Systemic or Contact? It Changes Everything
One kills from the roots up. The other only burns what it touches. Choosing the right mode of action determines whether your weeds stay dead or grow back within weeks.
The Core Difference
Think of it this way: systemic weed killers are slow but thorough. Contact weed killers are fast but superficial. Each has situations where it’s the better choice, and understanding this distinction is essential for choosing the right weed killer for your problem.
Systemic Weed Killers Explained
Systemic herbicides are absorbed through leaf tissue and then move through the plant’s phloem (the internal transport system that carries sugars from leaves to roots). This is called translocation, and it’s what makes systemic products so effective against deep-rooted perennial weeds.
Glyphosate is the most widely used systemic herbicide. Once inside the plant, it blocks an enzyme (EPSP synthase) needed to produce essential amino acids. The plant starves from the inside out. Because the chemical travels to the roots, the entire plant dies — not just the visible foliage.
Why the slow speed matters: You won’t see instant results. The first signs of damage (yellowing, wilting) typically appear after 5-7 days, with complete death taking 2-3 weeks. This is actually a good sign — it means the herbicide is being transported throughout the plant rather than just burning the surface. If you see instant browning after using a product labelled as systemic, the weed was probably already stressed or you may have over-applied. Check our guide on how long weed killer takes to work for detailed timelines.
Systemic products are essential for:
- Bindweed — roots extend metres deep, contact products have zero long-term effect
- Horsetail — extensive underground rhizome network
- Ground elder — spreading root system that regrows from fragments
- Couch grass — underground runners produce new shoots
- All persistent perennial weeds where root kill is the only long-term solution
Contact Weed Killers Explained
Contact herbicides work by destroying plant cell membranes on the surface. Products like pelargonic acid and acetic acid (vinegar) strip the waxy coating from leaves and rupture cell walls. The result is rapid desiccation — leaves turn brown and crispy within hours.
The appeal is obvious: you spray and see results almost immediately. But the limitation is equally clear. Nothing travels to the roots. For annual weeds (chickweed, groundsel, annual meadow grass) this doesn’t matter — kill the top growth and the shallow roots die too. For perennial weeds, the roots simply push up new shoots within a few weeks.
Contact products work well for:
- Annual weeds on paths and patios
- Quick cosmetic burndown before an event or viewing
- Organic or chemical-free approaches where persistence isn’t desired
- Situations where you want to minimise soil residue
- Areas near water where systemic products may not be permitted
Contact products are poor choices for:
- Any perennial weed with an established root system
- Weeds you need to kill permanently
- Large-scale clearance (they only kill what they directly coat)
When to Use Each Type
A common mistake is using contact products on perennial weeds and then wondering why they keep coming back. If you’ve been through this cycle, our weed killer not working guide covers the fix in detail.
Can You Combine Both Approaches?
Yes, and it’s often the most effective strategy for heavily weeded areas:
- First, apply systemic glyphosate to the entire area. Wait 2-3 weeks for full effect
- Clear dead growth and wait for any survivors to regrow
- Re-treat persistent regrowth with a second systemic application
- Use contact spray for maintenance — once the tough perennials are dead, contact products handle any new annual weed seedlings cheaply and quickly
This approach works well for clearing overgrown gardens or renovating neglected allotments. The systemic application does the heavy lifting, and contact products keep things tidy afterwards. For information on weather considerations when spraying, see our rain guide.
Frequently Asked Questions
How do I know if a product is systemic or contact?
Check the active ingredient on the label. Glyphosate, triclopyr, MCPA, 2,4-D, mecoprop-P and dicamba are all systemic. Pelargonic acid, acetic acid and diquat are contact. If the label says “fast acting” or “visible results in hours,” it’s almost certainly contact. If it says “kills roots” or “prevents regrowth,” it’s systemic.
Why does my systemic weed killer seem to work like a contact one?
Over-application is the usual cause. If you drench the leaves, the surface tissue dies so quickly that it can’t transport the herbicide to the roots. Apply a thorough but not dripping coating. The goal is absorption, not burn.
Are natural weed killers always contact?
Currently, yes. All natural and organic weed killers available in the UK (vinegar, pelargonic acid, fatty acid products) work by contact only. There is no organic systemic herbicide on the market. This means natural products work well on annual weeds but require repeated applications on perennials.
Does rain affect systemic and contact products differently?
Systemic products are more vulnerable to rain because they need hours to be absorbed through the leaf and transported internally. Rain within 6 hours washes them off before they can work. Contact products act faster — once the leaf surface is visibly affected (usually within 30 minutes to an hour), rain matters less. However, if rain falls immediately after application, both types are compromised.
Which is safer for pets?
Contact products based on pelargonic acid or acetic acid are generally considered the safest option — they break down very quickly and leave minimal residue. Systemic glyphosate is safe for pets once it’s dried on the leaves (typically 2-4 hours). For detailed guidance, see our pet safe weed killer guide.
Can I use systemic weed killer on a selective basis in my lawn?
If you mean systemic-selective (like MCPA or 2,4-D), yes — that’s exactly what lawn weed killers are. If you mean systemic-non-selective (glyphosate), you can spot-treat individual weeds using a weed wiper or gel applicator, but broadcast spraying will kill the grass.
Need a Systemic Weed Killer That Gets the Roots?
Our glyphosate-based weed killer is systemic, meaning it travels to the roots and kills the entire plant. Effective on bindweed, couch grass and all perennial weeds.
