Glyphosate Weed Killer : Friend or Foe?

What is Glyphosate?

Glyphosate is a systemic herbicide – meaning it’s absorbed through leaves and transported throughout the entire plant, killing it from the inside out, right down to the roots. This is what separates it from homemade weed killers like vinegar or salt, which only burn the parts they touch.

First developed in the 1970s by Monsanto under the brand name Roundup, glyphosate quickly became the world’s most widely used herbicide. The patent expired in 2000, so now you’ll find it sold under dozens of brand names – Gallup, Resolva, Doff, Rootblast, and many more. Check the label of almost any effective weedkiller and you’ll see glyphosate listed as the active ingredient.

How Glyphosate Works

When you spray glyphosate onto a weed, it’s absorbed through the leaves and green stems. From there, it travels through the plant’s vascular system – the same network that moves water and nutrients – until it reaches every part of the plant, including the roots.

Glyphosate works by blocking an enzyme called EPSP synthase, which plants need to produce certain proteins essential for growth. Without these proteins, the plant slowly starves and dies. This process takes time, which is why glyphosate isn’t the fastest weed killer, but it’s thorough.

The fact that glyphosate is translocated to the roots is crucial. It’s why glyphosate can kill deep-rooted perennials like bindweed and ground elder that simply regrow when you use contact-only treatments.

What Weeds Does Glyphosate Kill?

The short answer: almost everything. Glyphosate is non-selective, meaning it will kill any plant it’s applied to. This makes it incredibly versatile but also means you need to be careful around plants you want to keep.

Glyphosate is particularly effective against:

• Annual weeds – chickweed, groundsel, annual meadow grass
• Perennial weeds – dandelions, docks, nettles, thistles
• Deep-rooted invasives – bindweed, ground elder, couch grass
• Woody weeds – brambles, ivy, small saplings
• Tough cases – horsetail (with persistence), Japanese knotweed (multiple treatments required)

For a complete guide to identifying and treating specific weeds, see our common UK weeds guide.

What it won’t kill: Glyphosate doesn’t work on moss, algae, or lichens – these need dedicated treatments. It also won’t prevent new weeds from germinating. If you need lasting prevention, you’ll want a residual weed killer instead.

Is Glyphosate Safe? The Controversy Explained

Let’s address this directly. Glyphosate has been the subject of lawsuits, headlines, and heated debate. Here’s what you need to know.

The International Agency for Research on Cancer (IARC) classified glyphosate as “probably carcinogenic to humans” in 2015. This led to significant legal action, particularly in the US, where Bayer (who now owns Monsanto) has faced numerous lawsuits.

However, regulatory bodies in the UK, EU, US, and most other countries continue to approve glyphosate for use, concluding that it’s safe when used according to label instructions. The European Food Safety Authority, the US EPA, and Health Canada have all found no unacceptable risk to human health.

The reality is nuanced. Long-term, heavy occupational exposure may carry risks – this primarily affects agricultural workers using large quantities regularly. Occasional use by home gardeners, following the label directions, is considered safe by regulators.

Practical safety advice:

• Always read and follow the label instructions
• Wear gloves and avoid skin contact
• Don’t spray on windy days
• Keep children and pets off treated areas until the spray has dried
• Store products safely out of reach of children

How to Use Glyphosate Effectively

Getting the best results from glyphosate comes down to proper application. Here’s what works:

Choose the right conditions: Apply on a dry, calm day when rain isn’t forecast for at least 6 hours (24 hours is better). Glyphosate needs time to be absorbed before it’s washed off. Avoid very hot days when plants may have closed their stomata.

Use a sprayer, not a watering can: Sprayers give you control and even coverage. Watering cans waste product, risk runoff, and make it hard to target specific weeds. Dedicate one sprayer to weedkiller only – glyphosate residue can damage plants even in tiny amounts.

Coat the leaves thoroughly: Glyphosate enters through foliage, so you need good coverage. Don’t drench to the point of runoff, but ensure all leaves are wetted. For tough weeds like brambles, cut them back first and spray the regrowth when leaves are fully expanded.

Be patient: You’ll see yellowing in 7-14 days, with complete death in 2-4 weeks. Larger perennials and woody weeds may need a second application. Don’t be tempted to respray too early – you’ll waste product and may actually slow the process.

Dilution rates matter: Different products have different concentrations. A product with 360g/L glyphosate needs about 24ml per litre of water. Weaker formulations (90g/L) need up to 80ml per litre. Always check your product label.

Glyphosate vs Other Options

How does glyphosate compare to alternatives in our weed killer guide?

vs Contact weed killers (pelargonic acid, acetic acid): These burn foliage quickly but don’t reach roots. Good for annual weeds, useless for perennials. Pelargonic acid is often marketed as “natural” but still requires multiple applications.

vs Selective lawn weed killers (2,4-D, Dicamba): These kill broadleaf weeds while sparing grass – essential for lawns. Glyphosate would kill your lawn too.

vs Triclopyr: Another systemic option, particularly effective on woody weeds and can be used around grass. Often combined with glyphosate in professional products.

vs Residual weed killers: Residual products stay in the soil and prevent germination. Glyphosate has no residual action – it binds to soil and breaks down quickly, so you can replant treated areas within weeks.

For most gardeners dealing with established weeds on paths, patios, and beds they want to clear, glyphosate remains the strongest and most effective option.

The Bottom Line

Glyphosate isn’t perfect, and the debate around it isn’t going away. But for home gardeners facing tough weeds, it remains the most effective tool available. Used properly, it kills weeds that nothing else can touch – including the roots that allow them to keep coming back.

If you’re uncomfortable using glyphosate, your alternatives are manual removal (effective but hard work), smothering with mulch or membrane (slow but chemical-free), or accepting that some weeds will keep returning.

For most people, occasional, careful use of glyphosate to tackle problem weeds is a practical choice that regulatory bodies worldwide consider safe.

Frequently Asked Questions

How long after using glyphosate can I plant?

You can plant ornamentals, flowers, and vegetables 7-14 days after applying glyphosate. For grass seed, you can sow the same day since glyphosate has no soil activity and won’t prevent germination. However, waiting 7 days allows tough weeds to fully die before you disturb the soil. Always check your specific product label as some formulations contain additional ingredients that may extend the waiting period.

Is glyphosate safe for pets?

Once the spray has dried completely (usually 2-4 hours), treated areas are considered safe for pets. Keep dogs and cats away from freshly sprayed areas to prevent them walking through wet product and potentially licking it off their paws. Never use glyphosate near fish ponds or areas where pets drink, as it’s toxic to aquatic life and persists in water.

Does glyphosate stay in the soil?

No, glyphosate binds tightly to soil particles and is broken down by soil bacteria, typically within days to a few weeks. This is one of its advantages – you can treat an area and replant relatively quickly without residues affecting new growth. Unlike residual weed killers, glyphosate has no lasting soil activity.

Why didn’t glyphosate kill my weeds?

The most common reasons glyphosate fails are: rain within 6 hours of application washing it off before absorption, spraying on stressed or drought-affected plants with closed stomata, inadequate leaf coverage, or using a dilution rate that’s too weak. Large established perennials like brambles and ivy often need two or three applications several weeks apart.

Is glyphosate banned in the UK?

No, glyphosate remains legal and approved for use in the UK. The current authorisation runs until at least December 2025, following the European Food Safety Authority finding no critical areas of concern. While some local councils have voluntarily reduced or stopped using it in public spaces, it remains widely available for home garden use under brands like Roundup, Gallup, and Resolva.