Bordeaux Mixture

As colder weather causes leaves to fall throughout the garden, applications of Bordeaux mixture can prevent many common fungal diseases later in the year.

Bordeaux mixture is an excellent fungicide and bactericide for use on ornamental plants, vine fruits, and fruit and nut trees. Bordeaux mixture provides protection against these common diseases: 

• shot hole disease and leaf curl on stone fruits
• powdery mildew on grapes
• citrus blast
• walnut blight
• black spot
• peach leaf curl on almonds and other member of the Prunus family
• slugs and snails
• fireblight​

Bordeaux mixture tends to stay on twigs and branches, even in rainy weather, providing long term protection. At the same time, it is not a good idea to apply Bordeaux mixture after trees break dormancy because it can damage the leaves. Also, it can stain painted surfaces, so you will want to keep it away from buildings.

How to make Bordeaux mixture

While you can certainly buy ready-made Bordeaux mixture, it is far more effective when mixed and used while fresh. Be sure to wear protective clothing and safety goggles when mixing because this stuff is corrosive. 

To make your own Bordeaux mixture, you will need these ingredients, which can be found at most garden supply stores:

• Copper sulfate - Powdered copper sulfate, called “bluestone” should be used. Do not use fixed copper. Powdered copper sulfate mixes easily in warm water. Be sure to keep it dry until it is time to use. 
• Lime - Use fresh slaked lime (calcium oxide) or dry hydrated lime (calcium hydroxide). Only buy as much as you will use in the current season, in either case.

Lime is not as effective as it ages. Be careful when using slaked lime. As strange as it sounds, this stuff can heat water to boiling if it is added too quickly! To make your own slaked lime, slowly add one pound of quick lime to one gallon of water, using a wooden paddle to stir. The chemical reaction that will bring this solution to a milk-like consistency takes 1/2-2 hours to occur.

It is important to follow the correct procedure when making Bordeaux mixture. 

Start by mixing one pound of copper sulfate into one gallon of warm water, in a plastic bucket. Next, use your slaked lime, from above, or mix one pound of fresh hydrated lime into one gallon of water in a separate plastic bucket. Allow this mixture to stand for a couple of hours. Both of these “stock” solutions can be stored indefinitely in sealed containers. Because they are corrosive, label the containers clearly and keep them away from children.

When you are ready to mix these two together, start with 2 gallons of water in a heavy duty bucket. Then, follow these steps in order, stirring constantly with a wooden paddle:

1. Shake the copper sulfate solution energetically before adding one quart to the water.
2. Shake the lime solution before adding one quart to the diluted copper solution.
3. Stir for several minutes.
4. Pour into a sprayer.

Now you are ready to apply your very own Bordeaux mixture! Complete coverage is needed and the mixture will need to be agitated frequently as you work.

I know this sounds like a lot of work (and maybe even a little scary), but people have been making and using Bordeaux mixture successfully for decades. It really works. If a particular winter is especially rainy (we can hope), the Bordeaux mixture may need to be reapplied later in the season.

Once buds start opening in spring, protection can be continued with a fixed copper spray.