How to Kill Trefoil in Lawns

If you’ve noticed patches of small yellow flowers spreading through your lawn each summer, you’re probably dealing with trefoil. It’s one of the most common weeds in UK lawns and looks a bit like clover – and gets confused with it constantly – but trefoil has its own personality and requires a slightly different approach to get rid of it.

The good news? Trefoil is actually easier to control than its clover cousins. Understanding why makes all the difference to your success rate.

What Is Trefoil and How Do I Identify It?

Trefoil – most commonly lesser trefoil (Trifolium dubium) – is a low-growing weed with the classic three-leaflet structure that gives it its name. “Trefoil” literally means “three leaves.” You might also hear it called yellow suckling clover or, in Ireland, shamrock.

The easiest way to tell trefoil apart from white clover is the flowers. Trefoil produces small, compact yellow flower heads from May through to October. White clover, by contrast, has larger white or pinkish flowers. For help identifying other lawn invaders, see our guide to common UK lawn weeds.

The leaves differ too. Trefoil leaflets sit on short stalks and have a more elongated shape with rounded tips. White clover leaflets are rounder and often show a distinctive pale crescent marking – trefoil doesn’t have this.

Like clover, trefoil grows flat to the ground, which is why mowing doesn’t touch it. It spreads via creeping runners that root at intervals, and it produces seeds prolifically throughout summer. You’ll often find it alongside other low-growing lawn weeds like speedwell and creeping buttercup.

Trefoil vs Black Medick: Know the Difference

Another yellow-flowered lawn weed frequently confused with trefoil is black medick (Medicago lupulina). From a distance they look almost identical – both have three leaflets and small yellow flower heads. But they’re actually different plants with some key distinctions.

How to Tell Them Apart

The easiest way to tell them apart is to look closely at the leaf tips. Trefoil has rounded leaflet tips, while black medick has distinctly pointed tips, often with a tiny bristle or spine. Black medick leaflets also have a small notch at the very tip – look for this characteristic “indented point.”

Later in the season, the seed pods are a giveaway. Black medick gets its name from its seed pods, which turn black when ripe and have a distinctive coiled or kidney shape. Trefoil seeds stay within the faded flower head and are brown rather than black.

Does it matter which one you have? Not really for control purposes – both respond to the same selective herbicides and both indicate similar soil conditions (low nitrogen). But if your yellow-flowered weed doesn’t seem to be rooting at the stem nodes, you’re probably dealing with black medick rather than trefoil.

Why Is Trefoil Growing in My Lawn?

Trefoil doesn’t appear randomly. It’s telling you something about your lawn’s condition – specifically, that the soil is low in nitrogen and possibly quite dry or sandy.

Like all members of the pea family, trefoil can “fix” nitrogen from the air through bacteria in its roots. This means it doesn’t need nitrogen in the soil to thrive – unlike your grass, which depends on it entirely. When soil nitrogen drops, grass weakens and thins out, creating gaps. Trefoil moves straight in.

Other conditions that favour trefoil include:

• Compacted soil – trefoil’s root system handles compaction better than grass
• Dry, sandy soils – trefoil has a deep taproot that finds water grass roots can’t reach
• Mowing too short – low cutting removes grass competition but leaves trefoil untouched
• Thin or patchy lawn – any bare soil is an invitation

The Key Difference: Trefoil Is Annual

Here’s the crucial thing that makes trefoil easier to deal with than clover: it’s an annual weed, not a perennial.

This means each trefoil plant lives for just one growing season. It germinates in spring, flowers through summer, sets seed, and dies off in autumn. The following year’s trefoil comes entirely from those seeds.

Why does this matter? Because if you can stop trefoil from setting seed – even for one season – you dramatically reduce next year’s problem. The seeds do persist in soil for several years, so you won’t eliminate it completely in one go, but you’ll see a significant reduction each year you break the cycle.

Compare this to white clover, which is perennial and regrows from its root system year after year regardless of whether it seeds or not. Trefoil gives you an easier path to control.

How to Get Rid of Trefoil

Your approach depends on how much trefoil you’re dealing with and how quickly you want results.

For Small Patches: Hand Removal

If trefoil has only appeared in a few spots, you can pull it out by hand. Use a hand fork to loosen the soil first – trefoil has a surprisingly deep taproot for such a small plant, and if you snap it off, regrowth is possible.

The key with hand removal is timing. Get to it before the yellow flowers appear, or immediately after pulling, remove any flower heads from the area. You don’t want seeds dropping into the soil you’ve just cleared.

After removal, overseed the bare patch promptly. Thick grass is your best long-term defence against trefoil returning.

