Weed Killer Not Working? Here’s Why

Quick Diagnosis: What Went Wrong?

If you’ve used a weed killer and it hasn’t done what you expected, you’re not alone. It’s one of the most common gardening frustrations — and the cause is almost always fixable. Let’s work through the main reasons and how to get results next time.

Reason 1: Bad Timing or Weather

This is the single biggest reason weed killers fail. The product needs time to be absorbed through the leaves before rain washes it off. If it rained within a few hours of your application, there’s a good chance the weed killer never made it into the plant.

Here’s what to check:

• Rain forecast: You need a minimum of 6 hours dry weather after spraying. 24 hours is better. Check our guide to weed killer and rain for specific product timings
• Temperature: Weeds absorb herbicide fastest when they’re actively growing. Below 8°C, growth slows significantly. Apply between April and September for best results
• Time of day: Avoid very early morning (dew dilutes the spray) and hot midday sun (spray dries before absorption). Mid-morning or early evening on a calm day is ideal
• Wind: Even a light breeze causes spray drift, meaning the product lands everywhere except on the target weeds

For a detailed breakdown of the best conditions, see our seasonal timing guide.

Reason 2: Wrong Product for the Job

This catches people out more often than you’d think. There are two fundamental mismatches:

Selective vs non-selective: Selective lawn weed killers (containing MCPA, 2,4-D or mecoprop-P) only kill broadleaf weeds. If your lawn problem is couch grass, annual meadow grass or Yorkshire fog, a selective herbicide won’t touch them — they’re grasses, and the product is designed to spare grasses.

Contact vs systemic: Contact herbicides (like acetic acid sprays) only burn what they touch. The leaves die back, but the roots survive. For weeds in flower beds with deep or spreading root systems — bindweed, couch grass, ground elder — you need a systemic product like glyphosate that travels down to the roots.

Reason 3: Application Errors

Even with the right product on the right day, poor technique can let you down:

• Under-dosing: If you dilute concentrate too much, the dose may not be lethal to the weed. Follow the label dilution rates exactly
• Over-application: Surprisingly, this can also fail. With systemic products, drenching the leaves can cause rapid surface burn. The leaf tissue dies before the herbicide has time to travel to the roots. A thorough but not dripping application works best
• Poor coverage: Spray needs to coat the leaf surface. If you’re treating large weeds, make sure the spray reaches lower leaves too, not just the top canopy
• Mowing too soon: If you’re treating lawn weeds, don’t mow for at least 3 days before and after application. Mowing removes the leaf area the product needs for absorption
• Wrong growth stage: Treating weeds that have already set seed is too late for that generation. For buttercup and other annuals, timing before flowering gives the best results

Reason 4: Tough Weeds That Need Repeat Treatments

Some weeds are genuinely hard to kill. It’s not your technique or the product — it’s the weed itself. These species have evolved deep root reserves, waxy leaf coatings or rhizome networks that let them survive a single dose of herbicide:

The key with these weeds is patience. Treat, wait for regrowth (usually 4-6 weeks), then treat again. Each application depletes more of the root reserves until the weed can’t recover. Cutting between treatments actually helps — it forces the plant to use energy regrowing, making it more vulnerable to the next dose. For a full breakdown of how quickly you should see results, check our guide to how long weed killer takes to work.

Reason 5: The Product Has Expired or Degraded

This one gets overlooked. Weed killer doesn’t last forever:

• Shelf life: Most concentrates last 2-3 years unopened. Once opened, use within one season for best results
• Ready-to-use sprays: The diluted solution degrades faster than concentrate. If it’s been sitting in a shed for two summers, it may have lost potency
• Storage conditions: Freezing and extreme heat both damage active ingredients. A shed that bakes in summer and freezes in winter is the worst place to store weed killer long-term
• Pre-mixed solution: If you mixed concentrate in a sprayer and left it overnight or longer, the active ingredient may have started to break down, especially glyphosate in hard water

If you suspect your product is old or has been stored badly, it’s worth buying fresh rather than wasting time with a weakened formula.

What to Do Now: Step-by-Step Recovery Plan

If your weed killer application didn’t work, here’s what to do next:

1. Identify the weed. Check if you’re using the right type of product for the species. Our path weeds guide and under hedges guide can help with identification and product choice
2. Wait for regrowth. Don’t respray immediately. Let the weed produce fresh leaf growth (usually 3-6 weeks). Fresh, actively growing leaves absorb herbicide far better than stressed, damaged tissue
3. Check the weather forecast. Pick a day with 24 hours of dry weather ahead, temperatures above 10°C, and little wind
4. Apply correctly. Follow the label rate. Coat leaves thoroughly but don’t drench them. For pet safety, keep animals off treated areas until the spray has dried
5. Be patient. Systemic weed killers take 2-3 weeks to show full results. Don’t assume it hasn’t worked after a few days

Frequently Asked Questions

How long should I wait before reapplying weed killer?

Wait until you see fresh regrowth — typically 4-6 weeks. Respraying too soon is wasteful because there isn’t enough healthy leaf tissue to absorb the product. The new growth also draws energy from the roots, which helps the herbicide reach deeper into the plant’s system.

Can weeds become resistant to weed killer?

Yes, though it’s more common in agriculture than gardens. Repeated use of the same active ingredient can select for resistant individuals. The practical advice is to rotate products where possible and combine chemical treatment with physical removal. If glyphosate hasn’t worked after three proper applications, consider switching to triclopyr for woody weeds or dicamba for broadleaf species.

Why did my weed killer only kill the tops of the weeds?

You likely used a contact herbicide rather than a systemic one. Contact products (vinegar-based sprays, pelargonic acid, diquat) destroy the foliage they touch but don’t travel to the roots. For perennial weeds, you need a systemic product that’s absorbed and transported throughout the whole plant.

Does weed killer work in cold weather?

Poorly. Most herbicides rely on active plant growth for absorption and translocation. Below about 8°C, weeds grow so slowly that the product sits on the leaf surface without being properly taken up. April to September is the effective window in the UK. For more on seasonal timing, see our application guide.

Is it worth using a stronger concentration?

No. The label rate is the tested, effective dose. Increasing concentration can cause rapid leaf burn (which prevents systemic absorption) and is illegal under UK pesticide regulations. If the standard dose isn’t working, the problem is usually timing, weather or weed type — not strength.

My lawn weed killer didn’t kill the clover. What now?

Some clover species are resistant to standard 2,4-D herbicides. Products containing mecoprop-P or clopyralid are more effective on clover, though be aware of clopyralid’s compost persistence issues if you compost your grass clippings. A second application in autumn when clover is still actively growing often gives better results than repeated summer treatments.