The Lawn Weed Killer You’ve Probably Already Used
MCPA is one of the most widely used selective herbicides in the UK. If you’ve ever applied a lawn weed killer, there’s a good chance it contained MCPA.
What Is MCPA?
MCPA (4-chloro-2-methylphenoxyacetic acid) is a selective herbicide that kills broadleaf weeds while leaving grasses completely unharmed. It works by mimicking natural plant growth hormones called auxins, causing broadleaf weeds to grow uncontrollably until they exhaust themselves and die.
It belongs to the same chemical family as 2,4-D, and the two are often confused. They work in a very similar way, but MCPA is considered slightly safer for use on sensitive grass varieties and is the preferred choice in many UK lawn products.
What Does MCPA Kill?
Good control:
- Thistles (creeping and spear)
- Docks
- Buttercups (creeping and meadow)
- Daisies
- Plantain (greater and ribwort)
- Dandelions
- Ragwort
Partial or poor control:
- Clover — MCPA alone often provides incomplete control. Products containing mecoprop-P or fluroxypyr alongside MCPA are more effective
- Speedwell — resistant to MCPA. Needs a product with fluroxypyr or mecoprop
- Yarrow — tolerant of MCPA at standard rates
Won’t work on:
- Grasses (including weed grasses like couch grass) — MCPA is selective, it only affects broadleaf plants
- Moss — needs iron sulphate or a dedicated moss killer
How to Use MCPA
MCPA is a systemic herbicide, which means it needs to be absorbed through the leaves and transported throughout the plant to kill the roots. Getting good leaf coverage is essential.
Application tips:
- Timing: Apply between April and September when weeds are actively growing. See our full timing guide for more detail
- Don’t mow before: Leave the lawn unmown for 2-3 days before treatment — you want maximum leaf area for the product to land on
- Don’t mow after: Wait at least 3 days after treatment before mowing
- Weather: Apply on a dry, calm day with no rain expected for 6+ hours
- Temperature: Above 10°C for best absorption
- Watering: Don’t water the lawn for at least 24 hours after application
Most consumer lawn weed killers come ready to use or as a dilutable concentrate. Follow the label rates carefully — more product doesn’t mean better results and can damage the grass.
MCPA vs 2,4-D: What’s the Difference?
These two herbicides are closely related and often compared. Here’s how they differ:
In practice, the difference matters most to professional greenkeepers. For home lawn use, both are effective and you’ll often find them in combination products alongside dicamba or mecoprop-P for broader weed control.
Safety and Pets
MCPA has moderate toxicity and should be treated with respect. Keep children and pets off the lawn for at least 24 hours after application, or until the product has fully dried and been watered in.
As with all weed killers, store concentrates securely and follow the label instructions. MCPA is harmful to aquatic organisms, so keep it well away from ponds and watercourses.
Frequently Asked Questions
Is MCPA the same as glyphosate?
No. Glyphosate is a non-selective herbicide that kills all plants, including grass. MCPA is selective — it kills broadleaf weeds while leaving grass unharmed. They work in completely different ways and are used for different purposes.
How long does MCPA take to work?
You’ll typically see the first signs of wilting and curling within 7-14 days. Complete kill of the weed, including the root system, takes 3-6 weeks. Tougher weeds like docks may need a second application. See our guide on how long weed killers take to work for more detail.
Can I use MCPA on a new lawn?
Wait until the lawn is fully established — at least 6 months after seeding, or after the new turf has rooted firmly (usually 8-12 weeks). Young grass is more sensitive to herbicides and may be damaged.
Will MCPA kill clover in my lawn?
MCPA provides only partial control of clover. For reliable clover control, use a product that combines MCPA with mecoprop-P or fluroxypyr. These combination products are specifically designed to tackle the weeds that MCPA alone misses.
Is MCPA safe for the environment?
MCPA breaks down in soil within a few weeks and doesn’t persist long-term. It’s harmful to aquatic organisms, so always maintain a buffer zone from water. It has low toxicity to bees and birds when used as directed.
Can I reseed after using MCPA?
Wait at least 6 weeks after application before overseeding. MCPA residues in the soil can inhibit germination of new grass seed if you sow too soon.
Want a Weed-Free Lawn?
MCPA-based lawn treatments tackle thistles, docks and buttercups without harming your grass.
