Horticultural oil, also known as white oil, is an easy and inexpensive pesticide to prepare at home from readily-available materials. It is used to control scale, aphids, mealy bug, citrus leaf miner, mites and caterpillars on roses, ornamentals, citrus and other fruit trees.
The commercial white oil or horticultural oil insecticides sold in garden centres are petroleum based, they’re essentially just refined paraffin oil with an emulsifier (additive to make them water-solouble). They’re quite expensive, and also not the kind of thing you want in the soil, as they take years to break down in the ground.
A better option is to purchase a natural vegetable oil-based horticultural oil, or just make your own.
The benefit of natural horticultural oils is that they’re non-toxic pesticides that and perfectly safe to use. There is no withholding period, so crops that have been sprayed can be washed with water and eaten on the same day. After plants have been sprayed, there’s no toxic residue to harm beneficial insects such as bees or ladybirds, and it’s not harmful to earthworms either.
Homemade Organic Horticultural Oil Spray
The traditional horticultural oil was vegetable oil-based, and it’s survived the test of time, having been used for around two centuries. It worked back then, it still works now, and is very easy to make, in just a few minutes!
The formula for homemade horticultural oil or white oil is 80% (4 parts) vegetable oil combined with 20% (1 part) pure liquid soap.
The list of materials, and procedure for making horticultural oil are detailed below.
You will need:
- 2 cups of vegetable oil (1 cup = 284ml, 2 cups = 568ml)
- 1/2 cup pure liquid soap (1/2 cup = 142ml), use pure organic castile soap, made from olive oil if you want to keep it all natural
- Empty jar or wide-mouth bottle
- Plastic spray bottle
To make horticultural oil spray:
- Pour 2 cups of vegetable oil into a jar or wide-mouth bottle.
- Add 1/2 cup pure liquid soap
- Shake until the mixture turns white.
This is concentrated white oil which must be diluted with water for use!
Store the homemade white oil concentrate in a cool, dry place. The shelf life is approximately three months from the day it’s made.
This recipe will make 2-1/2 cups (710ml) of white oil concentrate, which, when mixed with water, will produce 71 litres of spray, which is quite a lot! If you won’t use that much in 3 months, make a smaller quantity by following the directions below.
How to make a smaller quantity of horticultural oil spray:
Using the formula for homemade white oil (horticultural oil), which is 80% (4 parts) vegetable oil combined with 20% (1 part) pure liquid soap, we can brew up as much as we need.
If we brew up 250ml of white oil concentrate, a bit less than a cup-full, this more manageable quantity can be diluted to make up 25 litres of spray.
- Pour 200ml of vegetable oil into a jar or wide-mouth bottle that can hold 250ml or more.
- Add 50ml of pure liquid soap
- Shake until the mixture turns white.
The advantage of making up a smaller quantity is that none goes to waste, and if more is required, it’s fairly quick to brew up another small batch of the homemade horticultural oil.
How to Use Horticultural Oil Spray
The application rate for horticultural oil is 10ml (two teaspoons) per litre of water. That’s tea-spoons, the little spoons in the kitchen drawer, not the larger table-spoons! Adjust the quantity to match the size of the spray bottle.
- Add 10ml (2 teaspoons) of the white oil concentrate into a spray bottle.
- Fill the spray bottle with 1 litre of water, and put the lid on.
- Shake the spray bottle well, then spray your plants or trees.
The best times to spray horticultural oils are morning or late evenings.
When spraying horticultural oils , spray on both sides of the leaves, branches and bark to target pests wherever they may be hiding.
Precautions When Using Horticultural Oils
Note: Only spray horticultural oils (both commercial and homemade) during mild weather. Do not spray horticultural oils on trees or plants when the temperature is around 30°C (86°F) or higher, as the oil will clog the pores in the leaves and cause damage to the plant.
Note: Only use horticultural oils on healthy, unstressed plants, do not apply to plants suffering from heat or moisture stress.
Note: Don’t use horticultural oils on plants that have hairy or soft leaves, as it will burn their leaves.
How Often Should Horticultural Oils Be Sprayed?
- For aphids, two-spotted mites (red spider mites) and whitefly on vegetables, strawberries and ornamental plants, apply two sprays, 3-5 days apart. Repeat again if pests reappear. Don’t use more than 3 sprays on plants over a 4-8 week period.
- For scale on fruit trees, apply two sprays, 7 days apart.
- For citrus leaf miner on citrus trees, begin spraying new summer growth as soon as it emerges, before the leaves reach 10 mm (3/8″) in length, and apply every 5-14 days as new flush growth is produced. Stop spraying when most of the new leaves produced within a flush cycle start to harden, or are more than 40 mm (1.6″) in length. Don’t apply more than 8 times per season.
How Does Horticultural Oil Work?
Horticultural oils works by coating insects and their eggs with a thin layer of oil, which blocks their breathing pores, suffocating them.
Since horticultural oils work through physical suffocation, and not through a chemical action, pest insects and mites cannot develop resistance to them, ever! They can develop resistance to pesticides that are poisonous to them though.
As long as the pest insect is sprayed directly with the horticultural oil spray, it will work. If any are missed, a second application usually gets them. Horticultural oil has to make contact with the pest it’s meant to control, and is therefore described as a contact pesticide.
If the pest is not visible, there’s no point spraying horticultural oils, except to deter citrus leaf miner adults from laying their eggs on the leaves. They find the oil layer on the surface of the leaves unnatural and avoid laying their eggs there.
For additional information on homemade pest control products, see the following articles: