The situation is actually more complicated: you can use the Bordeaux mixture within organic farming, but you must take certain precautions in the garden.
Bordeaux mixture, not as eco-friendly as it seems?
The Bordeaux mixture is a blend of copper sulfate and lime used in gardening as a fungicide, to kill the microscopic fungi responsible for plant diseases (late blight, peach leaf curl, apple scab).
- Precise dosing: Dose your Bordeaux mixture according to its use (tomatoes, potatoes, fruit trees) and the diseases to treat, never exceeding the copper sulfate doses specified in the recipe.
- Burn risk: Overdosed, your Bordeaux mixture will burn the foliage and your young shoots.
- Prevention vs Cure: In preventive use, use slightly less Bordeaux mixture than in curative use.
Regulation: What the copper legislation says?
In organic farming, the preparation is indeed permitted, but strict limits must be respected to prevent heavy metal buildup. European regulation has sharply reduced the thresholds: the maximum limit is set at 4 kg of copper per hectare per year (down from 6 kg previously), with smoothing flexibility over 7 years.
In practice, to keep garden tasks simple, many people avoid treating the soil more than twice a year with a solution that contains 20% copper metal. Some historical exemptions persist for certain types of cultivation under heavy weather pressure, such as vineyards.
Applying Bordeaux mixture with restraint: the dangers
Bordeaux mixture remains a less toxic option for immediate biodiversity than synthetic chemical fungicides, but it must be used sparingly.
Overdosing is common, while several studies have shown that reducing the amounts does not affect the effectiveness of the treatment against late blight.
Additionally, for certain crops, we must absolutely avoid excess copper. This is the case for Sauvignon Blanc or other wine grape varieties because of a negative impact on varietal aromas, with the mixture interfering with the yeasts responsible for alcohol fermentation.
Finally, remember to rinse your fruits and vegetables very carefully before consuming them if a recent treatment occurred.
Natural alternatives to Bordeaux mixture?
As an eco-responsible gardener, you can often replace the perennial Bordeaux mixture with ecological and biodegradable alternatives. It’s important to keep in mind that Bordeaux mixture is effective against certain very specific fungal diseases, whereas in everyday use it is sometimes employed far too universally.
Natural recipe against late blight (Baking soda + black soap)
To prepare 1 liter of spray solution for your plants:
- 1 level teaspoon of baking soda (modifies the leaf pH and prevents the fungus from settling).
- 1/2 teaspoon of liquid black soap (acts as a wetting agent to help the preparation adhere).
Use this preparation in unstable, heavy, humid weather, as a preventive measure or even at the first symptoms. Be careful not to overdose: more than 10 g/L of baking soda can burn the stomata of the leaves.
Other alternatives such as horsetail tea (very rich in silica to strengthen plant cell walls) or nettle tea also offer excellent preventive results.
Everything you need to know about Bordeaux mixture
When should you put Bordeaux mixture in the vegetable garden?
Bordeaux mixture is applied mainly in autumn (after leaf drop) and early spring (at bud break) to protect fruit trees. In the vegetable garden, it is used preventively on tomatoes and potatoes before the hot, wet summer periods that favor late blight.
Is Bordeaux mixture toxic to pets?
Although natural, Bordeaux mixture carries toxicity if ingested in significant amounts. It is strongly recommended to keep pets (dogs, cats) away during spraying and to wait for the product to dry completely on the leaves.
