Complete guide to black poplar trees

The name ‘black poplar’ was first recorded in the seventeenth century. Before this it was known as ‘popel’, ‘popelar’ or ‘popular’ to distinguish it from aspe (aspen) and abele (white poplar). Its fire-resistant timber was highly-prized and used to make many items, including bowls, flooring, baskets and fence poles. It was also depicted in works of art and literature. John Constable painted a black poplar in the background of The Hay Wain and poets such as William Cowper referred to black poplars in their work.

British black poplars are thought to be a subspecies (Populus nigra subsp. betulifolia), indigenous to north-western Europe. In the UK, black poplars are mostly found south of a line between the Humber and Lune estuaries. Known in past centuries as the water poplar, catfoot poplar, willow poplar or cotton-tree, it's now considered one of Britain’s most endangered native trees. There are around 7,000 black poplars remaining in the UK, the majority of which are male.

Despite its rare status, black poplars are not protected by Schedule 8 of the Wildlife and Countryside Act 1981, which prohibits picking, uprooting and destruction, or selling and advertising plants for sale. Instead, they benefit only from the protection afforded to all wild plants, and so may not be uprooted without permission of the landowner. In addition, some black poplars may be protected by Tree Preservation Orders.

How to grow black poplar

The black poplar is predominantly a wetland tree, growing on floodplains and riversides. Black poplars need to be planted in a large sunny area away from other trees to give them room to spread. Hybrid black poplar trees are often planted as timber or amenity trees. The majority of planted trees are male to avoid issues with the female catkins' prolific production of fluffy wind-born seeds.

Identifying black poplar trees

Black poplars have bright green heart-shaped leaves, although they can vary greatly in shape. Many black poplars planted in the UK are hybrids (Populus x canadensis) – a cross between the black poplar and the American eastern cottonwood (Populus deltoides).

Hybrids can be difficult to tell apart from native black poplars, though there are some key defining features. As well as its characteristic gnarled, riven bark and untidy, leaning form, Populus nigra has many bosses and burrs on its trunk, which are not present in hybrid black poplars. Hybrids often host mistletoe (Viscum album), which has not been found on native black poplars.

Black poplars could be confused with other related species such as white, grey and Lombardy poplar. Lombardy poplars (Populus nigra ‘Italica’) have leaves and twigs almost indistinguishable from black poplar, but they're columnar in form, growing to a maximum of 30m in height, with a spread of only 5-10m.

White poplars (Populus alba) are easily distinguished from black poplars when they're in leaf, as the leaves have five lobes and are whitish underneath. Grey poplar (Populus canescens) is a natural hybrid between white poplar and aspen. Black poplar bark lacks the diamond-shaped lenticels (raised pores that allow the exchange of gases) of grey and white poplars and aspen.

Black poplar trees flower in March and April. Male trees produce red catkins high up in the canopy and females produce green catkins that develop white fluffy wind-blown seeds.

Size, height and spread

Black poplars are large trees growing as high as 40m tall with a spread of around 20m. Mature trees have arching branches and a tendency to lean. Many of the UK’s black poplars were pollarded (where the young tree was pruned back to the trunk and three to five branches at a height of between 2-3m) to encourage new, straight growth which was cut for hurdles, baskets, arrows and bean sticks.

Value to wildlife

One of the benefits of the black poplar is as a wildlife habitat, especially for invertebrates such as longhorn beetles, bark beetles, flies, moth larvae (including the poplar hawk moth) and bugs. As a long-lived tree, it provides a place for many generations of birds to roost and breed. Owls and woodpeckers nest in holes in black poplars, jackdaws and other birds use the tree as a roost site, and birds such as finches eat the seeds. The catkins provide a source of pollen for bees, and honeybees have been known to build nests in hollow branches.

Growing black poplar

It's thought that black poplars were often planted as field and parish boundary trees. These days, conservation organisations and individuals are working with landowners to plant black poplars on estates, farmland and reserves to help save this declining species.

Where to plant black poplar

Black poplars are not suitable for most gardens due to their size. They grow quickly when young and develop vigorous root systems. Avoid planting within 40m of buildings so roots don’t cause problems with foundations or drainage. They should only be planted in areas where there's room for them to mature without competition from surrounding trees – black poplars are not generally thought to be woodland trees. When planting, consider the potential of black poplars to dry out surrounding ground. Female trees will produce fluffy seed, so this should be considered when selecting a planting site.

How to plant black poplar

Black poplars are available as bare-root trees in autumn and winter, and container-grown plants year-round. Plant black poplar as you would any tree. Dig a square hole and ensure the rootball sits at the same level it was in its pot or the ground (bare-root trees have a soil 'tide mark' to identify its previous planting depth), and then back-fill with soil. Firm gently and water well. Stake the tree using a stake and tree tie, with the stake fixed at a 45º angle.

How to care for a black poplar

Black poplar trees need very little care. Water well for the first two years after planting, especially in dry spells. Ensure the stake is secure and the tree tie not cutting into the bark. Mulch annually with well-rotted manure or compost, which helps prevent competition from grass and weeds.

How to prune black poplar

Populus nigra should not need regular pruning, but if you do need to do so, prune in late summer or early autumn (to avoid bleeding from cuts) to remove damaged, diseased or dead material. If black poplars are grown in hedges, they can be coppiced, and trees can also be successfully pollarded.

How to propagate black poplar

Like willows, black poplars root easily from cuttings. They're often cut as around 2m lengths of new growth (known as ‘truncheons’) which are taken and planted in late winter. Shorter cuttings can be taken in summer and planted in the ground or a pot, then transplanted into their final positions once they've rooted.

Pests and diseases

Black poplar trees attract a variety of insects which eat the foliage. However, these insects play a fundamental role in healthy ecosystems, so it's best to leave them if possible. Black poplars are also prone to fungal diseases such as bacterial canker, leaf rusts and honey fungus.