Adress
Zavelenberg Berchem-Sainte-Agathe
GPS coordinates :
50.8703 , 4.2983
Scientific inventory

Identity

Latin name :
Robinia pseudoacacia
French name :
Robinier Faux Acacia
Dutch name :
Gewone Acacia
English name :
Black locust
Family :
Fabaceae
Height :
15 m (estimation)
Targeted height :
This species can grow up to 30 m
Diameter of the crown :
14 m (estimation)
Trunk circumference :
Average 100 cm (group)
Expected circumference :
600 cm
Expected longevity :
Can live for 200 years
Origin / Indigenous
North America, eastern USA
Favorite soil :
Light, loose, sandy and fresh
Favorite climate
Temperate and continental

Usefulness and services of the tree :

Enhances the landscape :
+++
Enhances the biodiversity :
+++
Provide oxygen :
++
Purify the air :
++
Filter the water :
+
Prevents flooding :
+
Stores carbon :
+++
Softens the climate :
+++
Limits soil erosion :
+++
Does good, heals :
+++
Belgian Federal State Collection on permanent loan to the Meise Botanical Garden. <https://www.plantentuinmeise.be/en/home/> Hempel, Die Bäume und Sträucher dese Waldes, pl. 58, 1889

Features and characters of the individual

These trees are the guardians of the last agricultural plots within the urban area. They grow at the top of Zavelenberg, surrounded by cows and in the shadows of the city. In America, locusts were used to contain sloping terrain. But here, the roots are exposed, as if the sandy ground simply gave way. It’s as if this copse is trying to move…

The waders of the Zavelenberg

Clustered at the heart of the Zavelenberg, these individuals are part of its pastoral landscape made up of a small forest, pastures, hedgerows and a wet meadow. They stand in the middle of an area where a herd of cattle peacefully grazes. It is the last herd in Brussels, a vestige of a castle farm that disappeared in the 1950s and was replaced by social housing. What remains of this hedgerow landscape has been classified as a nature reserve. A dozen of the weeping beeches, lime trees and hornbeams that dot this expanse have been included in the Scientific Inventory of Notable Trees of the Region.

Controversial migrants

Their presence in this nature reserve is controversial because the species ‘Robinia’, commonly known as the black locust, is not native (to our country). It is from the south-eastern United States and was first brought to Europe in the late 16th - early 17th century. It was Jean Robin, a doctor and herbalist at the Faculty of Medicine in Paris, who established it in Europe (hence the scientific name of the species, ‘Robinia’). Often confused with African acacias, Robinia have been called ‘false acacias’ or ‘pseudo acacias’. It is true that their foliage is deceptive. With their leaves, composed of 10-12 oval leaflets with a smooth edge, they look incredibly like those of acacias, whose wood is of a lesser quality.

In three centuries, the locust tree has become particularly acclimatised here. It reproduces very easily: through dissemination of its very abundant seeds but also by its roots which produce many shoots over great distances. Today, it is often considered to be invasive, even a threat to biodiversity, because in some parts of Belgium, it competes with other tree species.

At Zavelenberg, its shadow could prevent certain native plants from growing at its foot. Oblivious to this controversy, the cows seem to appreciate the freshness of its foliage which is ample, rounded, light and airy.

Pioneers of uncultivated land

Cattle gather to graze or rest under the dome formed by the group of locusts. By treading beneath the trees, the cattle have ultimately eroded the soil to reveal their strange roots. So much so that our locusts now appear reminiscent of mangroves: those famous tropical trees or shrubs that seem to walk on legs at the edge of the sea.

Indeed, confronted with erosion, the locust trees had to develop a strategy to reinforce their stability. They have managed to create a whole system of stilts. Certain roots, just under the root collar, have turned into legs supporting the trees above the ground. Other roots are separated from the trunk, above the collar, and plunge towards the ground.

