Elm, <em>Ulmus</em>: planting, pruning and care

Description and botany

The genus Ulmus comprises 20 to 40 species depending on classification, consisting of deciduous trees or bush, very hardy, all native to the temperate Northern Hemisphere: Europe, China, Japan and North America. Elms belong to the family Ulmaceae like Zelkova and formerly the nettle tree (Celtis). In France, the elm was widely planted along roads and avenues from the 16th century much like the plane tree today, but has repeatedly suffered from a devastating disease, elm graphiosis, also known as Dutch elm disease.

Elm forms a handsome solitary tree with a broad domed crown sustained by large branches but also an imposing forest tree capable of reaching 30 to 40 m in height. It was indeed prized for its wood before graphiosis decimated it and was widely used to make artillery pieces during the Hundred Years’ War. An elm can live beyond 500 years. In Europe, three elm species commonly coexisted: Ulmus glabra (wych elm), Ulmus procera (English elm) and Ulmus minor (field elm), although scientific and vernacular names have often been confused.

The grey-brown bark of elms becomes channelled and cracks vertically or obliquely with age. Species such as Ulmus procera sometimes show corky ridges on the shoots, a particularly notable character in Ulmus minor var. suberosa. Elms have the particularity that the terminal shoot aborts, giving way to a lateral bud to continue branch extension, hence the sinuosity of its twigs.

Ulmus glabra – botanical illustration

The slender lateral twigs display a fishbone arista branching that gives a very graphic appearance. The scale-like buds are ovoid and asymmetrical.

Elms are generally deciduous but Asian species are semi-evergreen. The leaves are simple, ovate with an acute tip, alternate and arranged in two ranks along the shoot. The lamina is thick, doubly dentate and marked with parallel veins (sometimes branched) and shows a basal asymmetry that distinguishes the elm from the hornbeam or the beech without doubt. The leaves are rough or pubescent with a short petiole bearing stipules (tiny leaves) that fall quickly. The lamina has three points and the petiole is almost absent in Ulmus glabra; its shoots bear coarse hair and the tree produces no sucker, unlike Ulmus minor and Ulmus procera. The latter is distinguished by pubescent shoots sometimes adorned with corky ridges.

The irregular flowering appears before leaf emergence as clusters of apetalous green-purple flowers on one- or two-year-old shoots. Flowers are mainly wind-pollinated but some species such as Ulmus procera are completely sterile.

The flowers are followed by bicoloured winged fruits, the samaras, pale red in the centre and lime-green at the margin, turning brown at ripeness. They consist of a seed surrounded by a membranous wing 1 to 2 cm in diameter with a variable notch at the top depending on species. They are dispersed by wind or water in spring before leaf emergence.

Elm wood is heavy, resistant, hard and flexible, with an attractive pale to light brown colour (wych elm) or red (English elm) and a fine grain. The red, rot-resistant and frost-hardy wood of English elm (U. procera) was used in construction, to make watertight ducts, make bows or as firewood. Elm was also known for medicinal virtues, such as decoctions of bark used to treat burns and other ailments.

Leaves and winged fruits of the elm

Main elm varieties

Dwarf varieties

Ulmus hollandica Jacqueline Hillier - Elm

Small bush with contorted habit and warm autumn colours that enhances small spaces and Japanese-inspired gardens.

• Flowering time May
• Height at maturity 2,50 m

See the product sheet

Slow-growth varieties

Ulmus x hollandica Wredei - Elm

Elm variety with slow growth that will become over time a garden centrepiece with its 10 m tall columnar silhouette. Has small, undulating golden foliage very bright throughout the season. Place in partial shade to brighten area.

• Flowering time April, May
• Height at maturity 9 m

See the product sheet

Ulmus minor suberosa

Rare form of Field Elm with very slow growth becoming a large bush or small tree bearing branches covered with curious corky ridges, very decorative in winter. Foliage 10 cm long, bright green, takes superb colours in autumn. Tolerates cold well and adapts to dry, calcareous soils.

• Flowering time April, May
• Height at maturity 7 m

See the product sheet

Elm planting

Where to plant?

Elm grows in any well-drained soil, even dry and calcareous for the species minor (small elm, ormeau), although it prefers fresh, humus-bearing, rich and deep soil. Choose a very sunny position except for varieties with golden foliage.

