Castor oil has a reputation for being an effective mole, vole, and bird repellent as well as an easy-to-access pesticide with no side effects. If castor oil is so effective at what it claims to be, it feels like there should be a catch. “Will castor oil kill grass?” is a reasonable question to ask. Further, what is castor oil doing to your soil and will it really get rid of moles and grubs? These questions and more are answered in the article below.
Will Castor Oil Kill Grass?
No, castor oil will not kill grass. Even in large quantities, castor oil is a lawn-safe pesticide or pest deterrent. Not only will castor oil have no effect on the appearance of your lawn except for a mild shine directly after application, but castor oil also has little to no effect on the soil.
The only thing castor oil makes a difference in is lawn insect density and, by extension, the number of insectivores visiting your lawn for a snack.
What Castor Oil Will Do to a Lawn
When castor oil is sprayed on a lawn, it will stick lightly to plants, dirt, and other surfaces. Castor oil is a thin oil and shouldn’t give anything a sticky or oily appearance. Castor oil will give off a strong, unpleasant odor no matter what it touches. It will also give anything it touches a strong, bitter taste.
Castor oil will not harm full-grown worms and grubs that it comes into contact with, but it can coat and suffocate larvae and eggs, reducing insect populations. Winged insects may also be prevented from taking flight while they are coated with castor oil. Moles, voles, and birds that wish to feed on these types of insects will avoid doing so or reject the catch soon after tasting it. If there is nothing desirable to feed on, these animals will avoid your yard. Some have used this method to keep geese off their lawn.
Once applied, castor oil has no negative effects on plant life beyond its potential to reduce pollinator populations. The oil will break down in the soil in as little as one month, though some lawn-specific castor oil products have additives that will allow the oil to stick, rain or shine, for up to 6 months or more.
Related Article: What Kills Grass? A Guide to Harmful Substances
The Effects of Castor Oil on Your Soil
Castor oil has no notable effects on the quality of soil for any type of cultivation. This means that you should be able to use castor oil on your lawn in high quantities and not see any changes in major nutrient values or distribution, pH, or appearance of your lawn.
Does Castor Oil Kill Grass and Weeds?
As stated above, the answer to “Does castor oil kill grass?” is “No.” It’s also important to note that castor oil has no noted effects on the health of any other type of plant, whether it’s a tree, bush, or flower. At its worst, the only possible negative effect castor oil could have on a garden is a reduced number of pollinators. On the flip side of this, for those taking care of a lawn with no flowering plants, this could mean fewer weed seeds making their way into your lawn.
Does Castor Oil Damage Lawns?
Some herbicides and household substances that are safe for one grass type can be devastating to another. Just to be sure, you may be wondering, “Will castor oil kill St. Augustine grass?” Or, “Will castor oil kill pampas grass?” to be sure that castor oil will not harm your specific grass type.
Rest assured that castor oil is not at all harmful to any type of garden plant. Even moss and algae are safe from any harm when it comes to the application of castor oil. Further, castor oil is biodegradable and should break down completely within 6 months at a maximum, under typical lawn conditions. Castor oil has no lasting effects on soil quality, pH, texture, or nutrient value.
Spraying Castor Oil on Lawns to Control Pests
The most common use for castor oil in the garden or sprayed over a lawn is to control a variety of pests. Castor oil has a distinct, unpleasant odor that moles, voles, and even grubs detest and will avoid.
Further, castor oil can be fatal to insect eggs and larvae found on plants or in the soil. Castor oil is a good method of reducing fly, mosquito, flea, termite, beetle, and ant populations. While castor oil is good for controlling problem insects, the same fatal effects can be seen with butterfly, bee, firefly, and glowworm larvae.
Do You Need to Clean Up a Castor Oil Spill on Your Lawn?
Castor oil spills do not need to be cleaned up and the castor oil will be absorbed by the lawn over time with no negative effects. If you want to help a puddle of castor oil dissipate, you can add a little soap and water to help the castor oil dissolve. Be sure that any soap used is true, oil-based soap and not a detergent.
