Want Weeds to Stay Gone?
Residual herbicides prevent new weeds from germinating for months. But they only work in specific situations — and they’re not a substitute for killing existing weeds first.
Residual herbicides – also called pre-emergent herbicides – work differently from standard weed killers. Instead of killing existing weeds, they create a chemical barrier in the soil that prevents new weed seeds from germinating.
This sounds ideal, but there are important limitations. They don’t last forever (typically 3-6 months), they won’t kill weeds that are already growing, and they’re only suitable for areas where you never want anything to grow.
How Residual Herbicides Work
Standard weed killers like glyphosate kill actively growing plants. Residual herbicides do something different – they sit in the top layer of soil and prevent weed seeds from developing into seedlings.
When a weed seed germinates, it encounters the chemical barrier and dies before it can establish. The seed might sprout, but the seedling can’t develop properly and fails within days.
This means residual herbicides are preventative, not curative. If you have existing weeds, you need to kill them first with a systemic herbicide like glyphosate, then apply the residual product to prevent regrowth from the seed bank in the soil.
When to Use Residual Herbicides
Residual herbicides are only appropriate for areas where you want zero plant growth. Once applied, nothing will grow in that area for months – including anything you might want to plant.
Good Uses
Paths and driveways: Block paving, concrete, tarmac, and gravel driveways are ideal candidates. You don’t want anything growing here, and residual herbicides can keep joints and surfaces weed-free for months.
Patios: Patio weeds growing between slabs are a constant nuisance. A residual treatment after clearing existing weeds can prevent regrowth through the season.
Fence lines and boundaries: The strip along fences where weeds constantly appear can benefit from residual treatment if you’re not growing anything there.
Industrial and commercial areas: Car parks, loading bays, and storage areas where vegetation is unwanted.
Do NOT Use On
Lawns: Residual herbicides will kill grass and prevent it regrowing. Never use them on or near lawns.
Flower beds: They’ll prevent your seeds and bedding plants from establishing. Keep well away from planting areas.
Vegetable gardens: Obviously not somewhere you want soil-acting chemicals that prevent plant growth.
Near trees and shrubs: Roots can extend far beyond the visible canopy. Residual herbicides can damage established plants if applied within the root zone.
Areas you plan to replant: If you’re clearing ground for a new lawn, flower bed, or planting, don’t use residual herbicides. Use glyphosate instead – it breaks down quickly and leaves no residue.
Active Ingredients Available in the UK
Several residual herbicides are available to UK gardeners. Each has different characteristics and durations.
Diflufenican
The most common residual herbicide in consumer products. It’s a pre-emergence herbicide that prevents broadleaf weeds from establishing.
Duration: Typically 3-4 months of protection.
Best for: Paths, patios, driveways. Often combined with glyphosate in “path clear” type products for both knockdown and prevention.
Limitations: Less effective on established weeds – works best as a preventer. Can harm aquatic life, so keep away from water sources.
Flazasulfuron
A sulfonylurea herbicide that provides longer-lasting residual control. It inhibits amino acid production in germinating seeds.
Duration: Can provide 4-6 months of control.
Best for: Hard surfaces, gravel areas, fence lines. Used in professional groundskeeping.
Limitations: Non-selective – kills anything trying to grow. Slow-acting, so patience required. More expensive than diflufenican products.
Propyzamide
A pre-emergence herbicide particularly effective against grass weeds. Can remain active in soil for up to two years in some conditions.
Duration: 6-12 months or longer depending on soil conditions.
Best for: Areas with persistent grass weed problems. Often used in orchards and established shrub beds (applied carefully away from desired plant roots).
Limitations: Restricted use in some areas due to environmental concerns. Not widely available to consumers. Requires careful application.
How to Apply for Best Results
Getting good results from residual herbicides requires proper preparation and timing.
Step 1: Clear Existing Weeds First
Residual herbicides don’t kill established weeds effectively. You need to clear the area first using a systemic herbicide like glyphosate. Wait until existing weeds are completely dead (2-3 weeks) before applying the residual product.
