I was thinking my plant was small cause of the pot size and thought it would do better in the ground. But I read celery has very shallow roots, which is why people struggle to grow it.
Excessive heat, inadequate moisture, or lack of fertility will result in tough, strong tasting and stringy celery.
At planting time apply 10-10-10 complete fertilizer. Continue to fertilize regularly, every three weeks or so, with 1 tablespoon per plant. Sprinkle around the base of the plant, but not up against the stem, and water in.