Hello Mary,
I don't know if this is relevant but ants do produce Formic Acid. Could it be something like this that is damaging your plants? It might be worth trying treatment with a solution/suspension of garden lime although not all plants like lime.

I think you might be right. I thought ants just squirted the formic acid at their enemies, but searching for 'effect of formic acid on plants', I found that trees in rainforest are killed by ants there. Apparently, the ants bite the plants to inject the formic acid into them, and it then enters the plant's circulatory system and poisons them. Something similar must be happening here. That would account for the Pomegranate's leaves still falling off after I've repotted it.

I'm not sure what the ants are doing in my conservatory, though. The nests are in pots, not on a particular type of tree, so they aren't killing species which aren't useful to them. Perhaps it's some sort of instinct, and given the chance, they'd nest in all the pots and kill all the plants. I'm not keen on that idea.

The plants seem to like the soil (apart from the ants) as it is, so I'd rather not make it more alkaline. Perhaps I should put a bit of chalk in all the pots in future. I don't know what would deter the ants but not affect the plants.

Here is a link that might be useful: Ants kill trees in the rainforest

I don't know how ants kill plants but maybe you should put some ant killer down. I have used something called Nippon in the past when we had them in the greenhouse.

Hi Crazylady. Firstly the stuff about the acid is not true. I get ants too and thankfully they stay outdoors and out of my pots, mostly they burrow under the paving slabs, the compost heap and some dry beds. You hit the nail on the head when you mentioned your sandy soil. Ants adore dry sandy soil and love to make their nests in that kind of environment. Your plants are dying because the ants are making so many tunnels that the water just runs through and doesnt get to the roots which are also exposed due to their excavations, this is mae worse by your already sandy soil. The ants dont like damp wet compost like material and remove it leaving the soil deprived of nutrients. Its really rare for them to cause the death of plants and especially trees outdoors, are you sure your magnolia didn't die for some other reason? Anyway it sounds like you have an extreme invasion. I would recommend potting your plants in peat based compost and keep them watered well, the ants wont like this so much. Try placing some plants in dishes of water also, i.e. the ones that are infested. Nippon is a dust that you use as a barrier to prevent the ants coming into your house, but if they are already in there you will just prevent them from leaving! There is a poison that is available and apparantly the ants take it back to the nest and it kills the queen and the larva, you would need to look it up. If it was me Id concentrate on getting rid of them from my pots first and then in the winter I would dig in some well rotted manure, compost, or other humous material into the soil and water more frequently this should lessen their impact. At least you wont be troubled by greenfly or other pests too much as the ants are great little pesticides themselves. Good luck. PS On your soil it might be an idea to grow more "ant adapted" plants like mediteranean shrubs, lavenders etc that like dry conditions.

The ants will have an absolutely gianormos nest under them. They don't dislike veggie waste but adore it and tke it down to the lower levels.

You should go into the composting business by getting all the organic waste available locally and spreadng it out over the area affected. Outside will do. Water the area with your grey waste water it will be worth arranging that anyway in a dry sandy soil.

If you dig the soil out annually you will have the richest topsoil imaginable. You will have to treat it to remove the ants. I don't suppose there is a market in it though.

Here is a link that might be useful: There is a market here.

I have a type of ant in my garden that excavates near plants and builds the soil up in a crusty layer around the base of the stem. This can cause wilting because the air spaces around the roots of the plant mean they are effectively exposed to air rather than soil and so cannot take up water. If not found in time it could cause the plant to die - I water well and put Nippon down. I had assumed that this was an ant killer rather than a barrier.

ants protect greenfly and blackfly keeping insects that do eat them away as they milk the aphids

Thanks for all the replies. I don't think the problem is extreme drainage (which was suggested to me before). When I've tipped the pots out, I haven't noticed any particular tunnels, just a solid mass of ants and their eggs.

I mentioned the sandy soil as I know ants prefer an easy life. It isn't very sandy, just more sandy than chalky. None of my plants are potted in the soil, they are all in a mixture of JI and multi-purpose compost. I do have ants outside under paving slabs, but they don't cause any problems there. The conservatory isn't indoors, it's more a greenhouse attached to the house. The soil in the conservatory border also has the compost mixture in with the soil. Oh, and I don't have greenfly in the conservatory, either, just outside, where there are black ants. It's red ones in the conservatory.

If it was just too much drainage and lack of water, the plants would have recovered when I repotted them, wouldn't they? I'm still hoping the pomegranate will recover. I repotted it, and put chalk in the pot as well. The Passiflora had beautiful roots, but hasn't recovered.

I was looking for an explanation for something that isn't normal, and the acid thing seemed plausible.

I am just in the process of repotting an Acer. Ants and eggs have taken over half of the root ball. I've had this before with a potted Bay tree which I thought was scorched by the sun but on a closer look it was riddled with ants! I tried to save it with nippon ant killer but not sure if it made it worse. In fact, I suspect it probably finally killed it.
So this time I'm re-potting but think the only way I might save it is to wash ALL the soil in the root ball away and start again. A very long process and a LOT of water!
I think the comment about the soil being dry is probably why they like it there in the first place. It's not a problem in the winter when we have plenty of rain!

YAY! Somebody else who'll believe me! I have red ants all over my garden and certain plants which happen to be covered in ants are slowly dying. The perennial basil plant, which I planted a couple of months ago in a pot with lovely loamy potting soil is also a victim. The stem started to darken from the BOTTOM UPWARDS, almost like it's sucked up some poison - and only now the leaves are starting to brown. (It makes one think that the ants have turned the plant into part of their nest and live inside which I'm sure isn't possible???) If it was just dry, the leaves would've wilted first. AND we've just had an extremely wet summer (it's our Rainy season) where I'm surprised that the ants weren't flushed out,but the soil is swarming with red ants. The poison sounds much more plausible!

We have sandy soil and the ants are a real pest in the greenhouse killing tomato plants if not stopped in time. The best solution is the ant killer sold in soluble sachets by Doff and Nippon. Mix thoroughly using warm water and pour about a pint of the solution into the affected pot/ring. You will see the plant improve within 48 hours. I've just done my third tomato plant about an hour ago.

I had ants in a large outdoor pot that I wanted to plant with potatoes. Did not wat to use any poison. I had some left over onion sets, put a few of them in the pot and the ants disappeared. Worth a try for you?

My yard and garden are a mix of red clay and sand, and they've always been overrun with fire ants (South Carolina, zone 7b). They typically don't cause many problems to my plants, even when making homes in pots, but they decimated eggplant and potatoes every time I've tried to grow them. They destroy the root systems and eat holes through the baby potatoes, I'm curious what it is about these 2 plants in particular that entice the ants.

Ant problems? Instead of using a chemical poison you might try using cinnamon in and on top of your soil. Ants don't like it and it's a natural remedy.

also using vinegar on floors , patios, etc where you have your pots will destroy their scent trail.

Thanks Terri. Anything for caterpillars? They r eating the foliage above my axolotl pond so anything toxic is a no no.

The safest method is handpicking them off your plants and dropping them into a bowl of soapy water. Butterfly caterpillars and a few types of moths have host plant preferences. What type of foliage is being affected?