Why are there ants all over my plant?

Ants get blamed for a great deal they did not do. They are there because something else is producing sugar, and they protect that something from the ladybirds, hoverfly larvae and parasitic wasps that would otherwise deal with it. Removing the ants restores the natural control, which is why an ant barrier is one of the highest-value interventions in a garden.

Step by step

1. Follow the trail upwards.Ants going up and down a stem in a purposeful line are commuting. Follow them and look at the growing tips, the leaf undersides and the leaf joints. You will find aphids, scale insects or mealybug, usually in numbers.
2. Treat the sap-suckers, not the ants.Wash aphids off with a jet of water and repeat every few days. Scrape scale off physically. Dab mealybug with surgical spirit. Once the honeydew supply stops, the ants lose interest within days.
3. Put a barrier on the stem or the pot.A grease band or a band of sticky barrier compound around a trunk, a stem or a pot rim stops ants climbing. On fruit trees this is standard practice and it also catches winter moth females. It has an immediate effect on the predator population, because the aphids are suddenly undefended.
4. Check whether the nest is in the pot.A pot that dries out unusually fast, with fine dry soil at the surface, ants pouring out when it is watered, and a plant wilting despite watering, has a nest in the rootball. The tunnels create air gaps so water runs through without wetting anything. This is genuine damage.
5. Deal with a nest in a pot by soaking and repotting.Stand the pot in a bucket or a tray of water for an hour so the compost is thoroughly saturated, which drives the colony out. Then knock the plant out, wash the roots, and repot into fresh compost. Repeated drenching alone is often enough to persuade them to move.
6. Leave nests in the open garden alone where you can.Ants aerate soil, remove a great deal of pest larvae and are food for green woodpeckers and other birds. A nest in a lawn or a border with no plant suffering is not a problem, and pouring anything into it does more harm than good.
7. Move a nest that is genuinely in the way.Under a path slab, in a raised bed or against a house wall, repeated disturbance is the usual answer — turning the nest over with a fork every couple of days persuades them to move, and it is less destructive than a chemical treatment.
8. Do not confuse ants with the plants they nest on.Ants also collect the sugary coating on peony buds and on some seeds, and neither does the plant any harm. Peonies open perfectly well without ants and are not helped by them either, despite the persistent myth.
9. Keep sweet things away from patios and pots.Fallen fruit, spilled drinks, bird food and open compost all draw ants to an area and increase their interest in the plants nearby.

Tips

• Flying ant day, when winged reproductives leave the nest en masse in July or August, is normal and lasts a day or two. Nothing needs doing.
• Ants on a plant that has no visible pest often means scale insects, which are the easiest sap-suckers to miss. Look at the stems and midribs specifically.
• Watering a pot properly by soaking it in a tray rather than pouring from the top solves both the ant nest and the dried-out rootball at once.
• A grease band on a fruit tree needs renewing each year and needs the ties checked, since ants will bridge across a loose tie.

Common mistakes

• Treating the ants and leaving the aphids — The aphids are the reason the ants are there and are the pest doing the damage. The ants come back as long as there is honeydew.
• Pouring boiling water or chemicals onto nests in borders — It damages plant roots and soil life, and ant nests in the open garden are generally harmless or beneficial.
• Assuming ants have eaten the plant — Ants do not eat living plant tissue. Damage attributed to them is nearly always slugs, caterpillars or the sap-suckers they are protecting.

Questions people ask