For Larger Areas: Lawn Treatment

When trefoil has spread across significant portions of your lawn, hand removal becomes impractical. A selective lawn treatment is the answer.

Look for lawn weedkillers containing 2,4-D or dicamba – these active ingredients target broadleaf weeds like trefoil while leaving grass unharmed. For non-lawn areas where trefoil has escaped into borders, a glyphosate-based weedkiller will kill it completely. Products that combine weed-killing action with lawn feed are ideal, as they tackle the immediate problem (the trefoil) while addressing the underlying cause (nitrogen-deficient soil). As your grass gets stronger and thicker, it naturally crowds out future weed seedlings.

Apply treatment in late spring or early summer when trefoil is actively growing but before it’s produced masses of seed. The plants absorb the product most effectively during active growth, and you’ll prevent much of that year’s seed production.

One application often isn’t enough for heavy infestations. Expect to treat again after six weeks if trefoil persists, and plan for follow-up treatment the following spring to catch any seedlings from the soil seed bank.

Preventing Trefoil Long-Term

Killing existing trefoil is only half the job. If you don’t change the conditions that allowed it to establish, you’ll be fighting the same battle every year.

Feed your lawn regularly. A consistent fertilisation programme keeps nitrogen levels up, which benefits grass and disadvantages trefoil. Seasonal treatments in spring, summer, autumn and winter each play a role in maintaining year-round lawn health.

Raise your mowing height. Trefoil survives mowing by growing flat – but taller grass shades the soil surface, making it harder for trefoil seeds to germinate. Aim for at least 3 inches (7-8cm). This also encourages deeper grass roots, improving drought tolerance.

Overseed thin areas promptly. Every bare patch is a potential trefoil nursery. If you notice your lawn thinning, get grass seed down before weeds move in. Autumn is ideal for overseeding, but spring works too.

Aerate compacted soil. If your lawn gets heavy use, annual aeration helps grass roots breathe and access nutrients. Healthy grass competes far more effectively against weeds like trefoil.

Don’t let trefoil flower. If you spot trefoil and can’t treat it immediately, at minimum remove the flower heads before they set seed. This won’t kill the plants, but it prevents them adding to next year’s problem.

Frequently Asked Questions

What is the difference between trefoil and clover?

The main differences are flower colour and life cycle. Lesser trefoil has small yellow flowers and is an annual (dies after one season). White clover has white or pink flowers and is a perennial (comes back year after year from the same roots). Both have three-leaflet leaves, but clover leaves often show a pale crescent marking that trefoil lacks.

What is the difference between trefoil and black medick?

Both have yellow flowers and three leaflets, but black medick has pointed leaf tips with a small notch, while trefoil has rounded tips. Black medick doesn’t root along its stems like trefoil does, and its seed pods turn distinctively black when ripe. Both respond to the same herbicides and control methods.

Why does trefoil keep coming back every year?

Trefoil is an annual weed that reproduces entirely by seed – and it produces a lot of seeds. Those seeds can remain viable in soil for several years, so even if you kill all visible plants, new ones germinate from the seed bank each spring. Breaking this cycle requires preventing seed production for multiple seasons while improving lawn health.

Is trefoil harder to kill than clover?

Actually, it’s easier. Because trefoil is annual, you can significantly reduce it by preventing seed production. Clover is perennial and regrows from its root system regardless of seeding. Both respond to the same selective weedkillers, but trefoil gives you the additional strategy of breaking the seed cycle.

Will mowing get rid of trefoil?

No. Trefoil grows flat to the ground specifically to avoid mower blades – it’s evolved alongside grazing animals and regular cutting. However, mowing your grass higher helps indirectly by allowing grass to shade the soil and outcompete trefoil seedlings. Very short mowing actually makes trefoil worse by weakening the grass.

What time of year is best to treat trefoil?

Late spring to early summer is ideal – when trefoil is actively growing but before it’s set masses of seed. Treatment during active growth ensures the plant absorbs the product effectively. Avoid treating during drought, frost, or extreme heat. A follow-up treatment in autumn can catch late-season growth, and treating again the following spring targets new seedlings.

The Bottom Line

Trefoil might seem like a persistent nuisance, but its annual life cycle actually gives you an advantage. Kill the plants, stop them seeding, and improve your lawn’s health – do this consistently and you’ll see trefoil numbers drop dramatically within a couple of seasons.

The real secret is understanding that trefoil is a symptom of struggling grass. Build a thick, well-fed lawn and trefoil simply can’t compete. Your grass does the hard work of keeping weeds out – you just need to give it the right support. If you’re dealing with other lawn weeds like dandelions, the same selective treatments will tackle those too.