Mid-girdle above, full girdle below
It’s funny what people think they know about ants. I went to my local Farmer’s Market where there is a booth staffed by master gardeners. I wanted to see if they’d have suggestions. ‘Ants don’t eat plants’, I got. We looked it up in their pest bible and they learned that yes, ants sometimes will eat plants. The staffer said she’d just pull up the plants, kill the ants, and start over.
I’m stubborn. So I’m still fighting. I haven’t lost too many more kale plants to the ants, but they were still there after two treatments of diatomaceous earth. I never did try the petroleum jelly, cause they’re girdling the stem inside the ground, too. Instead, I headed out with my kettle of boiling water.
Yes, I was prepared to lose the plants – if only I could stop the roiling masses of ants attached to the stems. So I poured and poured, like I was watering each plant, but with death (muaah, ha ha). The air had a peculiar steamed-broccoli smell to it, but I was satisfied I’d made some damage. Next day I go out, the plants do not appear dead, and there are still ants. Really? Maybe it’s a big nest under there, and the ground was pretty cold. I try again.
But, three days later, they’re back. It’s on to the borax and jelly.
Sustainable Scientist 
have you tried spraying with vinegar? – erin
a friend recommended this to me – put small piles of cornmeal where you see ants. They eat it, take it 'home,' can't digest it so it kills them. It may take a week or so, especially if it rains, but it works. Also if you mix half baking soda and icing sugar – put in small container and again the ants eat the mixture which will not digest and it kills them.
I've got the same problem, the ants have killed off two of my kale plants and have moved on to some of the others. I've tried the cornmeal thing and coffee grounds but it hasn't slowed them down. Have you had any success yet?
Well, the problem seems to have taken care of itself. I suspected my pest issues were related to the unusual weather – it was so wet there was nothing much to eat besides my tender plants. In addition to the ants, I had baby slugs galore and baby roly-polies eating down my young curcurbits once their true leaves emerged. Now that it's a teensy bit warmer, other things are beginning to flourish in the yard (if not exactly the garden), so I think the pests are dispersing. I lost most of the kale, but a few survivors, enough, are standing tall, and the new young kale I planted later is untouched. I stand at guard…