i would look for cutworms ... in my z5 ... no clue if that is possible in your 8 ...

check out the link.. flip over to the web side ...

ken

Here is a link that might be useful: link

Leonardocorn,

I agree with Ken. Cutworms would be my likely suspect. Google cutworms for more info. At this stage, a cardboard collar around the lone survivor might be your best bet. And use your finger or a stick to root around in the soil and find and destroy the cutworm(s). Slugs or snails would be secondary suspect if you see anything like a slime trail.

ZM

Completely gone = could be bunnies or squirrels. Agree it sounds like cutworm, though. You may also want to investigate "damping off" and see if that could be what happened. Keep trying, though, you'll get some!

If you include your zone in your profile info, folks will be better able to assist with more specific ideas, and better understand suggestions and contributions you may make.

Here is a link that might be useful: GardenWeb Zone Finder

I have had problems with groundhogs eating my sunflowers. In my case, they didn't become a problem until the plants were about 2 feet tall or so and taller. The groundhogs would pull the still pliable stems over and munch the leaves off.
Once the stalks/stems started to become more rigid, they could only eat what they could reach and then the damage was minor.
Not sure if groundhogs are definitely your problem, but may be a possibility. I haven't had to deal with cutworms myself, so I can't speak on that idea.

I had the same experience overnight last night--but my sunflower seedlings were in little pots on the porch. I suspect a groundhog, as we have an active one near the house, or squirrels.

You might try spraying with Liquid Fence or another critter repellent.