Patience Pays Off
Sprayed your weeds yesterday and nothing’s happened? Don’t panic – and don’t respray. Most weed killers need 7-14 days to show full results. Here’s exactly what to expect and when.
The speed of weed killer depends entirely on the type you’re using. Contact herbicides burn foliage within hours. Systemic products take days or weeks to kill roots completely. Knowing which you’ve applied – and what to expect – prevents wasted product and unnecessary frustration.
This guide covers realistic timelines for every type of weed killer, explains why some weeds take longer than others, and helps you troubleshoot when things aren’t working. For help choosing the right product, see our complete guide to weed killers.
The Quick Answer
Contact weed killers: Visible results in 1-24 hours. Foliage dies but roots survive.
Systemic weed killers: First signs in 7-10 days. Complete kill in 2-4 weeks.
Lawn weed killers: Weeds begin distorting in 7-14 days. Full results in 3-4 weeks.
Tough perennial weeds: May need 4-6 weeks and repeat treatments.
Systemic Weed Killers: 7-14 Days
Systemic weed killers like glyphosate are absorbed through leaves and transported throughout the plant. The herbicide needs time to reach every root tip before it starts causing damage. Rush this process and you’ll kill the top growth while leaving roots alive underground.
What to Expect: Day by Day
Days 1-3: Nothing visible. The herbicide is being absorbed and transported.
Days 4-7: First signs appear. Leaves may look slightly dull, wilted, or develop a yellow tinge.
Days 7-14: Progressive yellowing and browning. Leaves curl and droop. Growth stops.
Days 14-21: Plant appears dead. Stems collapse. Leaves turn brown or black.
Days 21-28: Complete die-back. Roots are dead. No regrowth.
Why It Takes So Long
The delay isn’t a flaw – it’s essential. Glyphosate works by blocking an enzyme plants need to produce amino acids. Without these building blocks, the plant slowly starves. Faster-acting poisons would kill leaf cells before the herbicide could spread, leaving roots untouched.
Triclopyr works similarly for woody weeds, mimicking plant hormones to cause uncontrolled growth that exhausts the plant’s resources.
Contact Weed Killers: 1-24 Hours
Contact herbicides destroy plant cells on contact. Products containing pelargonic acid or acetic acid (vinegar) work this way. They’re fast and satisfying to watch – but they don’t kill roots.
What to Expect
1-4 hours: Leaves begin to look wet, then dark and wilted.
4-12 hours: Foliage turns brown or black. Stems collapse.
12-24 hours: Complete top-kill. Plant looks dead.
1-3 weeks later: New growth emerges from surviving roots.
When Contact Weed Killers Work Best
Contact products are ideal for annual weeds that don’t have persistent root systems. Seedlings in paving cracks, young chickweed, and annual grasses die completely because their roots are too shallow to survive.
For perennial weeds with established roots, contact weed killers provide temporary clearance only. You’ll need to reapply every few weeks or switch to a systemic product for permanent control.
Lawn Weed Killers: 2-4 Weeks
Selective lawn weed killers target broadleaf weeds while leaving grass unharmed. They’re typically hormone-based herbicides that cause weeds to grow abnormally until they exhaust themselves.
What to Expect
Days 1-7: Little visible change. Herbicide is being absorbed.
Days 7-14: Weeds begin to look distorted. Leaves twist, curl, or develop unusual shapes. Growth becomes stunted.
Days 14-21: Weeds turn yellow, then brown. They shrivel and collapse.
Days 21-28: Dead weeds decompose into the lawn. Grass fills the gaps.
Daisy rosettes take around 3 weeks to disappear completely. Dandelions with deep taproots may need 4 weeks or a second treatment. Clover can be particularly stubborn, sometimes requiring specialist products.
Tough Weeds: Why Some Take Longer
Not all weeds respond at the same rate. These factors affect how quickly weed killer works:
Root Depth and Energy Reserves
Perennial weeds store energy in their roots. The deeper and more extensive the root system, the longer it takes for herbicide to reach every part – and the more energy the plant has to resist.
Bindweed roots can extend several metres deep. Even after the visible plant dies, dormant root sections may send up new growth weeks later. Expect 4-6 weeks for initial die-back and be prepared for follow-up treatments.
Couch grass spreads through underground stems that can regenerate from tiny fragments. Complete control takes multiple applications over a growing season.
