I buy supermarket celery, and don't mind when it is slightly bitter, though I prefer the milder stalks. I like the leaves in salads and for cooking.
For the first, and perhaps last, time, a couple of weeks ago I bought a large, plump bunch of local celery at the last farmer's market of the season here near Boston. At home, I removed almost all the leafed stem-lets and bagged them separately. I trimmed the root end, halved the stalks crosswise, and put them into water in a large straight-sided lidded glass jar. The leaves and stalks are by far the most bitter celery I have ever encountered. Using a sweet dressing makes the leaves barely edible, but I haven't cooked with them for fear of ruining the stock or soup.
I tried eating the raw stalks. Normally I do not de-string celery but this stuff demands it. It's almost impossible to bite through the strings, and when I peel them, I get the whole outer layer of the stalk coming off, not just thin strings. The lighter part that remains is only slightly less bitter than the exterior.
Will steaming, stir-frying, or any other method of cooking mellow the celery? I suspect that even if I added something sweet like honey, or put it in a sweet marinade, the bitterness would still be overpowering. I don't want to ruin a bunch of other ingredients but would hate to throw the celery out if I can salvage it.