When to plant?

Plant elm preferably in autumn, especially when supplied as bare roots. A potted plant can be planted year-round except during frosts, provided it receives good watering in summer.

How to plant?

This plant is very easy to grow. Choose a small specimen so it can establish deeply.

• Dip the pot into a bucket of water to thoroughly moisten it, or praline the roots with mud if the specimen is bare roots.
• Dig a wide hole at least three times wider than the rootball.
• Add a few handfuls of sand and gravel to ensure good drainage around the roots.
• Apply a dose of ground horn if the soil is sandy.
• Place the plant in the planting hole.
• Insert a stake without damaging the rootball.
• Replace the soil and firm down gently.
• Tie the trunk to its stake.
• Water and mulch.

Elm ‘Jacqueline Hillier’ in mixed border

Elm care and pruning

• Keep soil cool by mulching and water well and deeply during summer.
• Add compost at the edge of the crown in autumn.
• Carry out standard formative pruning by gradually raising lower branches. Remove any suckers.

Apart from the terrible disease Dutch elm disease which affects fairly large specimens, elm has few enemies. Watch for aphids that sometimes produce rather spectacular reddish, swollen galls, and mites of the genera Eriophyes or Tetraneura which create harmless pustules on the leaves.

Dutch elm disease, or Dutch disease, caused devastation among elm populations in the Netherlands around 1917. The epidemic spread to North America in the 1940s and then to Western Europe in the 1970s via the United States. It is a fungus, Ophiostoma ulmi, that blocks the vessels conducting crude sap and causes yellowing followed by sudden desiccation of the tree’s crown. This fungus is carried by small beetles, the bark beetles (scolytids), which tunnel galleries beneath the bark to lay their eggs. It seems the future of elms is once again assured thanks also to maintaining a certain genetic diversity within local populations and to cross-breeding with resistant species such as Ulmus parviflora.

Elm propagation

Propagation of elm is very delicate because on one hand seed germination is poor, and on the other hand grafting of cultivars remains specialist matter. Separation of shoots from stump or of suckers can be considered if species produces suckers, as with Ulmus minor, or else propagation by cuttings in mid-summer.

Sowing

• Do not delay sowing after seed harvest as they germinate quickly. Protect them with fine mesh if you sow them outdoors
• Place pot in partial shade while they germinate then in sun during following year.

Propagation by cuttings

• Take tips of semi-ripe shoots 15 cm long.
• Remove leaves near base of cutting.
• Insert these to two-thirds of their length, taking care they do not touch.
• Firm gently all around to remove air pockets and ensure good contact between potting compost and cutting.
• Keep them covered and in shade, for example by placing a cut transparent plastic bottle over them.
• In autumn, separate cuttings that have rooted and pot them, keeping under cold frame until spring.
• Then grow your young plants outdoors until final planting.

Uses and companion plants

These small elms with striking silhouettes or exceptional foliage, such as ‘Wredei’ deserve a prime place in the garden. Bear in mind, however, that exposure must be sheltered from full sun for golden cultivars and from wind. You can position them as solitary specimens in a modest garden, or at the centre of a bed of low or creeping bushes such as Cotoneaster horizontalis, Euonymus, creeping junipers. The Elm can also be grown in a rockery or in a container to produce pretty sinuous bonsai such as Chinese elm (Ulmus parvifolia), surrounded by Pieris, ornamental grasses or box.

An example of a planting combination: Ulmus hollandica ‘Jacqueline Hillier’, Pieris japonica ‘Forest Flame’, Cryptomeria japonica ‘Vilmoriana’ (or ‘Compressa’) and Hakonechloa macra ‘Naomi’

Larger specimens such as hollandica Wredei can also be placed at the back, in a gently sunny part of the garden.

Enhance a small tree’s remarkable character by forming a clump of three specimens to mark an avenue or entrance to a larger garden.

Create a contemporary feel by pairing the golden form of elm ‘Jacqueline Hillier’ with the purple of a purple hazel, a common hornbeam ‘Purpurea’ or a smoke tree.

Play on autumn’s fiery colours within a more informal, country-style setting by pairing it with small maples, deciduous viburnums, hornbeam ‘Orange Retz’ and Parrotia persica.