Ants under the patio and lawn: what actually works

Why do ants tunnel under your patio and lawn?

Ants search for food and a safe place to expand their nest. Beneath a patio or lawn is dark, protected and more damp than above ground. They feed on soil organisms and food scraps, control other insects and aerate the soil in the process. To them it is an ideal construction site. To you it is sinking patio tiles, holes in the grass and piles of sand.

The good news: ants are not aggressive towards humans and they do not eat plants. The bad news: their tunnels destabilize patio tiles, create damp patches and open holes where other pests move in. Keeping ants away from the wrong spot is smarter than trying to eliminate them forever, because that rarely works anyway.

Why boiling water fails completely

Many gardeners pour boiling water into ant nests. It kills some of the surface workers on the spot, but the real nest is never where you think. Ant colonies have tunnel systems that can reach 2 to 3 metres below the surface. You kill the surface workers, the queen sits elsewhere and the nest reestablishes itself within 3 to 4 weeks on the same site.

Besides, boiling water drains straight out of the jointing sand, can damage plant roots nearby and does nothing to prevent the ants from coming back. This is wasted effort.

Nematodes: biological control underground

Nematodes are microscopic worms that are deadly to the larvae and eggs of many insects. The species Heterorhabditis bacteriophora also targets ant larvae: it infects them with a bacterium that kills the larvae within a few days. You buy nematodes as a powder that you mix with water, and you apply them best in spring or early summer, as long as the soil stays above roughly 8 degrees Celsius and remains moist. If the soil is still warm enough in August, you can treat then too.

Lift the tile or a strip of turf where you see the mound and pour the nematode solution straight into the nest, so you reach the core immediately. One well-aimed treatment is often enough to wipe out an entire colony, although you will only see the result after one to two weeks. If you still notice activity after three weeks, repeat the treatment on the same spot. Not sure which approach suits your own patio? At gardenworld.app you can preview a design with your own garden before you decide.

Polymeric sand as a barrier: prevention is best

Ants love to dig through soft, loose sand. Use polymeric sand (jointing sand) which hardens instead. This sand swells with water and forms a hard crust that discourages digging. Ant nests are much less likely to form under hardened jointing sand than under standard loose sand.

If your patio is new or you are rebuilding it: choose polymeric jointing sand now. For existing patios, clean thoroughly first (pressure wash), empty the joints and refill with new polymeric sand. It takes a day of hard work, but it prevents years of trouble.

Relocating the nest: it actually works

Many gardeners fear touching ant nests. Unnecessary. Ants in our gardens are not poisonous, they do not bite seriously and they will not attack you. You can literally move their nest.

In August, locate the spot with the most activity: a small mound of sand on the lawn or between the tiles. Carefully slide your spade underneath and lift the whole nest out. You will see sand, larvae and workers. Place the nest 5 to 10 metres away, under a shrub or at the garden edge. The ants reorganize themselves there. This works in an estimated 70 to 80 percent of cases, because you give the nest warmth and shelter elsewhere.

Do this in July or August, when the soil is still warm. Relocating a nest in winter does not work.

No poisons where children and pets play

Some insect poisons promise a quick fix against ants. Forget them. They are toxic to your garden, your lawn and especially to children and pets who dig in the sand. The Netherlands and Belgium already restrict private use of many of these products. Besides, ants smell poison, detect the toxic particles and simply nest 2 metres away instead.

Better: mechanical solutions such as polymeric sand, nematodes and nest relocation are much safer, cheaper and ultimately more effective.

Why you should partly accept ants

Ants are also useful. They aerate the soil through their tunnels, help prevent compaction and consume dead organic matter. Many bird species live on ants. They are a sign of healthy soil life. A garden without ants is also a garden with less wildlife.

The goal is not to make your garden completely ant-free. The goal is to move them away from places where they cause trouble, such as under your patio, towards places where they are useful, such as the garden edge. Nest relocation and nematodes are a much wiser approach for that than poison.

Preventing soil compaction: that helps too

Ants prefer to nest in loose, well-drained soil rather than compacted ground. If you aerate your lawn regularly, for example by scarifying, avoid driving heavy machinery over it and add mulch, the soil compacts less. Healthy, well-aerated soil holds more soil life, which makes it overall less attractive for an ant colony.

Ants you can tolerate

Many gardeners have a genuine dislike of ants. Yet they are not only a nuisance. Once you apply nematodes in August and September, ants also feed on other soil pests, such as certain larvae you would rather not have in your borders either. They are, in a way, a part-time ally. That does not mean you want them under your patio, but it does mean you can simply leave them be further away, under a shrub or in a quiet corner of the garden.

Step by step

Step 1: Find the spot

Find where ant activity is strongest. You recognize a nest by small mounds of sand on the lawn or between patio tiles. These mounds grow as the colony gets bigger.

Step 2: Choose polymeric sand

If your patio has not been rejointed yet, choose polymeric sand now. This prevents new colonies. Remove the old sand first before adding the new.

Step 3: Buy nematodes

Buy nematodes against ant larvae, preferably in spring or summer. Budget around 20 to 40 euros per treatment. Mix them with water according to the instructions and water them in.

Step 4: Relocate the nest (optional)

Slide the spade under the nest, lift it out and place it 5 to 10 metres away. Do this in July or August, on a warm day.

Step 5: Check and repeat

Check after three weeks whether there is still activity. If so, repeat the nematode treatment on the same spot. A little residual activity is fine; getting rid of ants completely is rarely necessary.

Frequently asked questions

Are ants dangerous to humans? No. Ants in Dutch and Belgian gardens do not bite seriously and are not poisonous. They may pinch lightly if you directly disturb their nest. The more you simply leave them alone, the less threatened they feel.

Can I treat an ant nest myself with poison? Technically yes, but it is wasted money and effort. Ants smell poison, detect the toxic particles and simply nest 2 to 3 metres away instead. Chemical control helps for a month or two at most, then they are back.

Do ants damage my plants? Not directly. Ants do not eat plant sap or leaves. Their tunnels can occasionally expose roots, but this is rare and mainly affects very young, vulnerable plants.

How long before I see results from a nematode treatment? Nematodes work gradually. After 2 to 3 weeks you already notice less activity. The full effect shows after 3 to 4 weeks. Do not expect an overnight miracle, but a clear and lasting decline.

See it first in your own garden

Want to see how your patio and lawn could look without those sand mounds and holes? Upload a photo of your garden to gardenworld.app and view a free preview of your improved patio or lawn design. The sharp, detailed plan with measurements and plant suggestions costs just 7.99 euros once, no subscription.