Celery really is tricky to grow decently, at least in my experience... the seeds need to be started early (I've started them in mid March to plant out around the end of May), the plants need a lot of water and need to be kept evenly moist, etc etc. I've been able to get nice big plants, but the individual stalks have tended to be pretty stringy (I think this is because I don't blanch them by covering for a while to keep the sun off), and often hollow (which I think is from uneven watering). I think the problem with growing the roots into plants with the idea of harvesting something that looks like "store-bought" celery might be that it's a biennial plant and would likely start going to seed at that point... I think my next attempt will be in self-watering containers, and I'm also just going to keep harvesting the outer stalks as they grow, like leaf lettuce.
