We live in Montreal and this is our third year gardening in a 10'x20' community plot. This year, most of our plants are not growing very well compared to previous years. The zucchini plants in particular seem to have stalled at just a few leaves each, but the lettuces and carrots we planted from seed have mostly failed to sprout and most other things, even the nasturtiums, are just not looking very robust. The tomatoes, dill, red cabbage and snow peas seem fine, but everything else is failing to thrive. There are even fewer weeds in the area around the zucchinis than elsewhere in the garden. I have noticed that there are a LOT of ants around our plot this year. They are the regular small brown kind, which I think are usually supposed to be beneficial to gardens as they will attack other insects, but the whole area is riddled with ant holes and the surface of the soil has that flat-but-grainy look that anty areas tend to have. Last week there were two separate times where I noticed a single broccoli plant drooping for no obvious reason, but when I looked closer, there were ants swarming the base of the stem, seemingly using it as an entry point to their tunnels. In each case, I flushed out the area with water and the problem has not returned, although those two plants are now well behind the others in their development. I have not seen any aphids or other things that might be particularly attractive to ants. Before planting, we tilled the soil and tried to mix it around to evenly distribute the compost we were adding, so I think it should be consistent throughout the plot. From looking around on the internet, it seems like ants are usually harmless in a garden (unless they are carpenter or fire ants), but I can't think of anything else it could be. I saw a few forum posts that talked about ants spreading diseases between plants, and one about ants eating seeds, but those types of posts were an overwhelming minority. The plots around ours seem fine, but they are separated from ours by hard-packed-fine-gravel walkways. I'm hoping someone here will know: if our garden plot is basically one giant ant colony, could this damage the plants or stop seeds from coming up?
It could. Too much of a good thing is not a good thing. I would try diatomaceous earth to get rid of ants, see How Long Does It Take to Kill Ants With Food Grade Diatomaceous Earth? I would also try to find out what could lead to such a sudden increase in the number of ants in your garden plot but not in the others; what have you been doing differently? Where is your compost coming from? Is it from a different source than your fellow gardeners use? By the way, is your garden organic?
We've always had some ants, usually in one or more corners or along a particular side of the plot--and when we would dig into that area, it would seem like there were more ants than soil--but they always stuck to the edges and didn't seem to affect the plants in any particular way. The compost comes from our home composter, which also has some ants in it, but the garden is several blocks away. We definitely don't have enough compost to really prepare the soil well; at the end of each summer, I carry over all that we have, which is a full year's worth of our decomposed kitchen scraps, but it never looks like very much in the context of the garden. So the soil might be somewhat depleted too. This is only our third year having the plot. I don't know how it was treated before we came along, but I'm pretty sure the garden has rules against anything non-organic. We've never added anything but plants and the aforementioned compost. Our plot is against the edge of the garden, so we tend to get extra weeds, vines, and tall grass that creeps in, but I don't know if that has anything to do with the ants.
Agree with Sundrop. Too many ants can be a problem. Most ant hills I've encountered are essentially plant-free miniature deserts: File:Ant hill cm01.jpg - Wikimedia Commons
@Andrey Zharkikh posted a photo (from Utah) on flickr that demonstrates that: 2016.07.04_11.26.43_IMG_7148
Fact #1: The vegetable garden is growing poorly. Fact #2: There are lots of ants. Consider the possibility that these 2 observations may only be coincidental; that the ants may not be responsible for the plants' poor performance. How could the activity of ants affect the health of plants? Since they are not being eaten above ground, you have to ask if the ants are robbing the soil 0f nutrients? Are they eating the roots? Disturbing the roots? Spreading diseases? Poisoning the roots? Not likely. Have you dug anything up to see how the roots look? I suspect you have a nutrient problem. You cannot assume that the compost you apply yearly is providing enough nutrients - nitrogen in particular. It would be interesting to give your garden a shot of a fast-acting, nitrogen-rich fertilizer and see what happens. It can't hurt! If you feel you must use diatomateous earth, try to limit it to spot treatments. While it is effective killing ants, it also kills many beneficial insects and it does persist in the soil. PS I have several thatching ant nests in my Vancouver Island garden but I have never seen any evidence that they are anything but beneficial overall. (They are the most fascinating creatures.) In my experience, they do not create mini-deserts but, in fact, future compost piles . . . there are always plenty of grasses and other weeds lapping at the edges. I believe ants do far more good than harm and should be cherished rather than eradicated unless absolutely necessary.
I always wondered, when looking at my Strawberry plants struggling for survival whenever ants decided to make their nest underneath. And I came to the conclusion that where there is an infestation of ants it could be all or any of the above. No plant, as demonstrated in the links in the posts above as well, can survive for too long when in the center of the ants nest. Most community gardens are organic. "fast-acting, nitrogen-rich" sounds strangely like synthetic agrochemical. I am curious, is it what you have in mind?
Until reading Sundrop’s question about which fast-acting nitrogen fertilizer to use, I hadn’t considered man-made fertilizer – rather something like liquid fish fertilizer, blood meal or alfalfa meal, maybe worm castings. Thinking about it further, perhaps a very small amount of urea or ammonium sulphate applied to a representative few of the plants would provide an even quicker diagnosis to determine if a lack of nitrogen is at the root of the troubles in AmandaD's garden. Of course, the best idea is to have a proper soil test done at an accredited lab (home soil test kits are not accurate enough to be relied on) and go from there. There are so many variables, including soil type, temperature and pH so it is always more difficult to correct suspected nutrient deficiencies than to be proactive and create the best organic soil possible before planting anything. Whatever the underlying problems may turn out to be, I am unconvinced that ants are soley to blame, keeping in mind too that healthy plants are much less susceptible to disease and insect problems.
Another possibility to check out is root aphids. They are usually accompanied by ants and produce symptoms similar to those described.
After reading vitog's post I did a little research on the Net to find more about root aphids. It is what I have found about Yellow Meadow Ants that fits AmandaD's description: “males 3–4 mm and workers 2–4 mm. Their colour varies from yellow to brown . . . The Yellow Meadow Ant feeds on the honeydew from root aphids, which they breed in their nests.” Yellow meadow ant - Wikipedia, the free encyclopedia
For the sake of making sure my view isn't misinterpreted: I wasn't advocating for the eradication of ants in gardens, just noting that they could be troublesome.
