by Leslie Cox; Friday; January 27, 2017

Oils have long been used by gardeners as an effective control against insect pests. With the exception of neem oil, they kill insects by suffocating them. The oils also penetrate the shells of insect eggs and interrupt the respiratory and metabolic processes of the growing embryo inside.

The key virtue of horticultural oils is their ability to kill insects by smothering. Most other pesticides use chemicals to interfere with the biochemical functions in pest insects. These functions are similar to those in other animals, including humans…so, those chemical pesticides will make us sick as well.

As a control for diseases, horticultural oils work effectively in preventing the spread of viruses caused by infestations of pests. They also work well for controlling powdery mildew. Mix a little baking soda into your diluted horticultural oil spray to deal with this common fungus.

Be forewarned! There are some beneficial insects who will be harmed, along with the pests. Such good guys as ladybugs and green lacewings will usually fly off as soon as you start spraying. But there are soft-bodied beneficial insects, such as butterfly larvae and predatory mites, who simply cannot move out of the way fast enough. So keep this in mind, if you are resorting to using a horticultural oil as a pest control. Actions have consequences.

Now to delve into the different types of oils. There are petroleum-based oils, vegetable-based oils, and neem. The term “horticultural oil” is just the catch-all one to gather and group all of the various oils gardeners use on their plants under one heading.

Petroleum-based oils contain some heavy duty compounds, such as paraffin, naphthene and sulphur. Parrafin is effective for its toxicity to insects while having a lesser impact on the plants themselves. Naphthene, on the other hand, is more toxic to the plants and less effective on the insects. Its phytotoxicity can seriously impact on the growth of your plant…hence why gardeners have traditionally applied petroleum-based oils during the dormant months when the plant is not typically sprouting new growth.

Thankfully, because of numerous improvements in the oil refining processes, the petroleum-based horticultural oils on the market these days contain much fewer amounts of paraffin and naphthene compounds. So they are much less harmful to your plants.

But there is one more damaging compound ingredient to expose: sulphur. Sulphur, in high enough quantities, will burn both plants and humans. So, when buying your lime-sulphur oil for controlling lichen and moss growth on your shrubs and trees here in the rain-soaked Pacific Northwest, look for a high UR number. The “UR” stands for “unsulphonated residue” and its number rating refers to the sulphur content in the oil. The higher the number, the less sulphur there is in the oil.

And just to add onto the information overload here…viscosity of the oil is something else a gardener should consider. Viscosity is a scientific term for thickness. It is rated by how fast an oil can pass through a standard measuring device, such as a tube or ring. (Sorry my techie background is coming through.) All gardeners need to remember about viscosity and horticultural oils is: the thinner the more effective. So you are looking for a low “E” number on your oil which indicates a faster evaporation rate.

Vegetable-based oils refers to any oil which is derived from an oilseed crop, such as soybean, canola, or cottonseed. More on these later.

Neem oil, which comes from neem tree seeds (obviously), is used as both an insecticide to control insect larvae and eggs, and as a fungicide to control powdery mildew and black spot.

Okay…now that you have a little background on the different types of oils and their make-up…let us see if we can sort through some of the confusing terms which have been coined over the decades.

Petroleum-based oils…

Dormant oil: This is used on woody plants, especially fruit trees. Nowadays, the name refers to when the oil is applied (winter while plants are dormant) and the rate of application. New, better refined, and more lightweight oils on the market have replaced the more harmful, heavier dormant oils of the past.

Mineral oil: This is a lightweight, petroleum-derived oil which is an effective control of the corn earworm, among others.

Narrow-range oil: This is a lightweight oil which has been graded according to the temperature range over which the oil evaporates. The lighter the oil, the faster it evaporates over a narrower temperature range. Translated…the lighter the oil, the safer it is for use on plants. The term “narrow-range oil” is synonymous with “superior” oil.

Summer oil: This oil is generally the same as narrow-range and superior oils. It is used on leafy plants during the growing season…which is over the summer months.

Superior oil: This term is reserved for the newer, more refined oils which can be safely applied at lower rates to plants in leaf. Nowadays, all horticultural oils are superior-type oils. Directions on their labels will display the different application rates for use during whichever season you are dousing your plant: winter or summer.

Vegetable oils…

Soybean oil: This oil provides good insect control.
Canola oil: Slightly less effective than soybean oil.
Sunflower oil: Slightly less effective than soybean oil.
Corn oil: This oil displayed mixed results on insect control in field trials.
Neem oil…this oil is usually recommended for use on fruit trees, roses, and vegetables.

Precautions in Using Horticultural Oils

As with any chemical substance, there are valid precautions one should follow. The mixture may not be particularly harmful to you, the gardener, but it can have serious effects on your plants.

Some precautions to keep in mind for using horticultural oils:

• Always read the labels thoroughly. If any part of the label is missing through weathered use, or inadvertent tearing, look up the product online. You should be able to find a full list of the precautions on the manufacturer’s website. Or check with your local nursery. They should have this valuable information as well.
• Avoid applying the oils on rainy days, or if rain may be imminent. Wet conditions impact on oil evaporation, which you want to be fairly quick to avoid any burning on leaves.
• Avoid applying if there is even a hint of a breeze. Winds, even small ones, can cause the chemical spray to drift where you do not want it.
• Wait to apply the oils when the temperatures are above freezing. If applied during freezing temps, the oil emulsion will most likely break down when it hits the plant which results in uneven coverage.
• Conversely, do not apply the oils if temperatures threaten to top 35 °C (95 °F) or higher. At these temps, plants become stressed which may result in damage occurring when hit with the oil.
• Even though the newer, more refined oils are technically safe for use on plants through the growing season, do not spray on any plants that have new shoots growing!

Armed with your sprayer filled with horticultural oil, there are some plants you need to avoid spraying. Those noted in the following list are quite sensitive to the chemicals in these products:

• Acer palmatum – Japanese maple
• Acer rubrum – red maple
• Carya – hickories
• Cercis canadensis – redbud
• Cotinus – smokebush, smoke tree
• Cryptomeria japonica – Japanese cedar
• Juglans nigra – black walnut
• Picea – spruce, particularly Picea glauca ‘Conica’ (dwarf Alberta spruce)
• Pseudotsuga menziesii – Douglas fir
• Cupressaceae family – many cedars and junipers in this family find horticultural oils toxic

And finally, a word of caution about neem oil insecticides. If you are a greenhouse grower, these products have been very successful in keeping pests under control on ornamental plants. However, be careful in using them on a few carnation varieties, fuchsia, hibiscus, impatiens, ornamental olives, and some varieties of roses. Use due diligence in researching whether your species of little seedlings can tolerate a neem oil spraying. Always far better to be safe rather than sorry.