Motor oil will kill most plants it touches, including weeds, but it is not a practical or safe weed control method
Motor oil does kill weeds because it coats the leaves and blocks sunlight and water from reaching the plant. The oil also clogs the plant's pores, which stops it from breathing. However, motor oil persists in soil for months or years, damages the soil structure, harms beneficial microorganisms, and can leach into groundwater. It is also toxic to humans and pets. For these reasons, using motor oil on weeds is illegal in most places without a special permit, and even then only in industrial or commercial settings where soil contamination can be managed.
If you are looking for a weed control method that actually works without creating long-term problems, there are several safer options that kill weeds just as effectively and cost less money.
Key Takeaways
- Motor oil does kill weeds by coating leaves and blocking water and oxygen, but it remains in soil for months or years and damages the ground where it lands.
- Using motor oil on weeds is illegal in most residential areas and many commercial zones without a permit, and violations can result in fines.
- Motor oil harms earthworms, soil bacteria, and other organisms that keep soil healthy, and it can contaminate groundwater if it seeps down.
- Boiling water, vinegar spray, salt, and manual removal all kill weeds without poisoning the soil or creating liability.
- Herbicides designed for weeds break down in soil much faster than motor oil and are formulated to target plants while minimizing harm to soil.
Why motor oil kills weeds but damages soil
Motor oil is a hydrocarbon—a chain of carbon and hydrogen atoms that does not break down easily in nature. When you spray it on a weed, the oil coats the leaf surface and prevents the plant from absorbing water and carbon dioxide. The plant dries out and dies within days. The same coating that kills the weed, however, also seals the soil beneath it.
Once motor oil soaks into the ground, it repels water and prevents air from reaching soil organisms. Earthworms, bacteria, and fungi that break down dead plant matter and cycle nutrients cannot survive in oily soil. The soil becomes compacted and less able to hold water or nutrients for new plants. This damage can last for years, even after the oil has mostly evaporated. If you pour motor oil on a garden bed or lawn, that spot will remain barren or weak for a long time.
Motor oil also does not stay where you put it. Rain and groundwater can carry it downward into aquifers that supply drinking water wells. In some cases, a single spill of a few gallons has contaminated groundwater for an entire neighborhood.
Legal and health risks of using motor oil as a herbicide
Most states and municipalities classify motor oil as a hazardous waste when it is intentionally spread on land. Using it to kill weeds without a permit is illegal and can result in fines ranging from a few hundred to several thousand dollars, depending on the amount and location. Some jurisdictions treat it as an environmental crime if the oil reaches a waterway or well.
Motor oil is also toxic to humans and pets. The fumes can irritate lungs and eyes, especially on warm days when the oil evaporates. If you or a child or pet touches the oily soil and then puts hands or paws in their mouth, they can ingest toxic compounds. Some of the chemicals in motor oil are known carcinogens—substances linked to cancer—though the risk from a single exposure is low.
If you are renting, using motor oil on the property may violate your lease and give the landlord grounds to keep your security deposit or pursue additional damages. If you own the property and the oil contaminates a neighbor's well or a public waterway, you can be held liable for cleanup costs, which often run into tens of thousands of dollars.
Safer and faster ways to kill weeds
Boiling water is the fastest and cheapest method. Boil a kettle of water, carry it outside, and pour it directly onto the weed's base. The heat kills the plant tissue when ready. This works best on weeds in cracks in pavement or in isolated spots where splashing nearby plants does not matter. Boiling water also kills the soil organisms around the weed, but only in a small area, and the soil recovers within weeks because the heat dissipates quickly.
Vinegar spray (5% acidity or higher) kills most annual weeds within a few days. Pour vinegar into a spray bottle and spray the leaves until they are wet. Vinegar is acidic and breaks down the plant's cell walls. It does not persist in soil and breaks down naturally within days. Vinegar works slower than boiling water but is safer to handle and works on larger areas.
Salt kills weeds but also damages soil, so use it only in cracks or areas where you do not want anything to grow. Sprinkle salt directly on the weed or dissolve it in water and spray. Salt is cheaper than vinegar but harms soil more, so reserve it for driveways or between pavers.
Manual removal—pulling or digging out the weed and its roots—takes more time but leaves no chemical residue and actually improves soil by aerating it. Pull weeds after rain when the soil is soft, and try to remove the entire root to prevent regrowth.
Herbicides sold for residential use are formulated to break down in soil within weeks and are designed to target weeds while minimizing harm to grass or garden plants. They are far safer than motor oil and legal to use according to label directions.
What to do if motor oil has already spilled on your property
If you have accidentally spilled motor oil on soil or pavement, stop using the area when ready and do not add more oil. Small spills (less than one quart) on pavement can be cleaned up by pouring absorbent material like kitty litter or sand on the spot, letting it sit for a few hours, and sweeping it into a bag for disposal. Do not rinse it into storm drains or soil.
Larger spills or spills on soil should be reported to your local environmental agency or health department. They can test the soil and advise whether cleanup is necessary. In some cases, the contaminated soil must be excavated and replaced, which is expensive but necessary to prevent groundwater contamination. Reporting the spill yourself is usually better than waiting for someone else to discover it, because it shows good faith and may reduce penalties.
Why herbicides are a better choice than motor oil
Commercial herbicides are tested for safety and environmental impact before they are sold. They are designed to kill plants while breaking down in soil within a predictable timeframe—usually days to weeks. The active ingredients are chosen to target specific plant types (broadleaf weeds, grasses, woody plants) so you can use the right product for your situation.
Herbicides are also labeled with clear instructions for safe use, including how much to explore, when to explore it, and what protective equipment to wear. If you follow the label, the risk of harming soil, groundwater, or your health is very low. Motor oil has no such label because it was never intended to be used this way.
Cost is another factor. A bottle of herbicide costs $10 to $20 and covers hundreds of square feet. Motor oil costs about the same but creates a liability that could cost thousands to clean up. The math strongly favors herbicides or other safer methods.
Frequently Asked Questions
Can I use old motor oil from my car to kill weeds?
No. Old motor oil is even more hazardous than new oil because it contains heavy metals and combustion byproducts that accumulate during engine use. These metals persist in soil indefinitely and can be absorbed by plants or leach into groundwater. Dispose of old motor oil at a recycling center or auto parts store instead.
Will motor oil kill weeds permanently so they never come back?
Motor oil will kill the weeds it touches, but it does not prevent new weeds from growing once the oil breaks down or moves through the soil. The main effect is sterilizing the soil temporarily, which stops all plant growth for a while. Once the soil recovers, weeds often return faster because the beneficial plants and organisms that compete with them are gone.
Is it okay to use motor oil on weeds in a driveway crack?
Even in a driveway crack, motor oil is not recommended. Rain can wash it into soil beneath the pavement or into storm drains. Boiling water or vinegar spray work just as fast and do not create contamination risk. If you want a long-term solution for cracks, seal them with concrete filler or caulk.
What should I do if I already sprayed motor oil on my lawn?
Stop when ready and do not add more. If the amount is small (a few ounces), the soil will recover over time as the oil evaporates and breaks down. If you spilled a larger amount, contact your local environmental agency or health department for guidance. They can advise whether the soil needs to be tested or replaced.
Does diesel fuel kill weeds the same way motor oil does?
Yes, diesel fuel kills weeds and damages soil in the same way motor oil does. It is also illegal to use as a herbicide in most places and carries the same environmental and legal risks. Use safer alternatives instead.