevilleas). Other beautiful Grevillee species are cultivated in this country under the name Australian silver oak as a container plant. These include Johnson's silver oak (Grevillea johnsonii), juniper leaf silver oak (Grevillea juniperina) and everblooming Australian silver oak (Grevillea semperflorens).

Is a silver oak poisonous?

The silver oak (Grevillea) is declared as slightly poisonous for reasons of caution. The parts of the plant contain various substances that are harmful to health, which can trigger allergic reactions and symptoms of poisoning. However, no actual poisoning has been reported in the literature to date.