October 06, 2018
Got Pests? Try Botanical Oils
Into every beautiful and productive garden a little nasty pest activity may fall. Aphids may infest the kale, powdery mildew coat the leaves of zucchini, black spot mar the rose leaves, or little white flies flutter while their nymphs are sucking the sap from a shrub.
In the early days of modern pesticides, incredibly toxic chemicals were sold to home gardeners to manage pests. These days, the products we use are more likely to kill pests without threatening our health as well. A good example is oil-based pesticide.
Petroleum oil in water, with a sticker-spreader to keep it from settling out of solution, was long used to spray woody plants in winter. It was called “dormant oil.” If you sprayed it when leaves or flowers were present, it burned them. Because of this, other, often less environmentally friendly, chemicals were used in the summer.
However at some point, researchers learned that it was sulfur impurities in the petroleum oils that made it toxic to leaves. So they refined the sulfur out and sold the new products as “superior” or “summer” oils. You could safely spray them on leaves and flowers all summer, as long as the temperature was below 90 degrees F when you sprayed.
Oil kills by clogging the breathing pores of insects or actually exploding the spores and other cells of fungus. And as research continued, It was found that many kinds of botanical oils will also do the job. Products based on oil of canola, soy, linseed, sesame, even jojoba (a desert plant) began to appear (read the active ingredients list). Some of these sprays include aromatic oils, such as thyme and peppermint, which might offer some repellant action.
I encourage you to try these products, based on renewable, even edible, oil sources, in your garden. I have permanently stopped an aphid infection of kale with a single, thorough spray of a botanical oil product. However it is usually necessary to respray every week or two to continue the protection. Also, to control diseases, oils are most effective as a preventative or at the very first signs. If you spray early, you can kill spores on plant surfaces, but once a fungus grows deep inside the plant, where sprays can’t reach, it is often too late. Follow directions on labels and watch your plants.
Another oil product that kills insects and some disease spores is Neem oil. It also is toxic for insects to eat and kind of disgusting to them, so reduces feeding and even egglaying. However, the ingredient, azadirachtin, is a growth retardant for bees, so I prefer to try other oils first, or save neem oil for pests not listed on labels of botanical oil products. Azadirachtin breaks down in sunlight in 100 hours. Follow directions on label, including about interval between spraying food crops and eating them.
Here is a link to instructions for making "white oil" pesticide from mineral oil. https://www.gardeningknowhow.com/plant-problems/pests/pesticides/white-oil-insecticide.htm
However note that this uses 1 cup of mineral oil and 1/4 cup of dish soap (or Murphy's oil soap) to make a CONCENTRATE that you dilute, using about a tablespoon to a quart of water, to make a spray. So it would take a long time to use up a big bottle of the mineral oil this way unless you were spraying a lot of plants. Also note they advise avoiding bleach-based soap. Another search gave me the information that mineral oil is a petroleum product that contains sulfur compounds, so I think this homemade pesticide would be a "dormant oil" spray. That is, you could use it on dormant tree branches, but not on opening buds or on leaves. There are commercial oil sprays based on sesame oil, canola oil, and other organic oils that are far better for the environment. I suggest using those in your garden instead of any petroleum product.
Incidentally, a Google search for mineral oil uses turned up sites with many ideas for using it other than as a garden spray..
Posted by: Pam Peirce | October 01, 2019 at 01:25 PM
I have a big bottle of "heavy mineral oil" from about ten years ago and wonder if, rather than throwing it out, I can use it as a botanical oil.
Posted by: LPW | August 30, 2019 at 11:27 PM