Black locust (false acacia)
Robinia pseudoacacia
Botanical family: Fabaceae
Robinia pseudoacacia, native to North America, was introduced to Europe in the 17th century and has been widely naturalised since. Its rot-proof wood and its ability to fix nitrogen make it a pioneer tree valued for restoring degraded soils, but its use must be considered.
Full sun
Poor, well-drained, even stony or sandy
Low once established, tolerates drought
Very hardy (withstands -25 °C)
Overwinters in your zone
Role in permaculture
Like most Fabaceae, black locust captures nitrogen from the air, which lets it colonise poor soils. Its fragrant white flowering in clusters, in May-June, makes it one of the most renowned nectar trees of Europe (commercial 'acacia honey' is in fact largely from it); its very durable wood is also sought after for stakes and outdoor furniture.
Insects attracted or useful
Documented synergies
Relationships backed by a named source, with their real scope. A synergy without a named reference has no place here, no “it is said that…”.
The black locust fixes nitrogen by nodulating with the genus Mesorhizobium (identity confirmed by a sequenced genome), not with a classic Rhizobium. A double-edged effect: this nitrogen input favours mainly nitrophilous and ruderal plants, and the enrichment, highly variable with provenance and site, has not been measured in a permaculture-garden context.
scientific study
- · Wang et al. (2018), Standards in Genomic Sciences, DOI 10.1186/s40793-018-0321-3
Its clusters of pea-like flowers give the famous 'acacia' honey, an abundant nectar resource of late spring. Caveat: the flowering is brief (two to three weeks) and vulnerable to frost and rain, a one-off windfall, not a lasting one.
scientific study
- · Gismondi et al. (2018), Journal of the Science of Food and Agriculture, DOI 10.1002/jsfa.8957
Its leaves and litter release flavonoids (including robinetin) that slow the germination of neighbouring plants: in the laboratory, robinetin halves the growth of lettuce. The real extent in the garden, diluted litter, living soil, rains, is not established. A signal to know, not a field measurement.
scientific study
- · Nasir, Iqbal, Hiradate & Fujii (2005), Journal of Chemical Ecology, DOI 10.1007/s10886-005-6084-5
Seasons & availability
Black locust flowers in May and June; its hanging clusters of scented white flowers, the only edible part, are picked during those few weeks, the whole of the rest of the tree being toxic: bark, leaves, seeds and pods.
Available: May, June.
Guidance for the oceanic / temperate climate; it may vary with your region, altitude, exposure and growing method.
Sources
- Plants For A Future, « Robinia pseudoacacia » (pfaf.org/user/Plant.aspx?LatinName=Robinia+pseudoacacia)
- Missouri Botanical Garden, « Robinia pseudoacacia » (plantfinder.mobot.org/PlantFinderDetails.aspx?taxonid=280476)
Nutritional & medicinal properties
For educational purposes. Not a substitute for medical advice. Effects depend on the form, the dose and personal context.
Precautions & contraindications
Bark, roots and above all SEEDS are toxic (robin, phasin, toxalbumins); the pods look like peas and attract children. Eat ONLY the flowers, and in moderation.
Family behaviour: Legumes
In general, Legumes fix nitrogen from the air in most cases, thanks to the Rhizobium in their root nodules. They mainly enrich the soil for the following crops, as their residues decompose. Ideal in a rotation, before nitrogen-hungry plants such as nightshades.
Understanding families means learning one rule per family rather than every pairing one by one. Learn more →
Species & varieties
The best-known species and varieties in France.
Common varieties
Keep in mind
Black locust is classed among the invasive alien species in France by the INPN and many Botanical Conservatories: it suckers vigorously and can colonise natural environments to the detriment of the local flora, notably in dry grasslands and open habitats. To be planted with discernment, avoiding the surroundings of sensitive natural environments, and anticipating its ability to sucker in the garden.
A note on rigour: many companion pairings are gardening folklore; this note sums up what the sources say, where there are any.
Before you forage
This plant is often gathered from the wild. Only pick what is abundant, never a protected species, and never uproot a whole plant (roots) without the landowner's permission. Rules vary by country and region, check locally.
Key takeaways
- • Prefers full sun exposure.
- • Ideal soil: poor, well-drained, even stony or sandy.