Bordeaux mixture: what is it? How to use it ?


Bordeaux mixture has been used for nearly 200 years to treat crops against fungal diseases. These are capable of wreaking havoc in the vegetable garden among others, and rather than relentlessly treating with chemical antifungals, we resort to this famous copper mixture. This is where it takes on its full meaning. So let’s find out what exactly is the composition of this blue powder. Let’s also see how to properly use Bordeaux mixture to preserve good plant health, whether as a preventive or curative measure, without however overdoing it since Bordeaux mixture is sometimes criticized because of its proven toxicity.

What is Bordeaux mixture?

It’s about a micronized powder of turquoise blue color active against algae and pathogenic fungi. It is an algaecide and a fungicide. It is obtained by neutralizing with slaked lime a solution of copper sulfate. It is therefore the copper ions which constitute its active material. This cupric mixture, that is to say which contains copper, having been discovered by chance in the Médoc, was logically baptized Bordeaux mixture.

What to treat with Bordeaux mixture?

All areas of the garden can benefit from a preventive or curative treatment with Bordeaux mixture, whether it is the vegetable garden, the orchard, the vineyard or the ornamental garden. It is therefore of great help against:

  • Mildew,
  • Scab,
  • The peach leaf curl,
  • Screening,
  • Entomosporiosis,
  • Gummosis,
  • Moniliosis,
  • Bacterial canker, etc.

If it has been used for two centuries, it is because it is effective. It can be used in organic farming but in small doses.

This copper compound offers very good results since the germination of fungal spores is stopped after treatment. It is also useful in inhibiting the proliferation of bacteria in the garden. In addition, this copper compound is toxic to algae. It is for this reason that it makes it possible to overcome downy mildew, a disease responsible for an oomycete, kind ofalgae-mushroom (Plasmopara viticola).

On the other hand this blue powder must absolutely not be used to treat swimming pool water and it is of no use in the face of an infestation of mites or insect pests whether on legumes, fruit trees, hedges or flowering plants. You must know it.

How to use Bordeaux mixture?

We should’nt never use pure Bordeaux mixture. We absolutely dilute it in water in order to spray it where necessary, i.e. on plants susceptible to attack by pathogenic microscopic fungi or which are already affected by fungal disease.

The dosage to be observed must not exceed 20 mg of Bordeaux mixture per liter of water. But this is a maximum. In many situations, it is recommended to treat with only 10 or 12 g / L. The reason is simple: the plants treated with this highly dosed copper mixture are doomed to certain death and the treatment will not however be more effective on the fungal disease which is targeted. There is therefore really no point in overdosing this treatment, whether we are looking for a preventive or curative action.

Experienced gardeners are able to visually quickly realize that a plant has been treated with an excessive dosage: blue drops are visible at the tips of the leaves. If we notice this phenomenon, the only solution is to water the plants abundantly in fine rain in order to save them, but let’s not kid ourselves: the Bordeaux mixture will infiltrate the soil.

Rather than having a heavy hand, it is sometimes necessary to resort to a natural wetting product for increase the adhesion of the Bordeaux mixture on the plant parts to be treated. This prevents this contact fungicide from being washed away too quickly by the slightest downpour. In 1 liter of water with 10 to 20 g of Bordeaux mixture, you can add one or other of the following wetting agents:

  • 5 tablespoons of black soap,
  • 20 ml dormant oil or horticultural oil which is a mineral oil.

Some people opt for the addition of 10 cl of skimmed milk. However, it should be noted that this is not the best solution because bacteria and certain pathogenic fungi feed on milk proteins.

Is Bordeaux mixture toxic?

The toxicity of Bordeaux mixture is real for humans as soon as the dosage exceeds ” 0.3 g per kilogram of body weight As stated in numerous expert reports. It is also toxic to animals and the environment. It is therefore essential to strictly observe the doses. Let us not forget that the copper that Bordeaux mixture contains is washed away of rains and watering, then accumulates in the ground or in groundwater. The toxicity of copper is even greater in acidic soil than in neutral soil. We understand that every gardener must use Bordeaux mixture with great moderation.

In order not to fall into the excess, it is quite possible to replace the copper mixture from time to time by horsetail or nettle manure, but here again the dosages must be respected so as not to cause any adverse impact at the health and environmental level. The liquid manure should not be dosed at more than 8%.

And to avoid fungal diseases, as much adopt the right gardening methods such as spacing out your plantings sufficiently, making the right sizes to ventilate the heart of trees and shrubs, or even avoiding too wet the leaves of vegetable garden plants when watering.