Neem oil is a gentle horticultural oil that is produced as a by-product when the more powerful insect repellent azadiractin is extracted from raw neem seed oil.

Like other horticultural oils, neem oil works by covering its target in oil and smothering it. It should not be used as a general treatment for sick plants but can be helpful in addressing specific plant problems.

• It can be mildly effective in preventing fungal spores from germinating and for killing insects at some vulnerable stages of development, particularly insect eggs or crawler phases.
• To be effective, it must completely cover the target at the right stage of development, which varies with the pest.
• It is a mild remedy and needs repetitive application as the product remains effective for a very limited period of time.

There are limitations to what neem can do to manage pests and disease.

• Once a fungal infection has taken place, neem oil cannot cure it and its effect is only to reduce spore germination on other parts of the plant or on nearby plants.
• It will not cure bacterial issues.
• It is not helpful in attacking insects in an armored stage of development, like mature scale insects.

If you are concerned that your plant may have a root infection or insects in the soil, pouring neem oil into the plant soil will not help and will only create additional problems for your plant as oils will damage the soil structure and prevent the roots from absorbing water properly.

While neem is sometimes described as safe for beneficial insects, that is only because it is so mild as to have limited impact on any insects. It will, in fact, have the same effect on desirable insects as undesirable ones so be cautious when using the product outdoors, apply it in a targeted manner to specific insects, and don't use it  while a plant is flowering and pollinators are present. Once the oil has dried it will not affect arriving insects as it works by smothering.

Neem oil dries quickly but do not apply it to leaves while a plant is in bright sun or it may magnify the sunlight and burn the leaves. Indoors and out, don't apply neem oil to a very distressed plant.

Before using neem oil as a horticultural oil, check that the product you are using has the properties that you expect. Neem oil products list clarified hydrophobic extract of neem oil (or CHENO) as the active ingredient on their packaging. There are other more potent neem products called pure neem oil or cold-pressed neem seed oil that have much stronger chemical impact that interferes with the feeding and development of insects, friend and foe. It requires more care and personal protection to use.

If you need help determining what is troubling your plant and whether neem oil would be effective as a treatment, please email us with photos of what you have observed on your plant and a good description of the care, history and location of the plant at PlantInfo@NYBG.org.

Courtesy of NYBG Plant Information Service