The Strongest Weed Killer | Expert Advice & Test Results (2024)

Tested by Daniel Woodley. Fact checked by Elizabeth Smith. Published to Products on the 24th of September 2021. Updated: 27th February 2023.

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I’m not a fan of using powerful weed killers, as I know from experience that if I keep on top of my garden maintenance, I rarely need to use any chemicals at all.

However, there are occasions when only the strongest weed killer will do the job:

• Working with a very neglected garden.
• Large spaces where it’s not practical to use tools.
• Persistent weeds and invasive plants.

For tasks like these, there’s only one or two products that I would consider using.

My name is Daniel Woodley, I’m the co-owner of DIY Gardening, and this page is all about the strongest weed killers for specific scenarios.

Details & My Experience

I checked dozens of products before creating this guide and this is one of the strongest weed killers on sale and comes concentrated, so you get more mileage per bottle.

When I tested Doff, I found that this 1-litre product will, when diluted, treat up to 400 sq metres, so it is perfect for almost all domestic gardens.

Not only will Doff’s Weed Killer take out any known weeds, but it’s so powerful it will kill off most plants too; in fact, the only plants that could put up any resistance to this weed killer are ivy, the infamous Japanese Knotweed, bracken and tree roots but I’ve found a product that treats those too.

Doff is not a weed killer for lawns or flowerbeds where you have lots of plants growing as based on the results of my testing, it will take out everything. This is the strong stuff and I feel it’s best suited to persistent weeds or situations where you need to destroy all plant life in a large area so you can start from scratch.

I’ve used Doff dozens of times over the last few years but don’t take my wood for it, I’ve got photos to prove it works.

Specifics

• 1-litre bottle.
• Glycosulphate in liquid form at 90g per litre concentration.
• Treats up to 400 sq metres.
• Targets all plant life but works best on tender weeds.
• No gloves or face masks are supplied.
• You’ll need a dedicated garden sprayer (not expensive) or watering can to avoid contamination.

Pros

Perfect for large areas of overgrown weeds and unwanted plants such as nettles. I have personally used this product to clear long grass, weeds and plants growing in general wasteland.

It’s liquid-based and easily dilutable, I experienced no problems mixing or using this weed killer.

I have used it on hard surfaces without issue (patch test first), although it should be sprayed onto the weed rather than liberally over patios/driveways.

It contains Glycosulphate at 90g per litre, which is suitable for all known weeds.

In my test, it killed the roots as well as top growth. The weeds didn’t grow back.

While very powerful, it breaks down into harmless by-products in the soil.

Cons

As you can see from the images below, this weed killer can’t be used on lawns unless you want to kill the grass too.

It will kill most plants on contact, so it isn’t suitable for flowerbeds with plants in them.

I know from experience and from the tests I did (see images) that it can be slow to act as the plant must absorb the chemical, where it then causes the cells within the plant to collapse. This can take 1-3 weeks before noticeable results; although the weeds will be dead much sooner, they may look green for some time.

This product can be very harmful to wildlife, pets, children and adults, and care should be taken to follow the instructions, especially with regard to wind, rain and skin protection.

I have used stronger weed killers than this which contain the same chemical, just at higher concentrations (see next product on my list).

Test Results

Here are the results of my 21-day test, as you can see, Doff’s weed killer got the job done, but it took between 2-3 weeks.

Read more: My full review of Doff’s weed killer.

Details & My Experience

While Doff’s very effective weed killer contains Glyphosate at 90g per litre, Roundup Stump Killer contains the same chemical but at 441g per litre.

Needless to say, this is the nuclear option and based on the research I did, it’s by far the strongest weed killer on the market.

Despite its name, I’ve found that this product from Roundup can also be used on plants, including brambles, ivy, Japanese Knotweed and saplings as well as persistent weeds such as the notorious Horse Tail (which sprouts from underground tubers).

If you’re looking for a strong weed killer, choose Doff’s product, but for the strongest nuclear option, try Roundup Tree Stump Killer.

