Carpenter ants need all their cohorts to survive. And that includes the egg-laying queen. It's a very unlikely occurrence to receive the entire "family" in mulch.

Borax is usually a good way to stop an ant invasion but there’s other alternatives. This site has ideas on getting rid of ants in the garden. Best Ways to Get Rid of Ants in Your Garden It mentions Diatomaceous Earth and has a tutorial on how to implement it. The problem with Diatomaceous Earth is that it kills an ant from the outside rather than acting as a poison when ingested. So it forms a good barrier. Here is the explanation: Natural Ant Killer, Diatomaceous Earth for Ants Other ways are baking soda ant traps, natural spray and other alternatives. If you implement an ant trap correctly it can be as effective as a Terro ant trap. Try to make the borax or even a baking soda ant trap to make sure that it kills the nest and then form a barrier with Diatomaceous Earth.

You've seen the actual ants among that dark granular material or it is instead part of the remains of the plant structure, which you are mistaking for evidence of a nest? Carpenter ants nest in wood, if you are tearing into a nest eggs and larvae should become visible at some point. Otherwise scattered ants are foraging and not indicating the exact location of a nest. Except for when they come into a house while doing this and get onto counters etc. this behavior is of little comparative importance - the most significant thing about them being found near a wooden structure is that they might be nesting in it, gnawing out the wood. And killing a percentage of the foraging ants is not going to eliminate the nest they are coming from, which will continue to generate more ants. Ants make trails that they follow, if you are in fact seeing a number of carpenter ants moving around there look for them traveling along such routes and follow those back to the nest. They don't eat wood so the presence of their nests is marked by dust-like sawdust that they have generated and disposed of outside the entrance. If they are infesting dried conifer wood such as might be used in buildings this material will often be yellowish or otherwise pale.

We had those when we first built our home. I think it’s because we disturbed the soil. I asked my husband if it was a carpenter ant and he said to look for a tool belt on it. I was not amused. We never saw them again after that.