Honey locust

Honey locust: how do you recognise it?

Habit

Tree of 15–25 m with an open, broad crown and fine, airy foliage; comes into leaf late (May).

Leaf

Alternate, pinnate or bipinnate with many (20–30) small leaflets 1.5–3 cm long, without a terminal leaflet; golden yellow in autumn.

Bark

Dark grey-brown; wild trees have large, branched thorns on trunk and branches. Street trees are usually thornless.

Flower

Small greenish-yellow flowers in short clusters in June; inconspicuous, but loved by bees.

Fruit

Long (20–40 cm), flat, twisted brown pods; some thornless forms bear few or none.

Difference from false acacia

False acacia has fewer, larger leaflets with a terminal leaflet, and paired spines at the leaf base.

Flowering: June · Pruning: July–August and November–February

Identify by feature: Pinnate leaves · Pods · Thorns or prickles

Looks like it, but isn’t

• False acacia Robinia pseudoacacia: fewer, larger leaflets with a terminal leaflet, and paired spines at the leaf base. Difference from false acacia

Why grow it?

• Fine, airy foliage that casts light shade: grass and plants grow happily beneath it.
• Tolerates drought, heat and compacted urban soil: a tough tree for a hot, dry spot.
• Thornless forms such as ‘Sunburst’ (golden young leaves) and ‘Skyline’ are safe in a garden.
• The small flowers in June provide nectar for bees.
• Golden-yellow autumn colour, and after leaf fall the tiny leaflets simply disappear into the lawn.

Honey locust: where does it grow best?

• Full sun: that is where it grows best and where the leaves of ‘Sunburst’ colour most beautifully.
• Any well-drained soil, from poor sand to clay and chalk; tolerates drought, heat and compacted urban soil.
• Allow for a crown up to 12 m wide: the species suits a large garden, a park or a street; ‘Sunburst’ stays somewhat smaller.
• Lovely as a light shade tree by a patio: always choose a thornless form there.

Light

Full sun

Legal basis

Any garden soil

Moisture

Dry

Size

20 m tall, 12 m wide

Hardiness

Throughout Belgium, no protection needed

Goes well with

• Smoke-tree: A purple-leaved smoke-tree makes a lovely contrast with the golden leaves of ‘Sunburst’ and also likes a sunny, dry spot.
• English lavender: Flowers in the sun at the edge of the light crown and tolerates the same dry soil.
• Catmint: A drought-tolerant perennial that flowers for a long time for bees, even in light shade.

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

Honey locust: when and how to plant it?

1. Plant from November to March when the ground isn’t frozen; a container-grown tree can go in at any time of year, except in frost or drought.
2. Buy a grafted, thornless form (such as ‘Skyline’ or ‘Sunburst’): seedlings usually do have thorns.
3. Dig a generous planting hole, set the tree at the same depth as before and firm the soil well.
4. Put a stake on the windward side and tie the trunk loosely; remove stake and tie after 2–3 years.
5. Water generously after planting and during dry spells in the first year.

Care

Watering

In dry spells during the first two years; after that it copes even in dry summers.

Feeding

Not needed: it also grows on poor soil.

Leaf fall

The tiny leaflets fall into the grass and break down quickly: there is hardly any raking to do.

Pods

On fruiting forms, many pods drop in autumn and winter: clear them from paths and the patio.

Suckers

Cut off shoots below the graft, on the trunk or from the base, straight away: they come from the rootstock and often have thorns.

Tip: For a garden, always choose a grafted, thornless form: seedlings and wild trees develop long, branched thorns.

Honey locust: when to prune it?

Pruning months: July–August and November–February.

When: Dead, diseased and broken branches can be removed at any time of year. Formative pruning in winter or summer.

How:

• Prune little: a honey locust forms a fine, open crown by itself.
• Train a young tree on a single leader and remove competing leaders and crossing branches.
• Gradually lift the crown of a tree by a path or patio: remove one or two of the lowest branches each year.
• Wear thick gloves when handling wild, thorny trees.

Caution: The thorns of wild trees are hard and sharp: clear up prunings straight away, as they go through shoe soles and bicycle tyres.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Sowing

In autumn, take the seeds out of the pods, pour hot water over them and leave them to soak overnight; sow them in spring. Seedlings usually have thorns.

Grafting

Thornless and golden forms are grafted onto a seedling; that is a job for the nursery.

Honey locust: how does it get through the winter?

Hardy throughout Belgium: no protection needed.

• Fully hardy.
• In winter you recognise it by the long, twisted pods hanging in the bare crown and, on wild trees, by the branched thorns.

Diseases and pests

• Honey locust gall midge: Leaflets that swell into pod-like galls and drop off brown, especially on ‘Sunburst’. Unsightly but harmless to the tree.
• Thorny shoots: Shoots from the rootstock and seedlings have thorns. Cut them off as soon as you see them.
• Pods: Fruiting forms drop lots of pods. For a patio, choose a form that bears few or no pods.
• Drought in young trees: Small leaflets and early leaf drop in the first year after planting. Water until it is well established.

Through the year

• January: Formative pruning in mild weather; plant bare-root trees.
• February: Last month for winter pruning.
• March: Planting season ends.
• May: Late leafing: don’t worry if it is still bare.
• June: Small greenish-yellow flowers for bees.
• July: Summer pruning: remove crossing and low branches.
• August: Summer pruning still possible; water young trees in dry spells.
• October: Golden-yellow autumn colour; pods ripen and fall.
• November: Planting season starts; winter pruning possible.
• December: Winter pruning; clear pods from paths.

What can you use it for?

Garden and landscape

Airy street and garden tree that casts light shade; thornless forms for gardens.

Wood

Hard, durable wood.

For wildlife

The inconspicuous flowers provide nectar for bees.

Good for wildlife

Bees

Nectar and pollen for honeybees and wild bees.

Did you know?

• A popular street tree; the golden-yellow form ‘Sunburst’ is seen in many gardens.
• The name triacanthos means ‘three-thorned’: the thorns are often branched in three.
• Although it belongs to the pea family, it does not fix nitrogen from the air, unlike false acacia.
• The name honey locust refers to the sweet pulp inside the pods.
• The genus Gleditsia honours the German botanist Johann Gottlieb Gleditsch; triacanthos means ‘three-thorned’, after the branched thorns.
• The pods contain a sweet, sticky pulp, which gives the tree its English name, honey locust.