Nothing is worse than toiling hard in your garden for months only to have your crops decimated by fungal disease or an infestation of spider mites. Though it may be easy to reach for the strongest pesticide or fungicide you can find, there are many reasons to use a greener option.

There is growing concern about the toxicity of commercial pesticides to our bodies and the environment. Most have non-biodegradable properties and their residues remain in the soil, water supply, and our crops. This can have potential adverse effects on human health and scientists are looking for alternatives to reduce these negative impacts while maintaining a high yield come harvest. Essential oils may be the answer.

What are Essential Oils?

Essential oils are the volatile liquids extracted from plants through steam distillation. These oils, which contain the “essence” of the plant, have been used for thousands of years for their medicinal and cosmetic benefits. Their power lies in the hundreds of chemical constituents they contain. Each oil profile has anywhere from 200 to 800 different constituents in varying amounts, hundreds of which have yet to be identified and studied by scientists.

These include terpenes, alcohols, esters, aldehydes, ketones, phenols, and oxides. They have a wide range of antibacterial, antifungal, anti-inflammatory, antioxidant, anesthetic, sedative, expectorant, antiseptic, antispasmodic, antirheumatic, antiepileptic, and anticonvulsant properties. Each specific constituent also has its own unique attributes.

For example, thymol is a phenol found in thyme and oregano, and has been shown to inhibit the growth of microorganisms, such as E. coli, by disrupting the microbe’s cytoplasmic membranes and causing cell leakage. Without this membrane, the viability of the microbe decreases significantly.

Benefits of Essential Oils in the Garden

The most important benefit of using essential oils as pesticides is they are safer for humans and beneficial against pests. Recent studies indicate some of the chemicals in essential oils interfere with the octopaminergic nervous system in insects. Since mammals do not share this target site, most essential oils will be non-toxic to gardeners and their furry friends.

Pollinators are also less likely to suffer. Although essential oil sprays may cause harm if sprayed directly on beneficial insects, after 24 hours they no longer leave a residue that will harm bees and butterflies while they work.

Scientists believe pests will build up a resistance more slowly to essential oil-based pesticides than their synthetic counterparts because of the complex combinations of their constituents.

Essential oil-based pesticides are expected to have commercial applications in urban pest control, greenhouse crops, and high-value organic food production where the uses of other synthetic pesticides are limited.

Drawbacks of Essential Oils

One of the major drawbacks of using essential oils is maintaining consistency in the chemical profile of the oils. The levels of terpenes and other constituents can vary greatly depending on the region where they are grown, genetics, climate, and other seasonal factors.

For example, rosemary oil extracted from plants grown in two different regions of Italy will contain varying levels of alpha-pinene, ranging from 11 per cent to 36 per cent. Another study noted essential oils extracted from different chemotypes of garden thyme did not have similar toxicity against the bean weevil. These variations can cause problems for the commercial production of essential oil-based pesticides, especially for smaller businesses that do not have the funds to manufacture a consistent product themselves.

Although essential oils are produced in large quantities for the food and cosmetic industries, large-scale commercial production would require a substantial increase in the production of certain oils. This can be quite costly, as it can take thousands of pounds of plant matter to produce just one pound of essential oil.

On a large scale, this may not be cost effective when you consider essential oil-based pesticides require frequent application rates and a high concentration of the oils in the pesticides (as high as one percent active ingredient). Although not as safe, synthetic pesticides are less expensive and more effective in the short term.

Must-Have Essential Oils for Your Garden

For your home garden, however, it is easy to make your own essential oil pesticides and fungicides to treat common pests and diseases. Most essential oils have a negative effect on pests, however, the following oils stand out in the garden and are backed by plenty of science to support their effectiveness:

Peppermint

Peppermint is arguably one of the best essential oils you can add to your gardening routine. It works well against ants, aphids, beetles, caterpillars, flies, gnats, mosquitoes, moths, and will even repel spiders. If you store any grains, use peppermint soaked cotton balls nearby to deter beetles from getting into your supply. Peppermint also has fungicidal properties and works well when sprayed on plants with fungal issues.

Sandalwood

If your plants are facing environmental stress, spray them with a sandalwood solution. The terpene called santalol acts as a strengthening agent for plants and makes them hardier.

Studies have also shown sandalwood oil is effective in treating the twospotted spider mite and reduced the total number of eggs found on the infested plant’s leaves by 89.3 per cent.

Tea Tree

Tea tree has strong antifungal properties. Studies have shown tea tree oil effectively fights the fungi which causes Fusarium head blight in wheat, barley and oats. It has also been proven to treat barley leaf stripe, powdery mildew, anthracnose and leather rot in strawberries, early blight disease in tomato plants, Cercospora beticola on sugar beets, and Alternaria solani on potatoes. It also repels white flies.

To create your own tea tree fungicide spray, mix two tablespoons of tea tree oil with two cups of water and spray plants every three to seven days. Tea tree oil is sensitive to phototoxicity, so spray in the morning when the sun is not shining as brightly.

Orange

Orange oil will treat ants, aphids, beetles, caterpillars, chiggers, cutworms, fleas, flies, gnats, mosquitoes, moths, roaches, snails, slugs, spiders, and ticks. The terpene called d-limonene is the main constituent of orange oil. It will not harm humans, but will kill insects by dissolving their exoskeleton, which dehydrates and suffocates them. Many claim one application of an orange oil solution is all it takes to wipe out an entire colony of pests and prevent re-infestation. Ants are especially susceptible to orange oil because the oil destroys their pheromone trail so that new ants will not take their place. It is important to be aware this oil will also harm beneficial insects, so be careful where you spray.

Cedarwood

Cedarwood oil will take care of ants, aphids, beetles, caterpillars, chiggers, cutworms, fleas, flies, gnats, mosquitoes, moths, roaches, slugs, snails, spiders, ticks, and weevils. It is arguably the best essential oil for keeping gastropods, such as snails and slugs, off your plants.

Cedarwood oil is effective because it blocks the octopamine neurotransmitter in pests, which repels them from the area you spray. Octopamine regulates heart rate, movement, and behavior in pests. When they come into contact with cedarwood oil, they will suffocate and die. Mammals, birds, fish, amphibians and beneficial insects do not have octopamine neurotransmitters, so they are not negatively affected.

To make your own garden sprays, simply add eight to 10 drops of your oil to a quart of water, with a few drops of dish soap to act as a wetting agent. To make an all-purpose garden pesticide and fungicide, fill a 32-oz. spray bottle with water, then add 10 drops of sweet orange oil, 10 drops of cedarwood oil, and 10 drops of tea tree oil.

If you are looking for a natural, less toxic alternative to synthetic pesticides, try the essential oils that have been used successfully for thousands of years. Although it may be tempting to reach for the hard stuff, this route will have fewer harmful consequences in the long run.