Waxy or Tough Foliage
Horsetail has silica-rich stems that repel water-based herbicides. The waxy coating prevents absorption, making this prehistoric weed notoriously slow to respond. Crushing or bruising stems before treatment improves uptake.
Ivy’s glossy leaves shed spray easily. Adding a surfactant or spraying on a warm day when leaves are more permeable helps.
Time of Year
Weeds absorb herbicide fastest when actively growing. Treat in spring or early autumn for quickest results. Summer drought stress and winter dormancy both slow response times significantly.
Timeline by Weed Type
Why Your Weed Killer Might Seem Slow
If weeds aren’t responding as expected, consider these factors before reapplying:
You Haven’t Waited Long Enough
This is the most common issue. Systemic weed killers work internally, and visible damage takes time to appear. Respraying before the first application has finished working wastes product and can actually reduce effectiveness by burning leaves before full absorption occurs.
Rule of thumb: Wait at least 3 weeks before retreating with systemic products.
Weather Conditions
Cold weather slows plant metabolism, delaying herbicide action. Below 10°C, weed killers work significantly slower. Rain within 24 hours of application washes product off leaves. Drought-stressed weeds have tougher, less absorbent foliage.
Ideal conditions – warm, dry, actively growing – produce the fastest results. Treating in suboptimal weather extends timelines considerably.
Application Issues
Insufficient coverage means some leaves don’t receive enough herbicide. Thorough, even application matters more than total volume. Spraying in wind causes drift, reducing the amount reaching target weeds.
Wrong Product for the Weed
Selective lawn weed killers don’t work on grass-type weeds. Contact herbicides won’t kill perennial roots. Stronger products may be needed for established perennials that resist standard concentrations.
How to Speed Up Results
While you can’t fundamentally change how weed killers work, you can optimise conditions:
Treat actively growing weeds: Spring and early autumn deliver fastest results. Avoid treating stressed, dormant, or drought-affected plants.
Choose the right weather: Warm, dry days with temperatures between 10-25°C. No rain forecast for 24-48 hours.
Apply thoroughly: Wet all leaf surfaces without excessive run-off. Use a fine spray for even coverage.
Don’t disturb treated areas: Avoid mowing, strimming, or digging for at least 7 days after treatment. Let the herbicide work.
Consider timing: When you apply affects how quickly results appear.
When to Retreat
For most weeds, wait until you’re certain the first treatment has finished working – typically 3-4 weeks minimum. Signs that retreatment is needed:
- New green growth appearing from the base
- Partial yellowing but some leaves still healthy after 4 weeks
- Initial die-back followed by recovery
Persistent perennials like nettles and brambles often need 2-3 treatments over a season. This isn’t product failure – it’s the nature of deep-rooted weeds. Each treatment weakens the root system until it can no longer regenerate.
If you’re aiming to kill weeds permanently, patience with systemic products delivers better long-term results than repeated contact treatments.
Frequently Asked Questions
Why hasn’t my weed killer worked after 3 days?
Systemic weed killers typically need 7-14 days to show visible results. The herbicide is working internally even when you can’t see it. Contact weed killers should show results within 24 hours – if nothing’s happening, weather conditions or coverage may be the issue.
Can I spray again if nothing’s happening?
Wait at least 3 weeks before retreating with systemic products. Respraying too soon can burn leaves before full absorption, reducing effectiveness. If using contact weed killers on perennials, you can reapply once regrowth appears.
Why did my weeds come back after dying?
If you used a contact weed killer, roots survived and regenerated. Switch to a systemic product. If you used a systemic product, the roots may have been too extensive for one treatment – apply again when fresh growth reaches good leaf coverage.
Do stronger weed killers work faster?
Higher concentrations can improve effectiveness on resistant weeds but don’t dramatically speed up the process. The limiting factor is how quickly plants transport and metabolise the herbicide, not the amount applied. Using more than recommended is wasteful.
How long before I can replant after weed killer?
Most glyphosate products allow replanting after 7 days once weeds are dead. Check product labels – some residual herbicides prevent germination for weeks or months. For long-lasting weed killers, the extended prevention period means delayed replanting.
Is it normal for weeds to look worse before they die?
Yes. Hormone-based weed killers cause distorted, twisted growth before death. Glyphosate causes progressive yellowing from the growing tips. These visible changes confirm the product is working.
Ready to Clear Your Weeds?
Our professional-strength systemic weed killer reaches roots that DIY products miss. Expect visible results within 7-10 days and complete kill within 2-4 weeks.
