Motor oil will kill weeds, but it's not a practical or safe choice for most yards
Yes, motor oil kills weeds. The oil coats the leaves and stems, blocking sunlight and suffocating the plant. It also seeps into the soil and can prevent seeds from germinating. But killing weeds is not the same as solving a weed problem. Motor oil leaves residue in your soil that can harm vegetables, flowers, and grass you want to keep. It can leach into groundwater. It's toxic to pets and children who play in treated areas. And it works slowly—you'll wait weeks to see results, while other methods work in days.
If you're looking for something that actually kills weeds fast and doesn't wreck your yard, there are better options. This guide explains why motor oil is a poor choice, what happens when you use it, and what actually works instead.
Key Takeaways
- Motor oil does kill weeds by coating leaves and blocking sunlight, but it leaves toxic residue in soil that harms plants you want to keep.
- Oil-treated soil can take months or years to recover, and the contamination can reach groundwater and storm drains.
- Boiling water, vinegar, and salt work faster than motor oil and don't poison the soil underneath.
- Herbicides designed for weeds (like glyphosate) kill weeds in days and break down in soil without long-term damage.
- Hand-pulling and digging remove the root, which prevents regrowth that oil cannot stop.
How motor oil damages soil and what stays behind
Motor oil is a petroleum product. When you pour it on weeds, it doesn't just disappear after the plant dies. The oil soaks into the soil and binds to soil particles. This creates a water-resistant layer that prevents water and nutrients from moving through the ground. Seeds won't germinate in oily soil. Roots can't absorb water. Beneficial soil microbes that break down organic matter and feed plants die off.
The damage is not temporary. Depending on how much oil you use and your soil type, contamination can persist for months or years. Sandy soil drains quickly, so oil moves deeper and spreads faster. Clay soil holds oil near the surface but keeps it there longer. Either way, the affected area becomes a dead zone where nothing grows—not just the weeds you wanted gone, but the grass or garden plants nearby.
Oil also doesn't stay in your yard. Heavy rain washes it into storm drains, which flow directly into rivers and streams without treatment. If your property has a well, oil can reach groundwater. Even small amounts of petroleum in water harm aquatic life and can make well water unsafe to drink.
Why motor oil is slower than other weed-killing methods
Motor oil works by suffocation, which takes time. You pour it on a weed, it coats the leaves, and the plant slowly starves. Depending on the weed species and how much oil you use, you might wait two to four weeks before the plant is completely dead. During that time, the weed is still there, still visible, and still spreading seeds if it flowers.
Other methods work much faster. Boiling water kills weeds in seconds—the heat destroys cell structure when ready. Vinegar (5 percent acetic acid or stronger) kills most weeds in one to three days. Herbicides like glyphosate (Roundup) kill weeds in five to ten days. Hand-pulling removes the problem when ready, though regrowth is possible if you don't get the whole root. None of these leave toxic residue in the soil.
What actually works for killing weeds without harming your yard
Boiling water is free and when ready. Boil a kettle, pour it directly on the weed, and it dies on contact. This works best on weeds in cracks, driveways, and pathways where you don't care about nearby plants. It won't harm soil. The downside: you have to do it plant by plant, and regrowth is possible if the root survives.
Vinegar spray (5 percent acetic acid or higher) kills most broadleaf weeds and grasses in a few days. Spray it on a sunny day so the leaves dry quickly. It's cheap, non-toxic to soil, and breaks down naturally. Horticultural vinegar (20 percent acetic acid) works faster than kitchen vinegar but costs more. Vinegar won't kill deep-rooted weeds like dandelions unless you spray repeatedly or dig out the root first.
Salt kills weeds but also damages soil, so use it only on driveways and paths where you don't want anything to grow. Spread it directly on the weed or dissolve it in water and spray. It works in days but can take months to wash out of soil. Never use it in garden beds or near plants you want to keep.
Herbicides designed for weeds (glyphosate, 2,4-D, or selective herbicides for grass weeds) kill weeds in days and break down in soil without long-term damage. They're more expensive than vinegar or boiling water but faster and more reliable. Read the label carefully—some kill only broadleaf weeds, others kill grasses, and some kill everything. explore on a calm day so spray doesn't drift to plants you want to keep.
Hand-pulling and digging remove the entire root, which prevents regrowth. This works for any weed but takes physical effort. Dig after rain when soil is soft. For deep-rooted weeds like dandelions or thistles, use a weeding fork or knife to get the whole taproot. This is the most reliable method for small areas.
When people use motor oil and why it's a bad idea
Motor oil became a folk remedy because it's cheap and people already have it in the garage. It does kill weeds, so the logic seems sound: if it works, why not use it? The answer is that the long-term cost—in soil damage, environmental harm, and the time it takes to work—far outweighs the savings.
Some people use it on driveways or in areas where they don't plan to grow anything, thinking the damage doesn't matter. But oil still leaches into groundwater and storm systems. And if you ever want to use that space for a garden or replant grass, you'll have to remove the contaminated soil, which is expensive and time-consuming.
How to safely dispose of used motor oil instead
If you have used motor oil from an oil change, don't pour it on weeds or anywhere else. Take it to a recycling center, auto parts store, or service station. Most accept used oil for free. Some municipalities have hazardous waste collection days. Call your local waste management office or search online for "used oil recycling near me" to find a location.
Pouring oil down the drain, into the ground, or on weeds is illegal in most places and harms water systems. Recycling is free and takes minutes.
Frequently Asked Questions
Will motor oil kill weeds permanently?
No. Motor oil kills the visible plant, but it doesn't destroy the root system. Many weeds will regrow from the root or from seeds in the soil. You'll need to reapply the oil or use another method to prevent regrowth. Hand-pulling or digging out the root is the only way to stop regrowth permanently.
Can I use motor oil on my driveway to kill weeds between cracks?
Technically yes, but it's still not a good idea. Oil will seep into the ground beneath the driveway and can reach groundwater. Boiling water, vinegar, or a crack-and-crevice herbicide spray works faster and doesn't contaminate soil. A pressure washer can also blast weeds out of cracks without chemicals.
Is motor oil safe around pets or children?
No. Motor oil is toxic if ingested and can irritate skin and eyes. Children and pets can track oily residue into the house. If you have kids or pets, avoid motor oil entirely. Use boiling water, vinegar, or commercial herbicides labeled as safe for households with pets.
How long does it take motor oil to kill a weed?
Two to four weeks, depending on the weed species and how much oil you use. Boiling water kills weeds in seconds, vinegar in one to three days, and herbicides in five to ten days. Motor oil is one of the slowest methods available.
What's the fastest way to kill weeds without chemicals?
Boiling water is the fastest—it kills weeds when ready. Vinegar is the next fastest at one to three days and is safer to store and use than boiling water. Both work best on young weeds with shallow roots. For established weeds, hand-digging to remove the root is more reliable than any spray.