Ask the head gardener: How can I keep ants off my plants?

Telegraph

13-06-2025

Dear Tom,
I have infestations of ants in the polytunnel in my garden – they have destroyed the tender plants. I have tried all the various organic methods: paprika, chilli powder, white vinegar and drowning the nests; alas, to no avail. Any other ideas?
– Liz, Cornwall
Dear Liz,
I'm so sorry to hear that Liz; growing plants can be challenging enough at the best of times, let alone adding a nest of hundreds, if not thousands, of ants into the mix. The presence of ants in our gardens or around our homes can be quite unnerving, but in reality, they provide an important part of a wider ecosystem, acting as food for many birds and mammals. Ants in lawns can be tolerated by simply removing the mounds of soil as they appear on the surface; it is best to remove the soil before mowing to avoid smearing the turf. When ants build a nest in a compost heap, that extra level of aeration can help accelerate the composting process, and therefore they cause little damage. Powders and boiling water aren't always that effective – although these methods can make us feel better for a fleeting period of time, because we feel that we've done something about it, very little else is achieved.
Ants can cause harm when plants are particularly young, or low-growing to the ground, such as alpines or container plants, where the excavation of the soil can physically disturb and damage a plant. The warm and dry conditions that are created in your polytunnel will create an ideal environment for ants.
There are a few techniques that you can adopt to discourage ants from nesting in your polytunnel, however. First, you can make the ground conditions less than ideal, forcing the ants to relocate to a less sensitive part of the garden. This is a more realistic approach compared to complete eradication, which is counterproductive when it comes to your garden ecosystem.
It is a challenge to keep areas of soil biologically active in protected environments with fertile and moist conditions throughout the year, where no natural rainfall can hydrate the soil. Many of us come to plant our tomatoes and cucumbers in open ground in greenhouses and polytunnels in the late spring, and find that our soils are more like sawdust than the rich, moisture-retentive soil that we require. This is often the result of a degree of neglect over the winter. I would suggest keeping any open soil within the polytunnel regularly mulched and watered to make that area less hospitable and inviting for ants to take up residence.
Secondly, you can look at applying nematodes (microscopic parasites) to the area, such as Steinernema feltiae (available online), which will prey on the larvae but not kill the adult ants. The combination of moist ground and the presence of nematodes is likely to deter the ants and encourage them to relocate, let's hope to an area outside of the polytunnel, which is less sensitive. Finally, if you have staging or tables within your polytunnels, apply a layer of petroleum jelly to your table legs or staging to prevent or discourage ants from climbing; this will keep them from accessing your young plants in pots, where the excavation can do a lot of damage.
Generally, in a garden, ants do little damage to plants that are planted in the ground, but they encourage and protect aphids and other sap-sucking pests in their production of honeydew. This process is sometimes described as the ants 'farming' the aphids. The presence of ants can be detrimental to the plant in the way that they protect the aphids from predators such as ladybirds and allow those populations to build up to the point that they can weaken the plant by reducing its vigour and photosynthesising capabilities. Keep an eye on aphid populations within your garden and use SB Plant Invigorator once or twice a week when you see an outbreak, to keep those numbers under control.