For tough weeds like bindweed, horsetail, or ground elder, you may need multiple glyphosate applications before the residual stage. These weeds regrow from deep roots, not just seeds.
Step 2: Choose the Right Time
Early spring (March-April): Apply before the main weed germination season. This catches the spring flush of annual weeds.
Late summer (August-September): A second application can prevent autumn germination and winter annuals.
Avoid applying during very dry periods – the chemicals need some moisture to form an effective barrier in the soil.
Step 3: Apply Correctly
Coverage: Ensure complete, even coverage. Gaps in application mean gaps in protection where weeds will establish.
Rate: Follow label instructions precisely. Under-application gives poor results; over-application wastes product and may cause runoff.
Surface preparation: For hard surfaces, sweep clean before application. For gravel, ensure good contact with the soil beneath.
Step 4: Maintain the Barrier
No residual herbicide lasts forever. Plan for reapplication every 3-6 months depending on the product. Heavy rain can shorten effective duration by washing active ingredients deeper into the soil.
Realistic Expectations
Residual herbicides are useful tools, but they’re not magic solutions. Here’s what to realistically expect:
Duration: Most products provide 3-6 months of control, not years. Marketing claims of “up to 9 months” typically represent best-case scenarios.
Effectiveness: They work well on annual weeds germinating from seed. They’re less effective against perennial weeds regrowing from roots or rhizomes.
Maintenance: You’ll still need to reapply regularly. Residual herbicides reduce the frequency of treatment but don’t eliminate it entirely.
Cost: Regular reapplication means ongoing costs. For small areas, manual removal or occasional glyphosate treatment may be more economical.
Alternatives to Consider
Before committing to residual herbicides, consider whether alternatives might suit your situation better:
Regular glyphosate treatment: For many situations, periodic treatment with strong systemic herbicide when weeds appear is simpler and more flexible.
Physical barriers: Weed membrane under gravel, polymeric sand in paving joints, or proper pointing can reduce weed growth without chemicals.
Manual removal: For small areas, regular hand weeding may be less effort than maintaining chemical treatments.
Accepting some weeds: In less visible areas, occasional weeds may not justify the cost and effort of residual treatment.
Frequently Asked Questions
What is the longest lasting weed killer available?
Propyzamide can remain active for 6-12 months, but it’s restricted to professional use in many situations. For consumers, products containing diflufenican or flazasulfuron typically last 3-6 months. No weed killer lasts forever – all require reapplication.
Can I use residual weed killer on my lawn?
No. Residual herbicides prevent all plant growth, including grass. They will kill your lawn and prevent it recovering. For lawn weeds, use selective herbicides like 2,4-D or dicamba that kill weeds without harming grass.
Do I need to kill weeds before applying residual herbicide?
Yes. Residual herbicides prevent new weeds from germinating but don’t effectively kill established plants. Clear existing weeds with glyphosate first, wait until they’re dead, then apply the residual product.
How long after applying can I plant?
This varies by product – check the label. Generally, you shouldn’t plant anything in treated areas for at least 3-6 months. For areas you plan to plant, use glyphosate instead – it breaks down quickly and leaves no residue affecting future planting.
Is salt a good long-lasting weed killer?
Salt can prevent plant growth, but it’s not recommended. You need large quantities to be effective, it damages soil structure, can spread to areas you don’t want affected, and is harmful to soil life. Commercial residual herbicides are more effective and controllable. See our guide to salt as weed killer.
Why do weeds keep coming back even after using residual herbicide?
Several reasons: the product may have worn off, coverage may have been incomplete, perennial weeds may be regrowing from roots (not seeds), or weed seeds may be blowing in from surrounding areas. Residual herbicides prevent germination – they can’t stop established weeds or those growing from roots.
Prevention, not cure. Residual herbicides stop new weeds germinating but won’t kill existing plants. For established weeds, you need systemic herbicides that travel to the roots.