My Experience

I’ve used Roundup’s Tree and Stump Killer on bracken and ivy that didn’t respond very well to an earlier treatment from a similar but weaker chemical. None of the bracken or ivy survived, and even the roots were killed off.

In my previous role as a landscaping project manager, I worked with teams of landscapers who had experience in removing Japanese Knotweed – one of the most invasive plants in the UK. The workers used the same chemical (Glyphosate), and it was at similar concentrations to Roundup. In addition to spraying the foliage, they also injected it into the hollow stems, and I was told that individual knotweed plants are often killed off after one treatment, while infestations take two treatments a year, repeated over two to four years.

Specifics

• 441g of Glycosulphate per litre.
• Liquid form, dilutable.
• 250ml bottle.
• No mask or gloves supplied.

Pros

The strongest weed killer you can buy domestically. I couldn’t find anything stronger on sale in the UK.

It will kill all known plants, as I’ve witnessed first-hand.

One 250ml bottle contains enough dilutable chemicals to create 25 litres of product.

Breaks down naturally in the soil.

I’ve seen experts use it to treat Japanese Knotweed and even Bamboo – plants that are notoriously difficult to eliminate. Although based on my experience of treating Japanese Knotweed, you’ll need to apply it twice a year for up to three years.

Cons

Takes time until results are visible – from my testing and experience, usually up to two weeks.

Potential health implications if misused. Questions have been raised about the safety of strong weed killers such as this.

Not appropriate for widespread, non-targeted use as it’s too strong.

Smaller bottle and less coverage than Doff’s product.

Not suitable for lawns, flowerbeds containing plants or anywhere desirable plants are growing.

Details & My Experience

Weedol’s Lawn Weed Killer comes in two versions, the watered-down “ready to use” product, which is a waste of money and plastic packaging and the strong concentrated version, which, when diluted, covers 330 sq metres of lawn.

As the name suggests, this weed killer is safe for lawns, and I have used it on lawns two months old without any problems.

My Experience

I tested Weedol, and I’ve had good results with it on:

• Daisies.
• Buttercups.
• Dandelions.
• Plantain.
• Clovers.
• + many more.

I inherited a weed-infested lawn a few years ago when I moved home, and I had great results by treating it with Weedol in the spring and Greenforce in the autumn. I managed to get rid of 90% of the weeds in the first year, with the rest in the second year and none returned, I didn’t even have to apply any lawn weed killers in year three.

Specifics

• 10.7g of fluroxypyr per litre.
• 5.4g of clopyralid per litre.
  
• 53.4g of MCPA per litre.
• Liquid-based and dilutable.
• 500ml bottle.
• 330 sq metres coverage.
• Apply no more than twice per year.
• No mask or gloves supplied.

Weedol Test Results

I’ve previously tested this and several other lawn weed killersand found that this was the best of the bunch.

Here are some before and after photos of the weeds, taken from the test I performed:

(Please ignore the brown grass, we had a heatwave here in the UK)

Pros

Can be used on new lawns that are at least 2 months old. When I used it on a new lawn, it didn’t harm the grass at all.

Contains three active chemicals that each treat a different set of weeds.

100% lawn safe, provided the lawn is at least 2 months old.

It contains a concentrated formula so you get more coverage and there’s less plastic waste.

Pet and child safe after 24 hours provided there’s no rain in that time.

In my test, I found it killed the weeds within 14 days.

Cons

Doesn’t kill every known weed or even every common lawn weed.

Based on my experience, if you want 100% lawn weed control, it should be used in conjunction with Resolva (Resolva in spring and Weedol in autumn). Resolva contains different chemicals that kill different species of weeds that Weedol struggles with.

To protect the grass, this weed killer isn’t designed to be the strongest on the market, and it may take several applications, 6 months apart, to control the weeds.

For maximum coverage, I’ve found that lawn weed killers are best applied by sprayer rather than a watering can, and this product doesn’t come with a sprayer included.

Overview & My Experience

With years of experience, I’ve found that even the strongest weed killer will be useless against moss, mould, algae and lichens as these are entirely different species and require a dedicated treatment product.

