The first warm, dry weeks of May is usually when anthills appear. One Saturday the lawn is flat, the next morning there are half a dozen small mounds of fine soil between the daisies, and within ten days you have eight or ten of them and a mower deck that keeps clogging on the loose earth. If you have lived with the same lawn for several years and never had this problem, you are not imagining things. Ants build up gradually, are deeply tied to soil temperature, and tend to explode in numbers during the second or third May of a settled colony.
You have two practical questions to answer. The first is why the anthills appear so suddenly in mid-spring. The second is what to do about them without killing the grass or poisoning the rest of the garden. Both have better answers than the usual advice of “pour boiling water on them and hope”.
Why May is the trigger month for ant activity
The black garden ant, Lasius niger, is the most common culprit on lawns in temperate climates. Colonies overwinter underground in a state of near-dormancy from roughly November through March, with worker activity dropping to almost zero once soil temperatures fall below 10°C (50°F). When the top few centimetres of soil warm back to about 12°C, which in most cooler northern gardens happens in the first half of May and in warmer southern regions can be as early as late March, the queen restarts egg laying and the workers begin tunnelling outward and upward to forage.
The mounds you see are not anthills in the way you might imagine. They are the spoil heaps from new tunnels. As workers excavate chambers and galleries, they bring fine soil to the surface and deposit it around the entrance hole. A single colony will often produce three to five surface mounds, all connected underground. That is why flattening one mound rarely solves the problem. The colony simply rebuilds at another existing exit a metre or two away.
May is the obvious trigger month because soil moisture and temperature line up almost perfectly. Wet enough to dig, warm enough to be active, but not yet so hot and dry that the colony seals itself off. By late June, if the weather turns properly dry, mound building usually slows down and the visible damage stops getting worse even if the colony remains.
The treatments that work without destroying the lawn
Before going near a chemical, decide how much of a problem the ants actually are. A handful of mounds on a large lawn is cosmetic. Twenty mounds across a 50 square metre patch where children play barefoot is a different conversation. Many lawn specialists, including the Royal Horticultural Society, recommend tolerating low-level populations because ants aerate soil, predate on lawn pests like leatherjackets, and almost always come back to the same site within a season if you wipe out one colony.
If you do want to act, the most effective non-chemical approach is biological nematodes. The species Steinernema feltiae, sold under brand names like Nemasys No Ants (around £18/$23 for a 12 square metre pack) and Defenders Nemasys (around £20/$25), is a microscopic parasite that targets ants and irritates them until the colony moves on. You apply nematodes mixed into a watering can on a damp lawn, ideally in the late afternoon when the surface is shaded. They need soil temperatures of at least 10°C, so May and June are the prime application windows. The treatment usually clears most mounds within two weeks, although the colony may relocate to a neighbouring bed rather than vanish entirely.
For a more immediate physical answer, the cold water method described in turf-care advice from Rolawn and others does work, particularly on small new mounds. Water the mound deeply with a hose for about three minutes, repeat every other day for a week, and the colony will usually relocate because ants prefer dry conditions. This is the method to use if you want the mounds gone before a garden party and you can dedicate ten minutes a day to the watering can routine. It will not kill the colony, just push it elsewhere, which may be enough.
Strong-smelling natural repellents like cinnamon powder, cayenne pepper, peppermint oil and lemon juice all show up in folk remedies and some of them have a real if modest effect. Cinnamon sprinkled directly on a mound at a rate of about one teaspoon per mound, repeated three or four times over a week, can drive ants from that specific entrance. None of these will eliminate a colony, but they are safe around children, pets and pollinators, and can stop a mound from getting larger while you decide on a stronger treatment.
Why you should think twice before reaching for boiling water
The boiling water method is the internet’s favourite ant remedy because it is dramatic and free. It also kills the grass within a 20cm radius of the mound, leaves a yellow scar that takes six to eight weeks to recover in May conditions, and is unlikely to penetrate deep enough to reach the queen in a mature colony. By the end of the summer you will typically have both the original anthills and a series of brown patches around them.
If you have decided that a chemical is the answer, ant powders containing permethrin or bendiocarb, sold as Nippon Ant Killer Powder (around £6/$8) and Doff Ant Killer (around £5/$6), are highly effective when puffed directly into the entrance hole. They are best used in the early evening so the active ingredient is carried back into the nest as workers return. Keep children and pets off the treated lawn until the next rainfall, and do not apply more than the label rate, which is usually around 30g per square metre.
The brushing routine that stops mowing damage
Whatever treatment you choose, the immediate practical problem in May is that loose soil from anthills clogs the mower deck and can blunt the blade much faster than usual. The fix is to walk the lawn five minutes before you mow with a stiff yard brush or a spring-tine rake, sweeping the mounds flat. Once the soil is dry on a sunny morning, it brushes out easily and disappears into the canopy. If you mow without doing this, you will end up with a striped lawn where loose soil has been smeared along the mowing lines.
The same brush routine works in reverse for prevention. Brushing the lawn lightly twice a week through May disturbs the surface of any new mound before it grows tall enough to clog the deck, and the constant interference often persuades small colonies to move into a flower bed where they cause far less of a problem.
When to call a professional
If you are dealing with more than 25 mounds across an average garden, or if the same mound has been treated three times and keeps coming back, you almost certainly have multiple connected colonies under the lawn rather than a single nest. At that point a professional lawn-care company will usually use a granular insecticide bait at a controlled rate, often combined with a soil drench, for around £80 to £150 per visit in the United Kingdom or $120 to $250 in the United States.
For most gardens this level of intervention is overkill. The bigger gain comes from accepting that a small number of mounds is normal in May, treating only the ones that are clearly disruptive, and getting into the brushing habit before each mow. Within three or four weeks the visible damage usually plateaus, and by mid-July the lawn will be back to looking like the lawn you wanted.
A final word on living with ants
If your garden is otherwise healthy, a small population of ants is one of the better pests to have. They eat the eggs and larvae of leatherjackets, chafer grubs and several other lawn pests that do far more damage than the ants themselves. They aerate the soil around their tunnels in a way that benefits root growth. They feed pollinator predators like greater spotted woodpeckers that visit suburban lawns to dig out late summer broods. A garden with no ants at all usually only stays that way because of repeated chemical treatments that affect the wider soil food web. Striking a balance between visible mounds and a functioning ecosystem is, for most gardens, the better long-term play than trying to eliminate every colony each spring.
