False acacia

False acacia: how do you recognise it?

Habit

Tree of 15–25 m with an open, irregular crown; often forms clumps through root suckers.

Leaf

Alternate, pinnate with 9–19 oval, smooth-edged leaflets and a terminal leaflet. Two sharp spines at the leaf base.

Bark

Grey-brown and deeply furrowed in a network pattern, often twisted.

Flower

Hanging clusters of white, highly fragrant pea flowers in May–June; loved by bees.

Fruit

Flat brown pods 5–10 cm that stay on the tree for a long time.

Difference from honey locust

Honey locust has much smaller leaflets without a terminal leaflet.

Flowering: June · Harvest and fruit: May–June · Pruning: June–August

Identify by feature: Pinnate leaves · Hanging flower clusters · Pods · Thorns or prickles

Looks like it, but isn’t

• Honey locust Gleditsia triacanthos: much smaller leaflets without a terminal leaflet. Difference from honey locust

Why grow it?

• Why people love it: the white flower clusters in May–June are a top plant for bees, the source of ‘acacia honey’.
• It gives the most durable home-grown timber: posts, play equipment and garden furniture without treatment.
• It grows on poor, dry sandy soil and in towns, where few other trees thrive.
• Why better not: it spreads into the wild through root suckers and it is poisonous. A native tree such as small-leaved lime (for bees) or sessile oak (for dry sandy soil) is a better choice.

False acacia: where does it grow best?

• Full sun; in the shade of other trees it dwindles.
• Any soil, even poor, dry sand: through root nodules it fixes nitrogen from the air.
• That is exactly why it doesn’t belong near heathland, dunes, nutrient-poor grassland or woodland edges: there it enriches the soil and crowds out the original plants.
• Allow for a crown 12 m wide and for root suckers many metres from the trunk.

Light

Full sun

Legal basis

Any garden soil

Moisture

Dry

Size

25 m tall, 12 m wide

Hardiness

Throughout Belgium, no protection needed

Goes well with

• Small-leaved lime: A native alternative for bees: it flowers in late June–July and gives lime honey.
• Sessile oak: A native tree for the same dry, poor sandy soil.
• Rowan: A small native tree with pinnate leaves, white flowers and berries for birds.
• Silver birch: Grows just as fast on poor sandy soil, but is native and not invasive.

Good and bad companions

Goes well with

• Tolerate the shade under this tree: oregon grape, rhododendron, oleaster, japanese skimmia, garden privet, camellia, laurustinus, box-leaved honeysuckle

False acacia: when and how to plant it?

1. Plant a native tree instead: small-leaved lime for bees, or sessile oak, silver birch or rowan on dry sandy soil.
2. If you still choose a false acacia, for example in a town garden far from natural areas, plant it from November to March, in full sun.
3. Don’t plant it near a field with horses or other livestock, or where small children play: bark, leaves and seeds are poisonous.
4. Don’t lay paving or make borders right next to the trunk: damaged roots produce even more suckers.

Care

Suckers

Mow or cut suckers every year, as low as possible, and pull up young shoots roots and all. This exhausts the roots.

Feeding

Never needed: it makes its own nitrogen.

Watering

Only in the first year during a long drought.

Pods

Clear up fallen pods where children or animals go: the seeds are poisonous.

Old trees

The wood is brittle: have old trees near paths or houses checked regularly for cracks and dead branches.

Tip: Want durable garden timber without treatment? Buy false acacia wood for posts and garden furniture, rather than planting a tree yourself.

False acacia: when to prune it?

Pruning months: June–August.

When: Dead, diseased and broken branches can be removed at any time of year. Preferably prune in summer.

How:

• Prune as little as possible: hard pruning or felling triggers masses of root suckers.
• The wood is brittle: remove dead and split branches in good time and check old trees for cracks.
• If you want to get rid of a false acacia, don’t simply fell it, but let it die off first by ring-barking it (see Problems).

Caution: Wear gloves: the twigs have sharp spines, and bark and leaves are poisonous. Don’t leave prunings where horses or livestock can reach them.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Better not

False acacia propagates itself all too well. Don’t pot up suckers or seedlings or give them away; plant a native tree instead.

