The ants aren't innocuous as they're creating nests in your planting medium. This can hurt the roots because the ants are aphid ranchers. Aphids take the juice out of the marijuana plants and formulate the sugars into a "honeydew." The ants will then squeeze the aphids to harvest the honeydew. At the same time, the ants are very protective of the aphids and they ward off any predators.
Aphids hide on the bottom side of the leaves. These winged pests give live birth at the rate of up to twelve offspring per day. Aphids suck the leaves dry of nutrients causing them to turn yellow and wilt. They can transmit viruses from plant to plant resulting in stunted growth and the delay of flower production. You don't want any ants protecting them.
Just by looking at it, you might think the ants are really "everywhere," but, if you follow their trails, you'll notice that only one or two spots are really being colonized. It is vital to the health of your marijuana plants that these ants be eliminated. You can do this in a number of ways that don't involve the use of poisons. You can place mixes of boric acids, sugar, and diatomaceous earth in the path of the ants. These substances will surround their exoskeleton, puncture it, and cause the ants to dehydrate. You can also put a sticky barrier on the plants so that ants can't climb up it. Tanglefoot is a product sold by nurseries for exactly this purpose. A pyrethrum solution is used by professionals to exterminate ants. Ant bait can also prove to be quite effective.
In general, though, cinnamon is the best ant exterminator and repellent. Ants are both repulsed and killed by it. When you sprinkle it or water it onto your soil, you'll notice ants running out of their nests wildly. Simply sprinkle some regular ground cinnamon onto the planting medium with a spoon. Then make a solution of 2 rounded tablespoons of ground cinnamon per gallon of hot water. Allow the water to return to room temperature, then put in a liquid pyrethrum insecticide as directed. Coco-Wet and other surfactants will make this solution even more effective. Use this solution as a drench. You should also add beneficial nematodes to the medium. These are microscopic organisms that attack insects like ants and keep them from coming back again.
If you're just looking to keep ants away from your marijuana garden, create a perimeter composed of diatomaceous earth and cinnamon powder with an inch-wide barrier. If aphids are still present, make sure you treat them as well. You can also ask your local gardening center for other products.