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'Lovely neem' oil is a gardener's ally

My lovely neem,

That intercepts sun’s scorching beam,

Yet bears the heat all day

Without the rain’s refreshing spray,

Thou charm’st the wanderer’s woe away

With soothing shade.

— “The Neem Tree” by Elsa Kazi (1881-1967)

German-born poet and playwright Elsa Kazi lived for a long time in Pakistan with her husband Imdad Ali Kazi, who was a founder of the University of Sindh at Jamshoro. It was there that Elsa was introduced to her “lovely neem.” She was so captivated by the tree that she wrote her ode on the bark of the tree trunk while standing beneath its sprawling branches of evergreen leaves.

By doing so, Elsa followed in the footsteps of ancient Siddhars of India, who chronicled many benefits of the neem tree on palm leaves. In fact, this native tree of the Indian subcontinent has been used to heal such a wide variety of ailments — from head lice to ulcers — that it is sometimes called the village pharmacy.

Since the end of the 20th century, gardeners in the U.S. and elsewhere have used the garlicky-smelling, bitter-tasting oil that’s in the seeds, leaves and other parts of the neem tree as a natural pesticide to protect their fruit trees, vegetables, herbs and ornamentals. Thousands of years ago, Indian farmers were the first to use neem for crop management by soaking the seeds in water and spreading them in their fields. Dried neem leaves were also used to repel insects from grain bins.

Neem oil is effective against more than 200 species of insects that chew or suck on plant tissue, including aphids, spider mites, whiteflies, leafhoppers, thrips and scale.

There are about 2,400 plants in the world that contain natural pesticides (a common one is pyrethrum, found in a species of chrysanthemum). However, neem oil reigns supreme for its far-reaching applications, not only in managing garden pests but also as an ingredient in toothpaste, sunscreen, soap and pet shampoo.

The neem tree’s botanical name is Azadirachta indica, which translates to “noble (or free) tree of India.” Its oil is a complex substance with more than 100 different elements. Research on isolating each component of neem oil began in the 1940s; however, it wasn’t until the mid-’80s that scientists isolated the most significant pesticidal element in neem oil, which was named azadirachtin.

This biological component is an effective pesticide because it alters the hormonal system of insects and mites, leading them to stop reproducing, metamorphosizing and feeding. Like other horticultural oils that are made from mineral or cottonseed oils, neem oil smothers insect eggs and larvae; thus, it can be used to manage insect pests at all stages of their lifecycle.

Neem oil has also shown effectiveness at killing fungal diseases such as powdery mildew, black spot, rust, leaf spot, anthracnose and verticillium wilt, as well as bacterial diseases such as fire blight.

Organic gardeners appreciate the fact that neem oil is biodegradable and does not leave behind harmful residues. In addition, neem oil has low toxicity for people and pets, although the oil causes skin and eye irritation if handled improperly, and stomach upset if consumed.

Neem oil also shows low toxicity to beneficial insects once the substance has dried on plants because most beneficial insects don’t eat plant tissue where the pesticidal substances accumulate.

Also, the properties of azadirachtin may improve garden soil by encouraging earthworm activity. Fertilizer made from neem cake, the byproduct of processed neem fruit and seeds, increases the availability of nitrogen in the soil for plants.

No wonder the neem tree is called noble. Here are several suggestions for using neem oil:

Use neem oil as one of several strategies to reduce insect pests and diseases. The goal is not to eradicate all the “bad” bugs in the garden, but to balance the ratio of pests and beneficial insects for optimal biological pest management.

Studies have found that multiple applications of neem oil may increase pest resistance to azadirachtin — a good reason to use neem oil only when needed.

Neem oil breaks down quickly in sunlight, so apply it in the evening. This is also when beneficial insects are less active.

Do not use neem oil on days when the temperature rises above 95 degrees.

Some gardeners recommend not using neem oil in the fall.

Do not apply neem oil just after watering the plants; damp conditions prevent the substance from drying and may cause it to burn plant foliage.

A mixture of sulfur and neem oil is toxic to plants; wait 30 days before applying neem oil after any products with sulfur have been used (such as for increasing the acidity of soil).

Over the last decade, many azadirachtin-based products have been patented for different methods used to extract the oil (see this week’s post on my blog for more about this debate). The composition of the oil changes depending on the type of extraction method used; therefore, different neem oil products may produce varying effects on garden pests.

Rhonda Nowak is a Rogue Valley gardener, teacher and writer. Email her at Rnowak39@gmail.com. For more about gardening, visit her blog at http://blogs.esouthernoregon.com/theliterarygardener/ and check out her podcasts and videos at https://mailtribune.com/podcasts/the-literary-gardener.