When to Apply Weed Killer (UK Timing Guide)

Weed killers work best when weeds are actively growing. That means warm temperatures, moist soil, and dry leaves. Miss this window and you’re fighting an uphill battle.

This guide covers the best seasons, weather conditions, and times of day to spray weed killer in the UK. Get the timing right and you’ll need fewer treatments. For more on choosing the right product, see our complete weed killers guide.

The Quick Answer

Best time of year: Late spring (May-June) or early autumn (September-October)

Best weather: Dry, calm day with temperatures between 10-25°C

Best time of day: Morning after dew has dried, or early evening

Wait for: At least 24-48 hours of dry weather after application

Best Time of Year to Apply Weed Killer

Weed killers need weeds to be actively growing. Dormant weeds won’t absorb the herbicide properly, which means poor results and wasted product.

Spring (March to June)

Spring is prime time for weed control. Soil temperatures rise above 6°C, triggering weed growth. By May, most weeds are in full swing.

Why it works:

• Weeds are young and vulnerable
• Active growth means better absorption
• You catch weeds before they seed

Best approach: Wait until late April or May. Early spring weeds may not be growing fast enough yet. You want to see active leaf growth before spraying.

Summer (July to August)

Summer can work, but it’s not ideal. Hot, dry conditions stress both weeds and your lawn.

Challenges:

• Heat above 25°C reduces herbicide effectiveness
• Drought-stressed weeds have waxy, resistant leaves
• Risk of scorching desirable plants

If you must spray in summer: Choose cooler days, spray in early morning or evening, and ensure the soil is moist.

Autumn (September to November)

Autumn is your second-best window. Cooler temperatures and autumn rains trigger a second growth spurt in perennial weeds like bindweed and couch grass.

Why it works:

• Perennial weeds are moving energy to their roots
• Herbicide travels down with this flow
• Fewer treatments needed the following spring

Best approach: Treat in September or early October while weeds are still actively growing. By November, growth slows too much.

Winter (December to February)

Winter treatment is largely pointless. Weeds are dormant and won’t absorb herbicide.

Exception: Mild winters may allow some growth. If you see active green weeds during a warm spell, you can spot-treat. But don’t expect full-lawn treatments to work.

Best Weather Conditions

Weather matters as much as season. The wrong conditions waste your effort.

Temperature

Ideal range: 10-25°C

Below 10°C: Weed growth slows, poor absorption. Above 25°C: Herbicide evaporates before absorption. Sweet spot: 15-20°C with gentle sunshine.

Rainfall

Before spraying: Moist soil helps, but wet leaves don’t. Wait for leaves to dry after rain.

After spraying: This is critical. Most weed killers need 24-48 hours without rain to work. Check the forecast carefully.

If rain comes within 6 hours of application, you may need to reapply once conditions improve.

Wind

Ideal: Calm or very light breeze

Wind causes drift, carrying herbicide onto plants you want to keep. Even a moderate breeze can move spray several metres from where you aimed.

If you can see leaves moving, wait for calmer conditions.

Humidity

Moderate humidity helps absorption. Very dry air causes rapid evaporation. Very humid conditions slow drying but can work in your favour.

Best Time of Day

Morning (After Dew Dries)

The classic choice. Apply after morning dew evaporates, typically by 9-10am.

Advantages: Whole day for absorption before evening dew. Usually calmer than afternoon. Cooler temperatures in summer.

Early Evening

Works well, especially in summer when mornings heat up quickly.

Advantages: Cooler temperatures. Often calmer winds. Avoids midday heat.

Disadvantage: Less drying time before evening dew. Choose evenings when dew isn’t expected.

Midday

Generally avoid. Midday sun causes rapid evaporation, reducing absorption. In summer, high temperatures stress plants and reduce effectiveness.

When Weeds Are Most Vulnerable

Weed killer works by being absorbed through leaves and transported throughout the plant. This process requires active growth.

Signs of Active Growth

• Fresh, green leaves (not yellowing or wilting)
• New shoots appearing
• Flowers or buds forming
• Plant looks healthy and vigorous

When Weeds Resist Treatment

• Drought-stressed with waxy, tough leaves
• Dormant in cold weather
• Already dying back naturally
• Recently mowed (less leaf surface)

Some weeds are notoriously resistant even with good timing. Horsetail has waxy stems that repel herbicides, requiring specialist techniques and multiple applications.

Lawn Weeds: The Timing Trick

For selective lawn weed killers, don’t mow for 3 days before and after treatment. You need leaf surface for absorption, and you don’t want to remove the herbicide before it works.

Timing for Different Weed Killer Types

Systemic Weed Killers (Glyphosate, Triclopyr, etc.)

These travel through the plant to kill roots. Glyphosate is the most common systemic herbicide, while triclopyr is particularly effective on woody weeds like brambles.

Timing: Apply when weeds are in full growth. Allow 7-14 days before expecting results. Don’t disturb treated areas.

Contact Weed Killers (Pelargonic Acid, etc.)

These burn what they touch but don’t kill roots. Results appear within hours.

Timing: Less critical, but active growth still helps. Can be used in slightly cooler conditions.

Residual Weed Killers

These prevent new weeds from germinating. Apply to clear ground. For more on these products, see our guide to longest lasting weed killers.

Timing: Early spring before weeds emerge, or after clearing an area. Need moist soil to activate.

Common Timing Mistakes

Spraying Too Early in Spring

March enthusiasm often leads to wasted product. Soil temperatures below 6°C mean minimal weed activity. Wait until you see genuine growth.

Ignoring the Forecast

That sunny morning looks perfect, but if rain arrives in 4 hours, you’ve wasted your time. Always check a detailed forecast before applying.

Mowing Before Treatment

Cutting your lawn removes the leaf surface herbicide needs to contact. Wait 3 days after mowing before applying lawn weed killer.

Treating Stressed Weeds

Weeds suffering from drought, heat, or frost won’t absorb herbicide properly. Wait for recovery or address the stress first.

Expecting Instant Results

Systemic weed killers take 7-14 days to show full effect. Don’t assume it hasn’t worked and reapply too soon. If you want to kill weeds permanently, patience with systemic products is essential.

Month-by-Month Guide (UK)

Frequently Asked Questions

What temperature is too cold for weed killer?

Below 10°C, most weed killers become less effective. Below 6°C, weed growth essentially stops and treatment is pointless. Wait for warmer conditions.

Can I apply weed killer before rain?

You need 24-48 hours of dry weather after application for best results. If rain is forecast within this window, wait for a better day.

How long after mowing can I spray weed killer?

Wait at least 3 days after mowing before applying lawn weed killer. You need leaf surface for the herbicide to contact. Also wait 3-4 days after treatment before mowing again.

Is morning or evening better for weed killer?

Both can work well. Morning (after dew dries) gives maximum drying time. Evening avoids midday heat but risks evening dew. Choose based on your local conditions.

Why didn’t my weed killer work?

Common reasons: applied in cold weather, rain washed it off, weeds were drought-stressed, or not enough time has passed. Systemic weed killers need 7-14 days to show results. For particularly stubborn weeds, you may need stronger products.

Can I apply weed killer in winter?

Generally no. Weeds are dormant and won’t absorb the herbicide. Exception: during mild spells with active green growth, spot treatment may work.

Tackling an overgrown garden? Timing becomes even more critical when dealing with established perennial weeds that have built up energy reserves over years.