Thus, like the mangroves, it looks as if the grove is about to start walking. In reality, it is the topsoil that is vanishing beneath their feet. But there’s no risk of the whole hill washing away or of these waders migrating elsewhere. They are sunk deep into the earth vertically and extend horizontally under the surface of the ground, well beyond the perimeter of their crowns. Thus, these trees are able to anchor themselves in this loose soil and fix it through a root system that is both rotating and contouring. They help to permanently shore up the embankment.

It is no coincidence that this group of locust trees has found its place on this ‘sand mountain’ : the meaning of "Zavelenberg". The soil of this former quarry is sandy, loose, and uncultivated. It is exactly the kind of soil that locust trees prefer. They are the only ones who are able to grow in such conditions.

Like peas in a pod

If the locusts manage to grow on the poorest soils it is because they have the ability to produce their own natural fertilizer. They do this thanks to their roots. Unlike most trees, they coexist not with fungi but bacteria. These bacteria are able to capture the nitrogen from the air that circulates in the soil and turn it into nitrates that are directly accessible to plants. In return, the tree supplies the sugars they need.

This coexistence is one of the characteristics of the Fabaceae family, which includes legumes such as beans and lentils. The locust blossom is telling in this regard: it is the spitting image of a pea blossom. And its fruit, large flat brown pods, are reminiscent of bean pods. They hang among the leaves in winter, which is how you can recognize the locust. They contain small flat discs: the seeds of the tree.

Thus, the locust, an outsized legume itself, helps to transform uncultivated soils into fertile soils. It produces the organic matter it needs and generously shares it with other plants around it. It is a great ally of our vegetable gardens and meadows.

The sweetest of flowers

So, locusts don't just have enemies. On the contrary: beekeepers appreciate this sweet-nectared tree. And for good reason: it flowers very abundantly. In May-June its crown is adorned with long bunches (10-20 cm) of creamy white, sometimes pinkish flowers. They are arranged (in groups of 15 to 25) along a soft and pendulous stem (the spine). They sway at the slightest breeze and their perfume is carried by the wind. It is slightly reminiscent of orange blossoms. It attracts bees from afar, up to more than a kilometer away. They come to feast on the huge quantities of nectar.

Long ago, this highly melliferous tree was planted widely for the production of a kind of honey that we all know well: the famous ‘acacia’ honey, with its clear, amber colour. This name comes from the similarity with the tropical tree. It belies the true origin of this fragrant honey: the black locust (false acacia). Its flowers sweeten our breakfasts. But not only that: think of honey-glazed donuts, for example, a tasty treat for any time of day.

Resilience

But be careful: get too close and you'll prick yourself. These trees get their claws out. It is best to approach with caution, as their branches are equipped with formidable thorns, which can measure up to 3 cm long. Perhaps that's another reason why locust trees are often unloved? But that’s not the view of carpenters and cabinetmakers, who have dubbed it ‘European hardwood’ because locust wood has a reputation for being extremely dense and durable. It's so tough, waterproof and rot-proof that it can make sturdy tool handles and durable fences.

Reinforcing the fences of the Zavelenberg to protect it from urbanisation: this could be a good use for this locust grove one day, after it dies a natural death in a good hundred years (check with bruno). Because the qualities and the resistance of these waders of the Zavelenberg demand respect and indulgence, the various managers of the nature reserve have decided to let them live. The grove will be spared the chainsaw. However, once it is gone, it will not be replaced in the future. If you walk around there, enjoy of the presence of these locusts: a symbol of the resilience of the last farmland in Brussels.

ILLUSTRATIONS

This portrait is enriched with an illustration from the Belgian Federal State Collection on permanent loan to the Meise Botanical Garden. See attached. Thanks to the library (heritage collection) for this contribution.

TWINNING

This portrait is enriched with photos from theBelTrees' collection

Photos: Priscille Cazin - Zerolutions/32shoot asbl
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Twinning: Dorpsplein, Wielsbeke (Ooigem) N50°53'35.62" E3°20'12.23" Photo: © Beltrees
Twinning: Morlanwelz, N50° 28' 07.4" E4° 14' 04.0" Photo: © Beltrees
© Bruciel 1953
© Bruciel 1971
© Bruciel 2004
© Bruciel 2015