Does Washing Up Liquid Kill Weeds?

Does Washing Up Liquid Kill Weeds?

No. Washing up liquid (dish soap) has no herbicidal properties whatsoever. It won’t kill weeds, damage weeds, or affect weeds in any meaningful way when used alone. It’s designed to cut through grease on dishes, not harm plants.

So why does it appear in so many homemade weed killer recipes? Because it serves a different purpose entirely – helping other ingredients work slightly better.

What Washing Up Liquid Actually Does

Washing up liquid is a surfactant. Here’s what that means for weed treatment:

Breaks surface tension. Water naturally beads up on waxy plant leaves rather than spreading out. Surfactants reduce this surface tension, allowing liquids to spread into a thin film instead of sitting as droplets.

Improves coverage. When you add dish soap to a spray solution, it coats leaves more evenly rather than running off. More leaf surface contacts the solution.

Helps penetration. By breaking surface tension, surfactants can help solutions penetrate the waxy cuticle slightly better than water alone.

Acts as a “sticker.” The solution adheres to leaves longer, giving any active ingredients more contact time.

Why It’s Added to DIY Recipes

You’ll find washing up liquid in recipes combining it with vinegar, salt, or both. The logic:

Vinegar runs off waxy leaves. Many weeds have waxy coatings that repel water-based solutions. Plain vinegar beads up and drips off before it can work.

Dish soap helps vinegar stick. Adding a squirt of washing up liquid helps the vinegar spread across the leaf surface and stay in contact longer.

Better coverage = slightly better results. A thin film of vinegar across the entire leaf causes more damage than droplets on small areas.

The Fundamental Problem

Improving delivery doesn’t fix the underlying limitation:

Vinegar is still contact-only. Even perfectly applied, vinegar only burns the tissue it touches. It can’t travel through the plant to reach roots.

Roots remain unaffected. The extensive root systems of established weeds sit completely untouched by surface treatments, no matter how well they coat the leaves.

Regrowth is inevitable. Weeds with taproots, rhizomes, or spreading root networks simply regrow from underground after surface damage.

Adding washing up liquid to vinegar is like polishing a butter knife before trying to cut steak. The knife is shinier, but it still can’t do the job.

Can It Harm Plants at All?

In high concentrations, washing up liquid can cause some plant stress:

Strips protective wax. Very soapy solutions can remove some of the waxy coating that protects leaves, potentially increasing water loss.

May cause mild burning. Some plants are sensitive to detergents, showing brown spots where concentrated soap contacted leaves.

Disrupts beneficial insects. Soapy water can harm soft-bodied insects like aphids – this is actually a legitimate pest control use.

But none of this constitutes “killing weeds.” At best, you might mildly stress some foliage. The plant survives easily.

Comparing DIY Additives

How does washing up liquid compare to the actual “active ingredients” in homemade recipes?

Washing up liquid: Zero weed-killing ability. Helps other ingredients spread. Useful as an additive only.

Vinegar: Burns leaves on contact. Can’t reach roots. Works on seedlings, fails on established weeds.

Salt: Dehydrates plants and poisons soil. Damages soil for years. Environmental problems.

Boiling water: Kills on contact through heat. No residue. Limited to small areas.

Washing up liquid is the least harmful ingredient in these recipes – because it doesn’t do anything to weeds.

What Actually Kills Weeds

For effective weed control, you need products designed for the job:

Systemic herbicides. Glyphosate-based weedkillers absorb through leaves and travel to roots. One application kills the entire plant.

Commercial surfactants. Professional weedkillers already contain optimised surfactants. No need to add dish soap.

Strong weed killer: Formulated to handle tough, established weeds with deep root systems.

Residual weedkiller: For paths and drives where you want long-term prevention, not just contact treatment.

These products work whether dealing with simple patio weeds or stubborn problems like dandelions in lawns.

The Verdict

Washing up liquid doesn’t kill weeds. It’s a surfactant that helps other liquids spread and stick to leaves. Adding it to vinegar creates a slightly more effective contact treatment – but contact treatments fundamentally can’t kill weeds with established root systems.

If you’re going to try DIY methods, adding a drop of dish soap to vinegar is reasonable. Just understand you’re optimising a treatment that still won’t work on anything more than tiny seedlings.