I’ve used Benzalkonium Chloride (BAC) hundreds of times to treat moss on hard surfaces and have had great success with it.

My first ever job was as a roofer, and we used BAC to remove lichens and moss growth on roof tiles; this was 25 years ago, and it’s still the best product on sale.

BAC is the key ingredient in many branded products, including:

• Patio Magic.
• BAC50 (a concentrated 50% product)
• Smartseal.
• Pro Kleen.

Benzalkonium Chloride is for use on hard surfaces only and shouldn’t be used on lawns or flowerbeds.

Lichens (black spots on patios) can take weeks to lift off after treatment, as even though they are dead, they still cling to the surface.

For lawn moss, try one of these strong products.

Summary

Glycosulphate is the strongest weed killer chemical on sale and will kill grass too, but most gardeners won’t need a product this strong as more targeted chemicals are nearly as effective.

Health concerns about Glycosulphate have led several countries to ban its sale, but the general consensus is that occasional use by those wearing PPE is safe.

Lawn weeds should be treated with products that contain some or all of these:

• Dicamba.
• Mecoprop-p.
• MCPA.
• Clopyralid.
• Fluroxypyr.

Moss, algae, lichens and mould on hard surfaces should be killed off with Benzalkonium Chloride.

I’ve had good results getting rid of lawn moss by treating it with a bacteria (bacillusspp) which eats away at the moss over several months (see my review of lawn moss products here)

I Analysed Hundreds of Weed Killer Reviews Online. Here’s What I found:

Despite having a wealth of experience and having previously used every product on this page, I also researched the topic and here’s what I found:

• Most of the negative reviews I found online were from people who didn’t wait long enough for the product to act.
• Some weed killers cause the cells within the plant to grow quickly until they collapse and die; this can result in an initial burst of growth that some reviewers mistook for the product not working.
• Weed killers are more effective during the growing season as the weed absorbs the chemical much quicker.
• I’ve seen weed killers take much longer to work during periods of dry weather in the summer.
• I saw lots of reviewers were misusing the product, for example, using a weed killer on moss.
• Many reviewers were applying the weed killers by watering can which is wasteful, I’ve found that a small 5-litre pump sprayer is far better, more accurate and creates less wastage/runoff.

Suggested Application Method:

To avoid contaminating watering cans, I suggest buying a garden sprayer and reserving it for weedkiller chemicals.

I tested and reviewed the most popular knapsack garden sprayers here.

Alternatives I Recommend

Chemical-free weed killing methods have become popular in recent years as consumers are now more aware of the dangers chemicals pose to human and animal health.

Electric weed burners are popular, as are gas-powered burners.

Both are suitable for weeds on hard surfaces but they don’t kill the roots so repeat applications are required throughout the growing season.

This page lists the best weed burners.

Boiling water is also effective but I feel it wouldn’t be practical for large areas, grass or flowerbeds.

Pelargonic acid is a softer, more gentle weed killer but from my experience and testing, it’s not as effective but is far more environmentally friendly. It’s even included in many pet-friendly weed killer products.

Also, there are plenty of tools designed to pluck weeds and their roots from the soil and lawns, and some of these gadgets are very effective.

I Don’t Recommend

There is a lot of nonsense on the internet about “homemade” weed killers and how they are safer than commercial products.

While attempts to reduce the usage of chemicals should be applauded, most of these recipes contain salt, which is poisonous to plants and persists in the soil for years.

Our co-owner, Hannah, published a guide to homemade weed killers where she debunks many of the claims made by bloggers and spread by the echo chamber that is social media.

Vinegar does kill some weeds but rarely the roots, and it’s not practical for more extensive or even medium-sized gardens.

Daniel has over 18 years of experience in the construction, home improvement, and landscape garden industries.

He previously worked as a project manager and has experience in managing teams of tradespeople and landscape gardeners on both small and medium sized projects.

Daniel is also a keen gardener and enjoys growing unusual plants and tending to his lawn.

More About Daniel Woodley.

Why Trust Us? Our Experience & Testing Methodology

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This guide to the strongest weed killers was published here at DIY Gardening

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