I do not know how to get the stalks to grow thicker , but if you were wanting to use some in soups or diced into salads , just cut some of the outer stalks near the base and use away . Just do not cut more than 1/3-1/2 of the plant at a time and give it time to recover. You will mostly find the celery you have grown to be a much stronger flavor than the store bought original . It is due to the different growing conditions.
