Neem oil in the home garden: correct dilution and when not to use it

For a 3000 ppm azadirachtin neem oil, use 2 to 3 ml per litre of water for most garden pests. Higher concentrations need proportionally less: around 1 ml per litre at 10000 ppm, and 0.5 to 1 ml per litre at 50000 ppm. Always add an emulsifier, spray in the evening, and never spray a plant that is already wilting.

Neem oil is the most widely used organic garden spray and also the most widely misused. Two mistakes account for nearly all the complaints: people dilute it by volume of oil without checking the azadirachtin concentration, and people spray it in the sun. Both cause leaf burn, and both are avoidable.

Azadirachtin is the active compound. Neem oils sold for gardens range from around 300 ppm to 50000 ppm, which is a difference of more than a hundred times. A rate that is correct for one product will either do nothing or scorch your plants with another.

When in doubt, start at the lower end of the range and test on a few leaves before spraying the whole plant.

Oil and water do not mix. Neem sprayed without an emulsifier separates in the tank, and the last part of the spray comes out as almost pure oil, which is what burns leaves.

1. Start with warm water, around 30 degrees Celsius. Cold water makes neem thicken and separate faster.

2. Add a few drops of a mild liquid soap or a commercial spreader per litre. Many neem formulations already include an emulsifier, in which case skip this.

3. Stir the soap into the water first.

4. Add the neem oil slowly while stirring continuously.

5. Use the mixture within four to six hours. Azadirachtin breaks down in water and the emulsion separates as it stands.

6. Shake the sprayer every few minutes while you work.

Never mix a large batch to keep for next week. It will not work and it may burn.

Cover the undersides of leaves. Aphids, whitefly, thrips and mites live there, and neem works largely by contact and ingestion, so a spray that only wets the upper surface misses the population.

Spray until the leaf is wet but not running off. Runoff is wasted product on the floor.

Repeat every seven days while pests are active. Neem does not kill eggs, so the second application catches whatever hatched after the first.

For prevention rather than treatment, every ten to fourteen days is enough.

These four situations account for most of the damage people attribute to the product.

1. In direct sun or above about 35 degrees Celsius

Oil on a leaf in strong sunlight causes burn. Spray in the evening after the sun is off the plants, or very early in the morning. In a heatwave, skip the application entirely and wait for cooler weather.

2. On a plant that is already stressed

A wilting, drought-stressed or freshly repotted plant has no capacity to tolerate an oil coating. Water it, let it recover for a day, then spray.

3. On seedlings and very tender new growth

Young seedlings burn easily. Either use half the normal rate or wait until the plant has several true leaves. Some plants with fine hairy leaves, and some ferns, react badly at any concentration, so test a single leaf and wait 48 hours before treating the whole plant.

Alongside a fungal bio-insecticide

Neem oil coats spores and reduces the effectiveness of products such as Beauveria bassiana. If you use both in a programme, separate them by at least five to seven days rather than tank mixing.

It works on soft-bodied insects and young stages: aphids, whitefly, thrips, spider mites, leafhoppers, mealybugs, young caterpillars and leaf miners. It also suppresses some fungal diseases such as powdery mildew through a coating effect.

It does not work well on hard-shelled adult beetles, mature scale insects under their protective cover, ants, or established caterpillar infestations. It does not control soil grubs or nematodes at spray concentrations.

The National Pesticide Information Center publishes a plain-language fact sheet covering neem oil's mode of action, toxicity profile and handling requirements. It is the best neutral reference available.

• Wear gloves. Concentrated neem irritates skin for some people.

• Keep the concentrate out of reach of children and away from fish tanks and ponds, as neem is toxic to fish.

• Store it cool and dark. Heat and light degrade azadirachtin, so a bottle left in a sunny window loses strength.

• Spray in the evening to avoid contact with foraging bees.

• Wash produce before eating, even though there is no mandatory waiting period.

Check three things in order. Was the concentration right for that product? Did the spray reach the undersides of the leaves? Has it been applied more than once?

If all three are yes and the pest is still increasing after three applications, you probably have a pest neem does not control well, such as mature scale or hard-bodied beetles. Identify the pest properly and change the approach rather than raising the neem rate. Higher concentrations cause burn without improving control.