Hardy Rubber Tree

About

Tear a leaf of Eucommia slowly in half and the two halves stay connected by fine white threads that stretch across the gap like cobweb. Those threads are gutta-percha, a hard latex, and no other hardy tree does this. It is the most satisfying identification test in this collection and it works on any leaf at any time of year the tree is in leaf.

The tree is the only living species in its family, the Eucommiaceae, with no close relatives anywhere. Fossil records show the family was once widespread across the northern hemisphere, and a single species in central China is what remains. Like the houpo magnolia it is regarded as extinct in the wild, surviving because the bark is valuable enough to be worth planting.

As a garden tree it is under-used and thoroughly good. Hardy to minus 25 deg C, untroubled by chalk, drought, pollution or poor soil, free of almost every pest and disease, with clean glossy elm-like foliage. It has no autumn colour to speak of, which is the only thing to be said against it.

Appearance and character

Leaves are alternate, elliptical, 8 to 16 cm long, pointed, with finely toothed margins and a slightly quilted surface between the veins, glossy dark green above. The resemblance to an elm leaf gives the species its name, ulmoides, and the leaf base is symmetrical where an elm’s is lopsided.

The rubber threads are in the leaves, the bark and the fruit. Tearing a leaf slowly across reveals them at once, and they hold the halves together for several centimetres. Children find this reliably impressive.

Flowers are inconspicuous, without petals, appearing with the leaves in spring on separate male and female trees. Female trees carry flat winged fruits like small elm samaras, 3 cm long, which persist into winter. Bark is grey, becoming ridged with age.

Where it is from

The species is native to central China, in Sichuan, Hubei, Shaanxi, Guizhou and Yunnan, in mountain forest between 300 and 2500 m. No wild population has been confirmed for a long time, and every known stand is planted or derived from planting.

The bark is one of the standard items in Chinese herbal practice, used under the name du zhong for two thousand years, and the plantations that supply it are the reason the species survives. Harvest strips bark from mature trees, and it is done in a way that allows the tree to recover rather than killing it.

It was introduced to Western cultivation around 1896 and has proved much hardier than its origin suggested. It grows well across Britain and northern Europe and is used as a street tree in parts of the United States.

How it grows

Growth is moderate, around 30 to 50 cm a year in reasonable conditions, and a tree reaches ten metres in about twenty-five years. The crown is upright when young and rounds out with age into a broad dome.

The tree is dioecious, with male and female flowers on separate plants. Neither sex offers much ornamentally and the fruit on a female tree is unobtrusive. Seedlings are uncommon outside its native range.

Roots are deep and fibrous rather than shallow and fleshy, which is part of why the tree tolerates drought, compaction and root disturbance far better than a magnolia does.

Through the seasons

Plant in November while dormant, in an open sunny position, staking for the first two years. Water through the first two summers.

Leaves come out in April and hold through until they drop in late autumn, usually without colouring. The flowers in spring pass unnoticed.

There is no seasonal maintenance to speak of. Prune in late winter only to remove crossing or damaged branches. The tree needs no feeding, no spraying and no attention once established.

Care difficulty

As easy as any tree in cultivation. It tolerates chalk, clay, sand, drought, cold, wind and urban pollution, and it has no significant pests.

The only genuine failure mode is waterlogging. Ground that stays saturated through the winter rots the roots, and that is the one condition to avoid.

Availability is the practical obstacle. It is uncommon in the nursery trade and usually has to be sought from a specialist, which is a shame for a tree this useful.

Watering

Water through the first two summers after planting to establish the root system. After that the tree is drought tolerant and needs nothing in a temperate climate.

Deep infrequent watering is better than frequent light watering during establishment, since it draws the roots down.

The tree tolerates dry conditions well and waterlogging badly, so if the soil is heavy, plant on a slight mound rather than in a hollow.

Warning signs, diseases and pests

Almost nothing attacks it. Scale insects appear on young stems occasionally and squirrels bark young trees in some gardens, and neither amounts to much.

Honey fungus is the disease that kills established trees, as with most garden trees, and Eucommia has no particular resistance. Thinning crown, white fungal sheets under the bark at the base and black bootlaces in the soil are the signs.

A tree that is failing without those signs is almost always in ground that holds winter water. Check the drainage before looking for anything more complicated.

Growing environment

Full sun and an open position. The tree grows in light shade and makes a poorer shape for it.

Soil can be almost anything that drains. It grows on chalk, clay, sand and poor urban soil with equal indifference, across a wide pH range.

Hardiness is not a consideration. The tree survives minus 25 deg C and tolerates wind and coastal exposure better than most large deciduous trees.

Feeding

None. The tree grows on poor ground without complaint and feeding produces no benefit.

A mulch of compost around a newly planted tree, kept clear of the trunk, helps through the establishment years.

Where a young tree is growing slowly, the cause is usually grass competition at the base or a stake left too tight. Both are easier to fix than anything nutritional.

Troubleshooting

How do I do the rubber trick? Tear a leaf slowly across the middle rather than snapping it. Fine white threads of gutta-percha stretch between the two halves and hold them together for several centimetres.

Is it related to the rubber plant? No. Ficus elastica, the rubber plant, is a fig, and Hevea brasiliensis, the commercial rubber tree, is a spurge. Eucommia is the only species in its own family and its latex is a hard gutta-percha rather than elastic rubber.

Why was it never used commercially for rubber? The yield is low and the latex is hard rather than elastic, so it behaves like gutta-percha, which is useful for insulation and golf balls rather than tyres. Extraction has never been economic.

Is it hardy? Yes, to about minus 25 deg C, and it tolerates wind, drought, chalk and pollution. It is one of the toughest ornamental trees available.

How big does it get? Twelve to twenty metres over many decades, with a broad rounded crown. Allow it space.

Does it have autumn colour? Little worth the name. The leaves fall green or dull yellow, which is the one criticism of an otherwise excellent tree.

Where do I buy one? From a specialist tree nursery. It is uncommon in general garden centres despite being easier to grow than much of what they stock.