How it spreads

Mainly through root suckers, which come up many metres from the trunk, especially after felling or pruning. Seeds germinate on bare, disturbed ground.

False acacia: how does it get through the winter?

Hardy throughout Belgium: no protection needed.

• Fully hardy.
• The pods hang on into winter: together with the deeply furrowed bark, they help you recognise it even in a bare crown.
• In winter you can spot suckers and seedlings by the spines on the twigs.

Diseases and pests

• Root suckers: Dozens of shoots around the tree, especially after felling, pruning or digging. Mow or cut them every year and pull up young shoots.
• Removal: A felled false acacia sprouts en masse. Ring-bark it first: remove a wide strip of bark around the trunk, but leave a narrow bridge. The tree then weakens slowly; remove the bridge a year later and only fell the tree once it is dead. Have large trees dealt with by a professional.
• Branch breakage: The wood is brittle: in storms, branches break and old trunks split. Have old trees inspected.
• Leaf miners: Small pale blotches and tunnels in the leaflets in summer, made by the caterpillars of tiny moths. Harmless to the tree.

Through the year

• March: Pull up last year’s suckers while the soil is soft.
• May: Flowering: bees in the white clusters; the flowers are edible.
• June: Flowering ends; summer pruning of dead and split branches; mow suckers.
• July: Summer pruning; mow suckers again.
• August: Last month for summer pruning; pods ripen.
• September: Clear up pods where children or animals go.
• November: Planting season: choose a native tree instead of a false acacia.

What can you use it for?

Wood

The most durable home-grown timber: posts, play equipment and garden furniture without treatment.

Food and drink

The white flowers are edible, for example as fritters.

For wildlife

A top plant for bees: the source of ‘acacia honey’.

Caution: Bark, leaves and seeds are poisonous. Spreads into the wild through root suckers.

Edible and useful

Flowers (May–June)

Pick the white, sweetly scented flower clusters in late May and early June, when most of the flowers have just opened. They taste sweet, of honey and fresh pea pods. Fritters are the classic; for syrup, pull the flowers off their stalks.

Storing

Use the flowers the same day, as they brown quickly. Syrup keeps for a year in sterilised bottles.

Caution: Only the flowers are edible. The bark, leaves, pods and seeds of black locust are poisonous: do not eat the flower stalks and do not eat too many flowers at once. Do not confuse black locust with laburnum (yellow flower clusters, very poisonous) or wisteria (blue-purple clusters, poisonous): black locust has white flowers, pinnate leaves with oval leaflets and paired thorns on the twigs. Pick well away from busy roads and sprayed fields, and never eat anything you have not identified with certainty. Black locust is an invasive non-native species: do not plant it near natural areas.

Eating and using is at your own risk: see the terms of use.

Recipes

• Acacia-flower fritters (Dish · 4 people · 30 min)
• Acacia-flower syrup (Syrup · ± 1 litre · 30 min + 24 hrs steeping)
• False acacia blossom liqueur (Liqueur · ± 800 ml · 20 min + 2 weeks steeping + 2 weeks maturing)
• Yoghurt cake with false acacia blossom (Dish · 1 loaf cake of 25 cm · 20 min + 50 min baking)

Good for wildlife

Bees

Nectar and pollen for honeybees and wild bees.

Did you know?

• Native to North America and naturalised here; on sandy soils it crowds out other species. Bark and seeds are poisonous.
• ‘Acacia honey’ actually comes from false acacia: true acacias grow in warm countries.
• The tree is named after Jean Robin, gardener to the French king, who grew it in Paris around 1600.
• Through root nodules it fixes nitrogen from the air; this enriches poor soil, after which nettles and brambles crowd out the heathland plants.
• The name honours Jean Robin, gardener to the French king. A false acacia he planted in Paris around 1601 still stands, propped up, in Square René-Viviani.
• Like other legumes it hosts bacteria in its roots that fix nitrogen from the air. This enriches the soil, so nettles and brambles thrive beneath it.