DIY Weed Killer Calculator

What each of the three ingredients does

Three ingredients, three jobs. Vinegar does the burning, salt does the drying, and dish soap makes the other two stick to the leaf long enough to work. None of them travels down to the root, which is why the mix looks dramatic on day one and disappointing three weeks later.

Vinegar is acetic acid, and at 5 percent, which is what a grocery store jug contains, it is strong enough to break down the cell walls in the leaves it touches. The damage shows up within hours as a bleached or brown patch. At 20 percent, sold as horticultural vinegar, the burn is faster and deeper, and the liquid can damage skin and eyes, so gloves and glasses are not optional.

Salt pulls water out of plant cells by osmosis. On its own it is a slow killer, and mixed into the spray it adds a second hit to anything the vinegar soaked. The problem is that salt does not leave. It stays in the top inch of soil until rain or irrigation washes it deeper, and plants that come later pay the bill.

Dish soap is a surfactant. Water beads up on a waxy leaf, and a droplet that beads rolls off without doing damage. Soap breaks the surface tension so the spray spreads into a thin film and stays there. A tablespoon per gallon is enough; more soap burns the leaf with detergent rather than acetic acid and adds nothing.

Mixing order matters less than people think, but the salt has to dissolve. Stir it into a cup of warm water first, then add that to the gallon, then the soap last so it does not foam up while you stir. Spray within a day, because the mix does not improve with age.

The standard mix and the stronger one

The classic recipe is one gallon of 5 percent vinegar, one cup of salt and one tablespoon of dish soap. That gallon covers about 300 square feet of weeds when you spray to wet rather than to runoff, and it costs about $2 in ingredients.

Two upgrades show up in extension publications. The first is stronger vinegar: 20 percent horticultural vinegar kills faster and does more damage to the crown, and 30 percent is sold for tough brush. The second is a dash of orange oil or a commercial surfactant in place of dish soap, which spreads better on waxy leaves like plantain and thistle.

What does not work is the version with extra salt. Doubling the salt to two cups per gallon does not kill roots, and it doubles the salt load in the soil, which is the part of this recipe with the longest consequences. If the weeds are still alive after a week, the answer is a second application or a different method, not more salt.

Hot water is the free upgrade. Spray at midday on a sunny day when the plant is heat stressed, and the same mix does more damage than it does at 7 in the morning. Rain within six hours washes the spray off and you start over.

Worked example: 300 square feet along a fence

Say you have 300 square feet of weeds along a fence line, a 1 gallon sprayer, and a jug of 5 percent vinegar. The calculator returns one gallon of spray, which is one gallon of vinegar, one cup of salt and one tablespoon of soap. No water goes in, because the vinegar is already the carrier.

The finished spray is 5.0 percent acetic acid. That number matters: if you dilute a gallon of vinegar with a gallon of water, you have 2 gallons at 2.5 percent, which is weak enough that many weeds shrug it off. Dilution is the most common reason a homemade spray fails.

Salt lands at 0.33 cups per 100 square feet, or about 2.7 oz. Over the 300 square feet that is 273 g of sodium chloride sitting on the soil surface. On a gravel driveway or a patio crack that is fine. On a garden bed where you plan to sow lettuce in six weeks, it is a problem.

Scale it up and the numbers get uncomfortable. A 1,000 square foot area at 300 square feet per gallon needs four gallons, which is four cups of salt, about 1,092 g, spread over the same ground. That is 0.40 cups per 100 square feet, which the calculator flags as high. The vinegar cost scales linearly and stays cheap; the salt cost scales into a soil problem.

Why it burns leaves but leaves the root alive

Vinegar is a contact herbicide, not a systemic one. It damages whatever it touches and stops there. Glyphosate, by contrast, is absorbed and carried to the root, which is why one application can kill a dandelion outright. Acetic acid has no such transport, so a dandelion with a taproot three inches down loses its leaves and keeps its root.

