Elm

Elm: how do you recognise it?

Habit

Tree of 20–35 m with a broad crown about 15 m across; much smaller as a hedge or shrub.

Leaves

Alternate, oval with a pointed tip, double-toothed; one half of the leaf starts clearly lower on the stalk than the other. Often rough to the touch.

Bark

Grey-brown, furrowed; some species have corky ridges on the twigs.

Flowers

Small reddish-purple flowers in clusters, as early as February–March, before the leaves.

Fruit

Round, flat fruits with a papery wing all round the seed.

Difference from hornbeam

Hornbeam has a symmetrical leaf base and a leaf that feels smooth.

Difference from hazel

Hazel has a round, softly hairy leaf and is shrubby.

Flowering: March · Pruning: November–February

Identify by feature: Lopsided leaf base · Winged seeds

Looks like it, but isn’t

• Hornbeam Carpinus betulus: symmetrical leaf base and a leaf that feels smooth. Difference from hornbeam
• Hazel Corylus avellana: round, softly hairy leaf; shrubby. Difference from hazel

Why grow it?

• Food plant of the rare white-letter hairstreak, a small butterfly you give a helping hand by planting an elm.
• Flowers as early as February–March, before the leaves: an early source of pollen.
• A sturdy tree that copes well with wind, including sea wind.
• Resistant varieties give the classic avenue and dyke tree a second chance.
• As a field elm hedge it stays small and usually escapes Dutch elm disease.

Elm: where does it grow best?

• Full sun.
• Almost any soil, ideally rich, moisture-retentive clay or loam; chalk is no problem.
• Copes well with wind: a classic tree for dykes, coastal villages and open countryside.
• Allow for 20–35 m in height and a crown of about 15 m: as a free-standing tree only for a large garden, a park or an avenue. In a smaller garden: as a clipped hedge or a narrow columnar variety.

Light

Full sun

Legal basis

Any garden soil

Moisture

Normal

Size

30 m tall, 15 m wide

Hardiness

Throughout Belgium, no protection needed

Goes well with

• Hawthorn: Together the classic mixed hedge or hedgerow of the countryside.
• Blackthorn: Early white blossom and blue sloes: a thorny partner in a hedge for birds.
• Field maple: A native hedging plant with fine autumn colour that, like elm, takes clipping well.
• Dog rose: Scrambles through the hedge, with pink flowers and red hips.

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

Elm: when and how to plant it?

1. Plant from November to March when the ground isn’t frozen.
2. Buy a resistant variety from a reliable nursery, such as ‘Columella’ (narrow and columnar), ‘Lobel’, ‘Dodoens’ or ‘New Horizon’. A seedling or a sucker taken from the wild is not resistant.
3. Secure a young tree to a stake and keep the base free of grass for the first few years.
4. For a hedge: about 3 plants per metre, cut back at planting time in winter so that they branch from the base.
5. Water after planting and during dry spells in the first few summers.

Care

Watering

In dry spells during the first few years; rarely needed after that.

Feeding

Not needed in ordinary garden soil.

Checking

In June–July, look out for branches that suddenly wilt and turn yellow or brown: the first sign of Dutch elm disease.

Suckers

Field elm in particular produces root suckers: mow or cut them off.

Elm wood

Don’t leave pruned branches or felled wood with bark lying around the garden (see the warning).

Tip: Clipped every year, an elm hedge keeps thin stems and so usually escapes Dutch elm disease: the elm bark beetle breeds mainly in the thick bark of older trunks.

Elm: when to prune it?

Pruning months: November–February.

When: Only in winter (November–February).

How:

• In spring and summer, fresh pruning wounds attract the elm bark beetle, which spreads Dutch elm disease: so never prune during the growing season.
• Give a young tree a single central leader, remove crossing branches and lift the crown gradually; clip a hedge into shape in winter.
• Cut out diseased branches well back into healthy wood, take them away and disinfect your tools after each tree.

Caution: Don’t stack elm firewood with the bark on: the elm bark beetle breeds in the bark of dead elm wood and spreads the disease from there.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Sowing

The fruits ripen in May–June and only stay viable for a short time: sow them straight away, on the surface of moist soil. A seedling is not resistant to Dutch elm disease.

Resistant varieties

These are propagated vegetatively by the nursery, from cuttings or by grafting: that is the only way to keep the resistance.

Elm: how does it get through the winter?

Hardy throughout Belgium: no protection needed.

• Fully hardy, even on the coast.
• Winter is the only good season for pruning; by late February the first reddish-purple flowers are already appearing.

Diseases and pests

• Dutch elm disease: Leaves on one branch or across the whole crown wilt in June–July, turn yellow to brown and often stay hanging; under the bark you see brown streaks in the wood. There is no cure: have the tree felled and clear away the wood with its bark promptly.
• Elm bark beetle: A small bark beetle that breeds in diseased and dead elms. The young beetles fly out and carry the fungal spores to healthy trees. Clear away dead elm wood.
• Leaf galls: Rolled-up leaves or bean-shaped galls on the leaves, caused by aphids. Harmless.
• Suckers: Shoots popping up in the lawn or border, especially with field elm. Mow or cut them off regularly.

Through the year

• January: Pruning, only now in winter; take away the prunings.
• February: Last month for pruning; the first flowers appear.
• March: Flowering before the leaves; still planting season.
• May: Fruits ripe: sow straight away.
• June: Watch for wilting branches (Dutch elm disease); no pruning.
• July: Check again for Dutch elm disease; water young trees in dry spells.
• October: Yellow autumn colour.
• November: Planting season; pruning can start again.
• December: Pruning and clipping hedges into shape.

What can you use it for?

Wood

Tough wood that lasts well when wet: once used for water pipes, wheel hubs and keels.

For wildlife

Food plant of the rare white-letter hairstreak, a small butterfly.

Garden and landscape

Only plant varieties that are resistant to Dutch elm disease.

Good for wildlife

Caterpillars

Host plant: caterpillars of butterflies or moths eat the leaves.

Birds

Berries, seeds or insects as food, or a place to nest.

Did you know?

• Dutch elm disease owes its name to Dutch women scientists who identified the fungus as its cause around 1920.
• Elm wood is tough and lasts well when wet: it was once used for water pipes, wheel hubs and keels.
• Amsterdam still has one of the largest elm populations in the world, thanks to a strict approach to Dutch elm disease.
• Dutch elm disease is spread by elm bark beetles: small beetles that carry the fungal spores from diseased to healthy trees.
• Amsterdam has tens of thousands of elms, including many along its canals. Thanks to strict control of Dutch elm disease, most of them have been spared.