- Wiping out a colony tends to hand the ground to incoming queens, which can start even more nests than the one you destroyed.
- The heaps are fine soil, so brushing them flat on a dry day before you mow is most of the job.
- Winter is the easiest time to lift and relay turf that years of ant digging have left raised and lumpy.
Why the Mounds Keep Coming Back However Many Ants You Kill
You walked out, saw little soil heaps scattered across the grass, watched the mower smear them into brown streaks, and decided the ants had to go. Most people reach for a kettle or a tin of powder at this point. Both do damage, and neither ends the heaps.
The fix is gentler and quicker. Brush the heaps flat on a dry day before you mow, so the mower has nothing to smear. Water a troublesome mound with cold water rather than boiling water, which nudges the ants to move on without scorching the turf. Where years of digging have left the surface raised and uneven, wait for winter, peel the turf back, take out the extra soil and lay the turf again.
That is the whole plan. The next sections explain why killing the nest backfires.
What Is Going On Under the Grass
The heap is soil. A nest holds one or more fertile queens that lay eggs in brood chambers, plus a crowd of smaller wingless workers. The workers maintain, guard and enlarge the nest, feed the larvae and gather food. Each time they enlarge the nest, they carry the dug out soil up and leave it on the surface. That is your heap. Ants do little direct harm to plants, but soil dumped on top of a lawn is a problem for the lawn.
Lawn ants do not all behave alike. The yellow meadow ant is very small and rarely seen unless you disturb the nest. It forages below the soil, feeding on small insects, and farms aphids on plant roots. It is completely harmless, yet it leaves small earth mounds on lawns and flower beds. Also called the mound ant, it is the worst lawn offender, though passing feet and mowers usually flatten its mounds, and it favours lawn edges, where there is less disturbance.
The cost of a heap is the patch of earth it leaves, and bare earth is where weeds get into the turf. That is the part worth dealing with.
Nests can last for many years. A colony is a long term tenant of the ground, and that shapes everything below.
Why Wiping Out the Colony Makes Things Worse
The belief goes: kill the nest and the heaps stop. The RHS says the opposite. Ants are best left alone unless a nest is a real nuisance, as a destroyed colony is likely to be replaced by incoming queens that claim the vacant territory and can found more nests than before.
Follow a queen through her year to see how. In humid summer weather, nests release winged ants, which are young queens and males. They often emerge in huge numbers, the event known as flying ant day. They take to the air and mate. The males die. The young queens then look for a place to start a new nest, and a mated queen no longer needs her wings, so she bites them off. Every established nest sends out a fresh batch each summer.
Now look at a cleared patch of lawn from a queen’s point of view. It is open ground.
Here is the detail most people miss. Treat every nest in your garden, and winged females flying in from neighbouring gardens in search of a nest site are more likely to survive, as worker ants prey on new queens and so keep the population in balance. The ants you already have are, in effect, defending the ground. Remove them all and you open the door for the next flight.
Poison brings its own damage. Gardeners’ World points out that sprays and powders contain insecticides that harm other insects, bees among them, and leach into the soil, where they harm creatures such as earthworms. Ants are also a food source for birds and amphibians.
Boiling water is the other favourite. It scorches the turf, leaving patches that weeds colonise. You trade a small heap for a dead patch with weeds in it.
Then there are nematodes, microscopic worms sold by some biological control suppliers. The nematode Steinernema feltiae is watered into the soil wherever ants push soil up, and adds that it can also affect non target insects. It does not kill ants. It aims to encourage them to move elsewhere. When ant plagued Which? Gardening members tried it, most were dissatisfied, as the ants that did relocate were still a problem.
Kitchen remedies fare no better. Gardeners’ World suggests sprinkling cinnamon near nests. David Price, associate certified entomologist and director of technical services at Mosquito Joe, says the internet is full of myths that do nothing to stop ants. On cinnamon he says: “They don’t like it but doesn’t repel or stop them.” On vinegar, one of the most common suggestions: “It can temporarily disrupt the trailing pheromones, but they will reappear within a couple of hours,” Price says.
How to Get Rid of the Mounds Themselves
Deal with the heaps and leave the colony alone.
Choose a dry day and sweep the dug out soil across the surrounding grass before you mow. Skip this step and the mower smears the soil across the lawn surface.
If a heap keeps returning to the same spot, break the hill up with a fork and water it regularly with cold water. Cold water will not kill the turf, but it disturbs the ants and encourages them to move on. Ants prefer dry conditions, and good watering tends to put them off. A nest buried under a paving slab is out of reach, as the water will not get to it, so this works best where you can get at the nest.
Beyond that, you usually do not need to bother. If ant activity has loosened plants, firm them back into the soil and water them well.
One exception needs care. Price draws the line at the sting: “Fire ants in the yard will sting, and are aggressive if the mound is disturbed. Other ant species in the yard pose no harm and can be left alone,” Price adds. If the ants in your lawn sting, keep your hands and boots off the mound and ask a pest controller. Red ants, mostly Myrmica species, can sting as well, though for most people that is a minor irritation. The common black garden ant and the yellow meadow ant are safe to brush.
Worm casts are the other soil heaps that dot a lawn in autumn, and our post on brushing earthworm soil heaps off your lawn covers clearing those.
What to Do in Winter With Raised, Lumpy Turf
Years of ant digging can leave a lawn with a bumpy surface. A broom flattens a heap, but it cannot lower a raised area. The advice is to peel back the turf in the raised areas, take out the excess soil and relay the turf, and the job is easier in winter, when ants are less active.
The steps come from the RHS page on repairing lawns. You need a flat bladed spade, a fork and a rake.
1. Cut through the uneven area with an H shaped incision using the spade, and peel back the turf.
2. Fork over the base and remove the excess soil.
3. Firm down the soil so it is level, then replace the two flaps of turf.
4. Check the lawn is level and adjust again if necessary.
5. Compress the turf edges with the back of a rake and brush a sandy lawn top dressing into the crevices where the turf was cut.
6. Water thoroughly.
The top dressing is a mix of sand, soil and garden compost, bought from garden centres or mixed at home. The same page names spring and autumn as the ideal times for lawn repairs, when the weather is damp and cool, so pick a damp, cool day for the job. For smaller dips after the relay, see our post on levelling a bumpy lawn.
The payoff is a level surface by spring, before ants reach their busiest months, April to October.
How to Stop It Coming Back (and When to Call Someone)
You will not stop ants for good, and you should not try. The colony under your lawn is also what keeps newcomers out.
What you can do is make the surface harder to spoil. Brush the heaps before every cut from April to October, so the mower never smears soil across the grass. Water in dry weather, as ants prefer dry conditions. Watch the bare patches under old heaps, as they are the way in for weeds, and firm loosened turf back down. Check the lawn edges first, where mound ants prefer to settle.
Know when to stop. Leave harmless ants alone and save your effort for the heaps. Our post on levelling ant hills has more on the levelling. Call a professional if the ants sting or you cannot tell what you are dealing with. Kill the nest and you could face a new one next summer, with a dead patch of grass beside it.