The result is predictable. Annual weeds that grow from seed, like chickweed, purslane and young crabgrass, die because the whole plant is above ground. Perennials with storage roots, like dandelion, thistle, bindweed, nutsedge and plantain, regrow from the crown in 2 to 4 weeks, often with more shoots than before because the top growth no longer shades them.

That table is why the recipe is best used on driveways, sidewalks, gravel and mulch beds where the goal is to clear what is there now, not to win a season long fight. For a lawn full of dandelions, the honest answer is a registered broadleaf herbicide or digging the taproot, not a stronger vinegar mix.

Salt stays in the soil

Salt is the ingredient with a second act. Sodium chloride does not break down, bind to soil or evaporate. It sits in the top inch until water moves it, and in a raised bed or a dry climate it can stay for years. The damage shows up as soil that crusts, water that will not soak in, and seedlings that yellow and stall.

The mechanism is osmotic and structural. High sodium pulls water away from roots, so plants look drought stressed even when the soil is wet. Sodium also disperses clay particles, which destroys the crumb structure that lets air and water move. In a sandy soil the effect is smaller; in a clay soil it can take a season of compost and gypsum to undo.

The thresholds are a rule of thumb, not a lab test. What they capture is the direction: every extra half cup per gallon adds 45 g of salt per 100 square feet, and that salt does not leave when the weeds do. A soil test that reports electrical conductivity is the only way to know what your bed can take, and most state extension labs run one for $15 to $40.

If the area has to stay plantable, drop the salt entirely and use vinegar plus soap. It kills annuals just as well, and the only residue is a temporary dip in pH that rain corrects. The salt is there to make the burn faster, not to make it work at all.

How to spray so it works

Application decides more of the outcome than the recipe. Four rules cover most of it.

• Spray at midday on a sunny day. The burn is driven by sunlight and heat, and an evening application on a cool leaf does half the damage.
• Wet the leaf, do not flood it. Runoff carries the vinegar and the salt onto the soil, which is the one place you do not want either. A fine mist until the leaf glistens is enough.
• Cover the crown, not just the leaves. The growing point at the base of the plant is the target. A plant sprayed only at the tips keeps growing from the middle.
• Repeat in 10 to 14 days. One pass kills what it touches. The second pass catches regrowth while it is small, which is when a contact spray works best.

Clean the sprayer the same day. Salt water corrodes metal nozzles and pumps, and a sprayer left with the mix in it can seize within a week. Rinse with clean water, run it through the nozzle, and store it dry.

Keep the mix off concrete you care about, and off any surface where it can run into a storm drain. Vinegar is acidic enough to etch unsealed concrete and to hurt aquatic life if it reaches a stream in volume. Spray on a calm day, and do not apply before rain is forecast within six hours.

When a registered herbicide is the better tool

There are places where the homemade mix is the wrong tool, and it is worth saying so plainly. A lawn with a broadleaf problem, a bed of perennial weeds, poison ivy near a path, and anything growing between pavers over a root system all need either a registered herbicide or a physical method. The DIY spray is a good fit for gravel, sidewalks, driveway cracks, mulch beds and bare ground where nothing is meant to grow.

If you use a registered product, read the label and follow it. The label is the legal document that sets the rate, the protective gear and the waiting period before you replant, and it is written for the specific weeds and the specific crop. The EPA pesticide program pages explain how those labels are reviewed and what the signal words mean.

For the gardening side, extension services have tested the vinegar question for years. Montana State University's review of vinegar as a weed killer covers the concentration that works, and Washington State University's salt and vinegar summary covers what the salt does to soil afterward. The University of Maryland's vinegar as a glyphosate alternative page is a fair read on where the comparison holds and where it does not.

One safety note that gets skipped in videos: never mix vinegar with bleach, and never store the spray in a drink bottle. Acetic acid plus bleach releases chlorine gas, and a clear bottle of cloudy liquid in a garage is how kids and pets get poisoned. Label the sprayer, and keep it out of reach.