HELP! My plants are being snapped at the base!

For the last five days I have found some of my healthiest plants snapped at the base and toppled over. No rot, no visible insects other than some stray whiteflies. The plants that have been hit so far are a stevia, two amaranth, a cosmos and a marigold. All the leaves are intact, and it seems to happen in the early hours of morning, as when I find them they're still fresh and the leaves extended, as if it just happened. I have dugged to see the condition of roots, and all looks normal! As a matter of fact, a stump remains above ground from the cosmos, which looks healthy. Since it started, I have sprayed neem oil around their bases, put snail pellets around, and powdered with DE, but if it's an insect it is undettered by these treatments. They're all in the same area (sun-part shade), in zone 10.

cut worms?

Doesn't sound like insects, my guess would be either a 2 or 4-legged sort of pest, maybe bunnies but they probably would have eaten the plants, so more likely something else that's not necessarily planning to munch anything but is walking through the garden and breaks things off along the way.

It does sound like cut worms. If the plants you mention are young seedlings, those are most susceptible to cut worms. As the plants grow and the stems toughen up a little the cut worms won't bother them. Best way I have found to prevent their damage is to surround the base of the seedling with a collar made from the top half of a styrofoam coffee cup, pushed 1/2" into the dirt around the base of the plant.

At this time of year I was assuming they weren't seedlings anymore, I still think it was somebody/something walking through the garden snapping things off accidentally. I had a family of raccoons at my old house that trampled through the garden on a regular basis and they broke a lot of my plants.

Well, you did mention them being snapped off rather than chewed off. Don't know what to say. Try sitting in the garden all night with a flashlight. :-)

Mmmmmm.... maybe a few glue traps in the area will reveal a culprit ( I think I would rather sleep, blmlb ;-) Thanks for all the leads. Will keep you posted.

There are a number of products you can buy for repelling small animals, they repel a range of critters so you don't necessarily have to know exactly what it is. I'm not sure how well they work, the ones for underground critters like gophers don't work too well, but perhaps the above ground pests are more easily deterred.