The most effective way to keep cabbage worms off your plants is to prevent the butterflies from laying their eggs. Netting works well. I have not heard of people having much success with companion plants as a deterrent.

Here is a link that might be useful: Net on brassica

Use BT. It's organic certified and effective.

One word of caution though -- if using to control hornworms, you'll want to use it when the larvae are small. If they get big, it's not very effective. I usually use it Day 1 on my tomatoes and peppers from the time I transplant outside. That way I can protect the "children" and finding the caterpillars when the plants are large is virtually impossible.

Good luck.

Kevin

I start the season with illusion tulle netting as a row cover. This year, after I removed the netting, I put down egg shell halves around the plants. I see cabbage butterflies around but didn't have any worms on my broccoli or cabbage.
Keski