Apium graveolens var rapaceum
A globular root vegetable that has a celery like flavour. Celeriac will grow successfully anywhere that celery grows. Takes approx 200 days to maturity, but roots can be lifted sooner if desired, the smaller roots will be more tender and less woody.
The edible portion is the swollen, knobby stem that forms at and beneath the soil surface. Requires peeling before cooking.