Hole in my collard green
Thank you for attaching the image. Something is certainly enjoying filling upon your greens! The most likely candidates causing the damage are caterpillars, slugs, and/or snails.
The best way to determine who they are is regularly scheduled search-and-destroy missions after dark. Go out with flashlight in hand, at 10 PM or so, and search both the top and reverse of the leaves. You might find any of the 3 listed above. When you find one, snip the soft-bodied ones in half, stomp, or drop into soapy water.
Another possibility is a caterpillar which feeds during the daytime. (It’s the youngster of the white cabbage butterfly you might have seen flying during the day.) They’re a velvety green, and match the color of the leaf. They often align themselves with the leaf veins. The youngest stages tend to feed from the undersides.
Uh, oh; I enlarged your image and found the culprit. (Well, one of them; more than likely, it has a number of relatives joining in the feast.) It’s the caterpillar of the white cabbage butterfly. (I’ve attached your image which I cropped; look just above the center, at the small, well-nibbled, nearly vertical leaf. The caterpillar is to the left of the mid-vein.)
So, daily search-and-destroy missions during the day will probably be sufficient and is the least toxic method for home-grown edibles. During the search, keep an eye peeled for the small, white, spindle-shaped eggs which stand vertically.
You could also use Bt (Bacillus thuringensis) which is widely available at garden centers but also realize that it’s effective only if the caterpillar is less than half-grown.
See http://ipm.ucanr.edu/PMG/r108301111.html; the links in the first paragraph reveal images of the egg, caterpillar, and adult.