Castor Oil Could Solve Your Pest Problems
With no damage to your lawn and little danger from handling the substance, castor oil is an effective first-line pesticide and pest deterrent. The only downside is that humans aren’t immune to the smell. As such, spraying your entire yard with castor oil might not be the best plan, unless you have a serious mole, grub, or other insect infestation. In most cases, applying castor oil to molehills, tunnels, or any places where insect damage is apparent is the best place to start.
How long does the castor oil odor last after it’s sprayed on the lawn? And how much should I use in a tank sprayer?
Hi Jeffrey,
Castor oil odor lasts for a few hours, though fresh castor oil should have a very mild smell that is typically descibed as “earthy” and is difficult to detect once applied to the lawn. Castor oil that is dark in color, cloudy, or that has a strong, rotten smell is likely to be rancid and should not be used.
In a sprayer, the typical ratio of castor oil to water is 1 Tablespoon of castor oil for every 4 ounces of water or 1 cup (8 oz) castor oil per gallon of water. The total amount of liquid will depend on the size of your sprayer.
I hope this helps 🙂
Can I use liquid dish soap to mix with castor oil ?
Hi Kwong 🙂
If you want to use castor oil as a mild pesticide or insecticide: Dish soap should not be mixed with castor oil as dish soap can render the castor oil inert or ineffective. Dish soap is a detergent that will break down oils, including castor oil. Without the oil component, castor oil isn’t very effective at preventing or eliminating pests.
However, if you want to get rid of castor oil and the smell that often accompanies it (maybe because you spilt some on your lawn accidentally or because you used it and don’t like the effects), dish soap is highly effective.
I hope that helps!
Hello, Thank you for the info. Castor oil has poor solubility in water. Are you saying about spraying an emulsion? The purpose of adding dish soap is to introduce surfactant to better solubilize the oil or rather stabilize the emulsion.
I tried spraying your recipe from standard 1-gallon sprayer and the oil phase separates with water and sits at the bottom of the sprayer. As a result, the oil starts plugging the spraying nozzle. What are your thoughts on this?
Hi Alex,
Yes, dish soap can be used as a surfactant and does a very good job of this in some situations. When mixed with some oily substances, such as castor oil, it can lessen the effectiveness of these products once they have been emulsified and result in the products being washed away during the next watering or rain.
Using the recipe mentioned in the comment above, it’s important to agitate the sprayer tank frequently or turn any mechanical agitators on. This keeps the oil and water mixed and provides a more even spray while preventing the oil from clogging the nozzle.
Alternatively, you could use a solubilizer or a hydrogenated form of castor oil though most extension offices recommend the agitation method.
Is all castor oil the same when wanting to use it as a mole/vole repellent?
Hi J,
There are two main types of castor oil: yellow and black. The yellow castor oil is the one most commonly used, and it’s the least processed. Just make sure that whatever you use, it’s 100% pure castor oil. I would stay away from any castor oil that might be mixed with other ingredients to avoid accidentally harming your yard.
Let me know how it goes!
Cheers, Tom.
Will castor oil affect grass seed germination?
I am aerating and have a terrible mole or vole population. I am thinking that I will apply the castor oil from RTS Molemax product before seeding.
Hi Rick,
Moles definitely don’t like the smell of castor oil. I’ve heard of people using the ready to spray MoleMax bottles and that they work very well. Bonide appear to state that you can use their MoleMax product at any time around seeding so you should be ok doing it before you spread. I’ve used this type of applicator before and they make for quick work. Products like MoleMax need to be repeated every 3 months so don’t forget the next application.
An alternative is to use a mole-repellent stake. This is a battery/solar-powered device that creates noise in the ground that moles hate. A crude homemade version is to put a beer can on the end of a stick and push it into the ground. The Mole hears the rattle of the can as it clanks on the stick in the breeze and turns the other way.
Good luck with the moles!
Tom.